The Skin, the Eye and the Ear
6.1a The physiology of the skin, the eye and the ear
This section focuses on the physiology of three of the sensory organs of the body: the skin, the eye and the ear.
The skin is considered as a physiological system in its own right, also known as the integumentary system (a term derived from the word integument, which means skin). In conventional practice, diseases of the skin are managed within the medical specialty of dermatology.
The eye and the ear are, technically speaking, complex sensory organs that form part of the nervous system. However, in conventional practice the diseases of the eye and the ear are managed within distinct specialties. Eye diseases are managed within the medical specialty of ophthalmology, and ear diseases are managed within the surgical specialty of otorhinolaryngology (meaning the study of the ear, nose and throat). This specialty embraces diseases that relate to two overlapping systems: the nervous system and the respiratory system.
In this section the diseases of the skin, the eye and the ear are considered separately, as if each is an individual system.
The physiology of the skin (the integumentary system)
The functions of the skin1
The skin is often described as the largest organ of the body. It may not be immediately obvious that the skin, like many of the deep organs, has a range of complex functions. The functions of the skin include:
•Protection of the body: the skin provides a mechanical barrier that protects the deeper tissues from mechanical injury, toxins in the environment and radiation from the sun.
•Regulation of body temperature: the skin contributes to the homeostasis of body temperature by means of mechanisms including sweating in the heat and erection of body hair in the cold (goose pimples). The blood vessels in the skin also are very responsive to body temperature – they dilate in the heat and contract in the cold.
•Formation of vitamin D: the skin is the main source of vitamin D, which is essential for the maintenance of the health of the bones. Vitamin D prevents rickets and osteomalacia. It is manufactured in the skin (from a steroid precursor chemical) as a response to the action of sunlight.
•Sensation: the skin forms the boundary between the external environment and the internal environment of the body. The nerve endings in the skin are adapted to sense light, touch, deep pressure, pain and temperature. In this way the skin can communicate to the internal tissues what is going on in the outside world.
•Excretion: the skin is able to excrete unwanted substances through the sweat. It is also able to secrete aromatic chemicals, which act together with the extensive and unique community of microbes on the surface of the skin to give each person a unique smell that contributes to the non-verbal communication between individuals.
•Absorption: although this function is conventionally believed to be limited, the skin can absorb substances from the external environment. This can be interpreted as another aspect of the sensory role of the skin, allowing the body to respond to what is happening in the outside world.
The structure of the skin
The skin is composed of two thin layers known as the epidermis and the dermis. Throughout the body these two layers rest on a deeper layer of subcutaneous fat. The fascia, considered part of the musculoskeletal system, is the silvery fibrous connective tissue layer that underlies the subcutaneous fat and separates it from underlying muscles and bones. This is the glistening surface tissue that is seen surrounding the carcass when the skin of animals, for example, has been removed in the process of food production.
The epidermis is an example of stratified squamous epithelium, whereas the thicker dermis is made up of loose (areolar) and fatty connective tissue. Figure 6.1a-I illustrates the structure of these two delicate layers of the skin and how they rest on the underlying fat. The upper part of the dermis forms folds called papillae, and it is these that give rise to characteristic skin markings such as the fingerprint. The bumpiness prevents easy separation of the epidermis and dermis.
Figure 6.1a-I A section of the skin, showing how the epidermis rests on the dermis, which in turn rests on a layer of subcutaneous fat
Figure 6.1a-II is a more detailed image of the stratified structure of the epidermis. The cells of the epidermis continually multiply from the germinative layer and grow upwards in layers. As they progress upwards the cells become progressively more hardened with the protein keratin. The uppermost layers, the stratum corneum and lucidum, consist of lifeless keratin-stuffed cells, which have a vital protective function. This layer thickens in response to repeated trauma, such as that which continually happens to the palms of the hands and soles of the feet. As dead epithelial cells are shed from the skin they are replaced by new cells growing upwards from the germinative layer.
The pigment called melanin (dark brown), also produced by the cells of the epidermal layer, gives the skin its color. Clusters of deeply pigmented germinative cells form moles and freckles.
The sweat glands and the sebaceous glands originate in the looser areolar tissue of the dermis. The sweat glands are shaped like a coiled tube. They excrete a watery fluid onto the skin via a duct that passes through the epidermis. The watery fluid contains waste products. However, the most important function of the sweat is temperature control rather than excretion of wastes.
The sebaceous glands secrete an oily substance into the hair follicles and thence onto the skin. The sebum has a protective role in that it waterproofs the skin and hair, and helps to keep them soft and pliable. It is also believed to prevent penetrating skin infections.
The hair follicles are specialized tubules of germinative epithelium that generate the cells and the protein keratin, which form the strand of hair. A hair develops at the base of a hair follicle, where numerous keratin-containing skin cells are compacted together. As the skin cells die, the keratin remains and this is pushed out of the follicle to form a single hair. The arrectores pilorum muscles are tiny smooth muscle structures that attach the base of a hair follicle to connective tissue fibers in the dermis. When the arrectores pilorum muscles contract, the hairs become erect and stand on end.
The nails are also composed of keratinized dead skin cells. They grow out of the nail bed, which is an area of tissue lined with the rapidly dividing epidermal skin cells of the germinative layer.
The dermis is also richly supplied with nerve endings and blood capillaries. A healthy blood supply gives the skin a reddish pink glow. Bile salts (yellow/green) from the liver and carotenes (red/yellow) obtained from the diet also contribute to skin color.
The role of the skin in the control of temperature
It is vital that the body maintains a steady temperature because the complex metabolic processes of the body work optimally within a very narrow temperature range. If the temperature rises, the metabolic processes speed up, and if it drops, they slow down. If the metabolic rate becomes too fast or slow, the fine balance between the different chemical reactions in the body can be lost and ill health will result, as evidenced in someone suffering either from a prolonged high fever or from hypothermia.
The skin plays only a minor role in heat production. The main heat-producing tissues are the skeletal muscles, the liver and the digestive organs. Nevertheless, the contraction of the arrectores pilorum muscles in the skin does also generate a certain amount of heat. These tiny but numerous muscles also cause the body hair to stand on end, a response that will tend to conserve heat in a furry animal.
However, the most important physiological mechanism for heat conservation in the human is the constriction of the capillary network in the skin. This is mediated by the sympathetic nervous system. When vasoconstriction occurs, the blood is held deep in the body, and the skin will start to feel cold, and look pale or even blue. A similar response occurs in acutely frightening or stressful situations.
The role of the skin in the loss of heat is also very important. By far the largest proportion of body heat is lost from the skin. This heat loss is minimized by vasoconstriction and the erection of body hair. Body fat and layers of clothing also reduce this loss of body heat. In contrast, the loss of heat is increased by the physiological responses of vasodilatation and sweating.
Dilatation of the blood vessels in the skin raises the surface temperature of the skin, and so heat loss to the outside environment is increased. Vasodilatation is mediated by the parasympathetic nervous system, and is controlled by the hypothalamus. Sweating, which occurs when the body temperature rises by a fraction of a degree, causes the skin to become moist. Moisture requires energy (heat) to evaporate, and thus in hot, dry weather the evaporation of the sweat has a cooling effect.
Information box 6.1a-I
The skin: comments from a Chinese medicine perspective
In Chinese medicine, the skin and body hair are considered to be controlled by the Lungs, and sweating by the Heart. Other Chinese Organs play a part in the healthy function of the skin as it is conventionally understood. The Spleen is attributed to the control of healthy flesh and muscles. Temperature control depends on a healthy balance of Yin and Yang. This function of the skin might be attributed to the Kidney (in particular, the Ming Men) and Triple Burner Organs. The production of vitamin D in that it is essential for strengthening the bones would correspond to a function of the Kidney Organ.
The eye and the optic nerve pathways in the brain
The eyes are sensory organs that are specialized to respond to the stimulus of light. The eyes contain the cell bodies and nerve endings of the sensory nerves known as the optic nerves (second cranial nerves). Optic nerve fibers carry visual messages to the brain to a control center in the thalamus. From here the information is taken further back via connector neurons to the visual areas in the occipital lobe at the back of the brain. Here the complex processing necessary for visual perception takes place. Before the optic nerve fibers reach the thalamus, some of the fibers cross to the opposite side. This crossing point, known as the optic chiasm, is adjacent to the site of the pituitary stalk (overlying the sphenoid bone of the skull). This explains why pituitary tumors often result in visual disturbance. Figure 6.1a-III illustrates the optic nerves and their pathways from the eyes, which cross before they reach the occipital lobe of the brain.
Figure 6.1a-III The optic nerves and their pathways
Although the eyes are situated at the front of the head, the information that they sense is processed at the back of the brain. Because of the optic chiasm, information from each eye is sent to each half of the occipital visual area. This allows for the binocular vision, the facility that gives what we see an appearance of depth.
The structure of the eye
The eye has a very specialized structure that allows it to perform the function of focusing and sensing visual images. This structure is illustrated in simplified form in Figure 6.1a-IV.
The outer lining of the eye is composed of three distinct layers. The outermost layer, the sclera, is dense and glistening white in color. The sclera becomes transparent anteriorly; this clear area is called the cornea. The next layer, the choroid, is dark in color. This blends into the ciliary body and the iris at the front of the eye. The deepest layer is the light-sensitive retina. The retina is not present anteriorly, as it stops at the border of the ciliary body. The lens in the eye is responsible for focusing the light that enters the cornea so that it forms an image on the retina, akin to the way a camera lens focuses light onto a film. The space that lies between the lens and the cornea is called the anterior chamber. This chamber contains a watery fluid called the aqueous humor (in this situation the term humor means fluid). The space in the body of the eyeball behind the lens is called the posterior chamber. This chamber contains a jelly-like fluid called the vitreous (glass-like) humor.
The ciliary body is responsible for suspending the lens in the correct position in the eye. The smooth muscle of the ciliary body controls the stretching and relaxation of the lens. This muscle control of the curvature of the lens enables the eye to alter its focus so that both distant and close objects can be seen clearly.
The iris controls the amount of light that enters the eye, in a similar way to the aperture of a camera. The smooth muscle fibers in the iris contract and relax to vary the diameter of the pupil, the central hole in the iris. The pupil tends to contract in response to bright light, and also when the eye is required to focus on a close object. The color of the iris depends on the amount of brown and green pigment it contains. The amount and composition of these pigments is genetically determined.
The retina contains specialized sensory nerve cell bodies called rods and cones. These contain a light- and color-sensitive pigment that alters in structure in response to light of different wavelengths. Each rod and cone has a long nerve axon, which sweeps backwards and leaves the eye within the optic nerve. The rods play an important role in the vision of low-intensity light, and it is thus the rods that enable night vision. They are not sensitive enough to distinguish between lights of different wavelengths (i.e. of different color).
The cones are sensitive to different colors of light, but do not respond to light of low intensity. This is why it is not possible to distinguish colors in a darkened room.
The blind spot (optic disc) is the point at which all these nerve fibers converge to form the optic nerve. It is a tiny area of the retina that is devoid of rods and cones. Therefore, it is literally a blind spot, as it cannot respond to the light that falls on it. The macula lutea (yellow spot) is the region of the retina that holds the densest concentration of sensory cell bodies, and is the region that responds to light coming from the center of the field of vision. It is at the macula where fine detail and bright clear colors are distinguished.
The globe of the eye sits embedded within fatty tissue in the bony orbit. The optic nerve extends from the back of each eye to pass posteriorly through the sphenoid bone towards the optic chiasm (optic nerve crossing) within the skull. Six strips of muscle extend from the walls of the orbit to attach to the sclera of each eye. These are the extrinsic or extraocular muscles of the eye. All these structures hold the eye in place (see Figure 6.1a-V).
In thyroid eye disease, the eyes become pushed forward so that the eyelids appear to retract. In severe cases, the eyelids can no longer fully close. This is the result of excessive growth of the supportive fatty tissue in the orbit, which is stimulated by an autoantibody that appears in some cases of Graves’ disease (see Section 5.1c).
Figure 6.1a-V The extrinsic muscles that hold the eye in the bony orbit
The physiology of sight
The lens enables an image of the outside world to fall on the retina. For this to be possible for all objects in the field of vision, whether they are close or distant, the lens has to alter in shape to allow a clear image of the viewed object to be formed at the retina. Simply speaking, the lens needs to be fat (bulging) in shape to focus on a near object and thin (stretched) to focus on a distant object. In a healthy eye, when the ciliary muscle is relaxed, the lens is stretched so that it can focus on distant objects without any effort. The ciliary muscle has to work to contract so that the bulging lens can allow the eye to focus on close objects, and this is partly why close work can tire the eyes.
The eye has to make two other adjustments to enable close vision. First, constriction of the pupils takes place. This reaction enables only a thin beam of light to pass through the lens. This aids the focusing of the image because a narrow beam of light is easier to focus. This principle is used in a pinhole camera. Second, the eyeballs converge inwards when viewing a close object. When extreme, convergence causes the viewer to appear cross-eyed. Convergence allows the image of the object being viewed to fall onto the center of each retina. Without convergence the viewer would experience double vision.
In distance vision (objects more than 6 meters away) constriction of the pupils and convergence are not necessary, and also, as mentioned, the ciliary muscle becomes fully relaxed. It follows from this that the most restful aspect of vision is distance vision (e.g. when gazing on a countryside scene), as none of the three adjustments for close vision is required.
Each eye receives slightly different images, and the differences are more pronounced for close objects. The visual center of the occipital lobe of the cerebrum is the location for the interpretation of the two slightly different images that come from each retina, thereby giving the viewer an appreciation of the distance of a viewed object. The ability to visually appreciate distances is called stereoscopic vision.
The extraocular muscles and accessory organs of the eye
The six strips of skeletal muscle that form the extrinsic muscles of the eye guide the coordinated movements of the eyeballs (see Figure 6.1a-V). These movements, which are upwards, downwards, sideways and rotatory, are usually coordinated so that the two eyes move together.
Figure 6.1a-VI illustrates the extrinsic muscles of the eye in the orbit and the muscles that control the movements of the eyelids. One of these muscles, the levator palpebrae, lifts the eyelid when it contracts, while the other muscle, orbicularis oculi, because its fibers run circularly around the eyelids, closes the eyelid when it contracts. Figure 6.1a-VI also shows how the conjunctiva forms a protective sac behind the upper and lower eyelids.
The eyebrows, eyelids and conjunctiva are adapted to provide mechanical protection for the eye. The lacrimal gland is a specialized organ for producing the tears. Tears are constantly produced by this gland (see Figure 6.1a-VII) to lubricate the conjunctiva. Tears drain away via tiny channels called canaliculi to the nasolacrimal duct (tear duct), and thence to the nose. This is why a watery discharge from the nose frequently accompanies an episode of crying.
Figure 6.1a-VI The position of the eye in the orbit and the accessory structures of the eye
Figure 6.1a-VII The position of the lacrimal gland and the tear duct
Information box 6.1a-II
The eye: comments from a Chinese medicine perspective
In Chinese medicine, the Liver is the Organ that opens into the eyes. The Liver Blood moistens and nourishes the eye and gives it the capacity to see. However, as Maciocia (1989) explains, many other Yin and Yang Organs affect the eye. In particular, Kidney Essence and the Heart Organ are important in maintaining the health of the eyes.3
The functions of the ear
The ear is the sensory organ responsible for hearing. The ear is also an important sensory organ in the control of balance. The nerve impulses leave the ear via the auditory or vestibulocochlear nerve (eighth cranial nerve), which passes from the ear to the brain through a tiny hole in the temporal bone of the skull. Impulses from this nerve are transmitted via control centers in the brainstem and the mid-brain to the auditory area of the cerebral hemispheres (for the perception of sound) and the cerebellum (for the control of balance).
The structure of the ear
The structure of the ear can be considered in three parts: the outer, or external, ear, the middle ear and the inner ear. This structure is illustrated in Figure 6.1a-VIII.
Figure 6.1a-VIII The structure of the three parts of the ear
The external ear consists of the pinna (auricle), the auditory canal (acoustic meatus) and the tympanic membrane (eardrum). The main purpose of the external ear is to provide a protective opening for the delicate inner structures of the ear. The hair- and wax-lined auditory canal protects the eardrum from trauma, foreign bodies and infection. The pinna also plays a role in directing sound waves into the auditory canal, although it is more obviously adapted for this purpose in species such as dogs, cats and rabbits.
The middle ear is a chamber lined with respiratory epithelium, which contains three tiny interlinked bones (ossicles) known as the malleus (hammer), incus (anvil) and stapes (stirrup). The ossicles provide a link between the eardrum (tympanic membrane) and another membrane that separates the middle from the inner ear (the oval window). The purpose of the ossicles is to transmit the sound vibrations that strike the eardrum through to the cochlea of the inner ear. Tiny movements of the oval window lead to vibrations within the fluid (perilymph) that bathes the membranous cochlea (see Figure 6.1a-IX). The largest ossicle, the malleus, can be seen through the eardrum when the eardrum is visualized by means of the hand-held otoscope (see Figure 6.1a-X).
Figure 6.1a-IX The structure of the middle ear
From the perspective of the development of the ear in the embryo, the link that the middle ear has with the respiratory system becomes clearer. In the embryo, the middle ear forms from an upward pouching of the nasopharynx, whereas the inner ear develops outwards from the primitive nervous tissue. The middle ear and the interlinked mastoid air cells are lined with ciliated respiratory epithelium. The air and the surface microbes that they contain are in direct communication with the air and the surface microbes in the nasopharynx. This explains why respiratory infections can so readily involve the middle ear and lead to deafness and earache.
The spaces of the inner ear contain two interlinked fluid-filled membranous sacs which, like the middle ear, are safely protected, being bathed in fluid (perilymph) within bony cavities in the temporal bone of the skull. The first chamber, the cochlea, is spiral in form and contains fluid called endolymph. The cochlea is responsible for the interpretation of sound vibrations (see Figure 6.1a-XI).
Figure 6.1a-XI The structure of the inner ear
Within the cochlea, the spiral organ of Corti lies along its length and is fully immersed in the fluid of the endolymph. This tiny structure consists of a side-by-side arrangement of sensory nerve bodies, which are able to respond to sound vibrations. Messages are sent via nerve axons to auditory centers in the brain, where they can be interpreted as sound. Sound waves of different frequencies (corresponding to sounds of different pitch) stimulate different cells, and so pitch can be distinguished by the ear. Therefore, the organ of Corti is to the ear and hearing as the rods and the cones are to the eye and sight.
The second chamber of the inner ear consists of three semicircular canals, which sit in three different planes at right angles to each other. The semicircular canals are in direct communication with the cochlea, and, like the cochlea, also contain a membranous sensory organ, which is bathed in perilymph fluid. Movement of the head leads to fluid waves in the endolymph within the semicircular canals that are interpreted to allow perception of the head’s position in space.
The semicircular canals provide important sensory information to aid the maintenance of balance. This information is processed, together with visual information and sensory information from the large joints of the feet and leg, in the cerebellum. As the head moves into a new position, particular waves of fluid movement are set up in the three canals, which are then sensed by sensory cells in the dilated ends of the semicircular canals. This sensory information can be interpreted in the brain in terms of the position and rate of movement of the head.
Information box 6.1a-III
The ear: comments from a Chinese medicine perspective
From a Chinese medicine perspective, the Kidney Organ opens into the ears. Maciocia (1989) states that, if the Kidneys are weak, hearing may be impaired and there may be tinnitus (ringing in the ears). He also describes that the Heart has an influence on hearing, with some types of tinnitus resulting from Heart Qi Deficiency.5
6.1b The investigation of the skin, the eye and the ear
The diseases of the skin, the eye and the ear are managed within the distinct specialties of dermatology, ophthalmology and otorhinolaryngology (ear, nose and throat), respectively.
Investigation of disorders of the skin includes:
•a detailed inspection of the skin rash, including exposure to a Wood’s light
•culture and examination of samples taken by swab from the area of the rash
•examination of skin scrapings and nail clippings
•examination of a skin biopsy
•patch or prick testing for allergies
•a physical examination to exclude underlying medical conditions
•blood tests to exclude underlying medical conditions.
Investigation of disorders of the eye includes:
•examination of visual acuity using Snellen charts
•examination of the visual fields
•examination of color vision
•examination of the eye movements
•examination of the structure of the eye by means of a hand-held ophthalmoscope
•examination of the structure of the eye by means of a slit lamp
•examination of the intraocular pressure by means of a slit lamp and tonometry
•examination of the blood flow to the retina using fluorescein angiography
•imaging tests, including ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI)
•culture and examination of samples taken by swab from the eye.
Investigation of disorders of the ear includes:
•visual inspection of the external ear and eardrum
•clinical assessment of hearing
•testing of hearing by means of audiometry
•testing of the flexibility of the middle ear by means of impedance tympanometry
These investigations are considered below briefly in turn.
Investigation of disorders of the skin
Inspection of the skin rash
Pattern recognition is crucial in the assessment of skin disorders. A skilled dermatologist will be experienced in the recognition of the variety of ways in which skin disorders can manifest. Some skin disorders have such a characteristic appearance that once the rash is seen the doctor can be sure of the diagnosis. A physical sign that points to only one possible diagnosis is described as pathognomic (meaning that the sign itself leads to the knowledge of the underlying pathology).
Inspection of the skin should be thorough and systematic. All parts of the body might be examined to look for aspects of the rash that may not have been discovered by the patient. The skin may then be viewed under a hand-held microscope called a dermatoscope, which can reveal subtle changes to the skin markings not clearly visible to the naked eye. The rash may also be illuminated using ultraviolet light from a Wood’s lamp. This light causes some rashes to fluoresce, and can allow pigmented rashes to be viewed with more clarity.
If the cause of the rash is believed to be infective, the rash, or its discharge, may be swabbed, and the swab sent for microbiological analysis.
Examination of skin scrapings and nail clippings
Skin scrapings and nail clippings are samples that can be obtained painlessly. Microbiological examination of these samples can reveal fungal infection and infestation by scabies mites.
Examination of a skin biopsy
Skin biopsy is a very useful technique in the diagnosis of skin disorders. If the problem is small and localized (e.g. a worrying mole) then a biopsy can both treat the condition by removing it, and also generate a sample for examination. This approach, known as excisional biopsy, is ideally performed in such a way that the diseased area is completely removed, with no portion left remaining. A wide excision may be required to ensure full removal of a suspected skin cancer such as a melanoma. If only a small ellipse of skin is removed, the wound can be sutured so that the resulting scar is almost invisible. However, a wide excision may require a graft of skin from another part of the body (usually the outer thigh) to permit full healing of the area from which the section of skin was removed.
If the skin rash is more extensive, an incisional biopsy can be performed to remove a long ellipse of skin from the edge of the rash. As is the case with excisional biopsy, the resulting wound will require closure by sutures. Examination of the incisional biopsy should reveal both normal and affected skin, and thus allow comparison of the two.
A punch biopsy removes a fine column of affected skin by means of a tubular sampling blade. This leaves a tiny hole, which may require a single stitch to stem bleeding.
Patch or prick testing for allergies
Patch testing is used to test for sensitivity to various allergens that can result in contact dermatitis (contact eczema). The test involves holding diluted samples of a battery of common allergens against a flat area of skin (usually of the back) for up to 48 hours by means of an adhesive strip of patches. The development of a thickened red area of skin under one of the patches is indicative of sensitivity to that allergen. Allergens that commonly cause positive reactions in the patch test include nickel, rubber and colophony (a common constituent of sticking plasters).
Patch testing is a specific test for contact eczema. The aim is to exclude a delayed allergic reaction (type 4 hypersensitivity) that manifests in dermatitis/eczema (redness, localized swelling and itch) after two days or more of exposure to the allergen.
Prick or scratch tests are allergy tests that are aimed at excluding the more immediate allergic reaction that leads to urticaria, edema and asthma (type 1 hypersensitivity). In prick or scratch testing, a diluted sample of allergen is allowed to penetrate under the epidermis via the prick or the scratch, and the skin is examined for the appearance of a characteristic wheal. This reaction tends to develop within seconds to minutes if the test is positive. Allergens that are commonly found to lead to positive reactions in prick or scratch testing include animal dandruff, pollen and dietary constituents such as egg and nuts.
Physical examination to exclude underlying medical conditions
Dermatologists have a thorough grounding in clinical medicine. If a rash is found that indicates a possible medical condition as the underlying cause, the dermatologist will proceed to examine the patient for other features of that disease.
Blood tests to exclude underlying medical conditions
Blood tests may be ordered to exclude a suspected underlying medical diagnosis. For example, the blood may be tested for autoantibodies if an autoimmune condition such as systemic lupus erythematosus (SLE) is suspected as the cause of the rash.
Investigation of disorders of the eye
Eye disorders are often first picked up during examination by an optometrist (also known as optician). The optometrist routinely tests for clear, unrestricted sight by means of tests for visual acuity, the integrity of the visual fields, color vision and eye movements. The optometrist will also examine the internal structure of the eye by means of a hand-held ophthalmoscope. The optometrist may also check the visual fields and also the internal pressure of the eye by means of tonometry (see below), and also take a high-resolution photograph of the retina to check for signs of disease.
A hospital ophthalmologist will also perform these investigations, but has access to other powerful tools of examination, including the slit lamp, fluorescein angiography and imaging tests.
Examination of visual acuity
Visual acuity (acuteness) is the medical term used to describe the ability of the eye to see a clear image of the viewed object. A normal level of acuity depends on the health of the cornea, lens, vitreous humor, retina and the optic pathways. However, even in a healthy eye, acuity is always limited by the size of the image that lands on the retina. It makes sense that the smaller the image, the less clearly its features can be seen. The different aspects of an image that falls on the retina simply cannot be distinguished if they are smaller than the size of single cones on the retinal surface.
Acuity can be tested simply by means of the Snellen chart, probably the most familiar test associated with the optometrist. The Snellen chart consists of rows of letters (or pictures for those patients who are unable to read) of progressively decreasing height. The patient is placed at a fixed distance from the chart and is asked, for each eye in turn, what is the smallest size of letter that they can read with confidence. This is then compared with what is known to be a normal and healthy level of acuity.
The optometrist will then proceed to explore to what extent correcting the vision by means of a lens will improve the acuity. The degree to which a lens can correct the vision depends on the exact cause of the impairment in acuity. If the impairment is due totally to a problem in the focusing of the image by the lens in the eye, a perfectly suited lens should be able to offer the patient a return to normal visual acuity. If, however, the problem is a result of, for example, cataract or retinal damage, a lens may not be able to improve vision very much.
Examination of the visual fields
The visual field is the term used to describe the width and height of the view from each eye when the person is looking straight ahead. In health, even when looking straight ahead, movement that occurs almost to the side of the head can be detected because of the extent of the visual field. The visual field is less extensive to the medial side of the eye because the bridge of the nose gets in the way. However, objects can be seen when they are held almost at a right angle to the direction of the gaze on the lateral side of the eye.
There is a small area of the visual field, slightly lateral to the area on which the eye focuses, in which the image of the object becomes less distinct. This area is the blind spot, and corresponds to the area on the retina overlying the origin of the optic nerve, known as the optic disc.
Mapping of the visual fields and the blind spot is performed by means of asking the patient to register when they have seen tiny lights on a screen. This test can reveal restrictions that are characteristic of certain diseases, including chronic simple glaucoma and damage to the optic nerve chiasm by pituitary tumors.
Examination of color vision
Color blindness can be diagnosed by means of pictures that are made up of many dots of different colors. Color blindness will prevent an observer from distinguishing certain aspects of the pictures.
Examination of the eye movements
The eye movements are assessed by asking the patient to follow a moving object in the field of vision while keeping the head still. During the test the examiner watches for divergence or convergence of the eyes, and questions the patient about any episodes of double vision. This examination can reveal the presence of a squint (strabismus).
Examination of the structure of the eye by ophthalmoscope
The ophthalmoscope is a hand-held instrument through which the examining practitioner can focus on the various internal structures of the eye, ideally with the patient in a darkened room. The ophthalmoscope should allow inspection of the cornea, the iris and lens, the vitreous and aqueous humors, the pattern of vessels on the retina, and the optic nerve head (the disc).
Ophthalmoscopic examination of the retina is the only means by which a doctor can make a direct, but non-invasive, inspection of part of the nervous system. In addition to visualization of the optic nerve head, examination with an ophthalmoscope can reveal the characteristic changes that occur in a wide range of eye diseases, including cataract, vitreous detachment, retinal detachment, diabetic retinopathy and the changes that result from high blood pressure.
Examination of the structure of the eye by means of the slit lamp
The slit lamp (or bio-microscope) allows the examining practitioner to obtain a high-quality view of the external and internal structures of the eye. Prior to a slit lamp examination, a few drops of a mydriatic (a drug that causes widening of the pupils) are placed into the patient’s eyes. This can have the effect of causing blurred vision until the drug wears off, and so the patient is advised not to drive until this side effect has faded. Once the mydriatic has taken effect, the patient sits in a chair in a darkened room and, to ensure immobility, places their head against a headrest attached to the slit lamp. The magnifying lens of the slit lamp can then be focused on the various structures of each eye in turn. A fluorescent dye called fluorescein can be instilled into the eye to reveal scratches and ulcers on the corneal surface. These damaged areas fill up with fluorescein, and will fluoresce green when viewed through the slit lamp.
Examination of the intraocular pressure by means of the slit lamp and tonometry
The fluid contents of the healthy eye are maintained at a steady level of pressure, which ensures that the tension of the lining of the eye is of a sufficient degree to maintain its spherical shape, but is not so high that the delicate nerve endings of the rods and the cones are damaged. Glaucoma is the name of the sight-threatening condition that results from excessively high intraocular pressure.
The intraocular pressure can be assessed by means of a sensitive pressure gauge called a tonometer. The measurement of the pressure can be made during a slit lamp examination during which the tonometer is held against each of the corneas in turn. This non-invasive examination, which takes a matter of seconds, is uncomfortable but not painful.
Examination of the blood flow to the retina using fluorescein angiography
Fluorescein angiography is a more invasive technique, which involves the injection of fluorescein into the circulation. The retina is then illuminated with blue light, which causes the fluorescein flowing through the retinal arteries to fluoresce with a bright green light. A photograph of the fluorescing vessels is then taken by means of a specialized camera. This examination can reveal more detail about the damage to the retinal circulation that can result from conditions such as chronic diabetes mellitus or atherosclerosis.
Imaging tests including ultrasound, CT and MRI scans
The ultrasound scan is a frequently used tool in ophthalmology to assess the health of the vitreous humor, the retina and the posterior coats of the eye. It is particularly useful when a cataract in the lens prevents slit lamp examination of the structures behind the lens.
CT and MRI are of value in assessing the shape of the orbit and the pathways of the optic nerves as they pass from the optic nerve head to the brain.
Specialized electrophysiological tests can be used to assess the responses of the nerves in the retina, optic nerve pathways and the visual cortex (at the back of the brain) to visual stimuli (such as flashing lights) presented to the eyes.
Culture and examination of samples taken by swab from the eye
Infections of the conjunctiva can be investigated by means of culture and examination of samples taken by means of swab from the conjunctival secretions.
Investigation of disorders of the ear
Visual inspection of the external ear and eardrum
The external ear (the pinna), the external ear canal and the outer aspect of the eardrum can be assessed by means of a hand-held instrument called an auriscope (or otoscope). The auriscope provides illumination of the external ear canal by means of a fiber-optic light source. The auriscope is inserted painlessly into the outer ear by means of a plastic conical speculum. The examining practitioner places gentle traction on the pinna of the ear to enable straightening of the external ear canal and so to permit visualization of the eardrum (see Figure 6.1a-X).
Examination of the external ear and external ear canal can reveal the changes caused by otitis externa and the excessive accumulation of earwax. Visualization of the eardrum will demonstrate certain problems affecting the middle ear, including acute otitis media, chronic secretory otitis media (glue ear), perforated eardrum and cholesteatoma.
Clinical assessment of hearing
As deafness is one of the major symptoms that accompany diseases of the middle and inner ear, clinical assessment of hearing can provide very useful diagnostic information. Voice and whisper tests are rough and ready methods of assessing the hearing in each ear. While simple to perform, they can be very useful in certain patient groups, young children in particular.
A more specific test involves the use of a tuning fork. A large tuning fork is used, which when percussed will produce a note of low pitch. The vibrating tuning fork is placed close to the external ear canal, and then held with its foot touching the central area of the forehead (close to the acupuncture point Yin Tang). The patient is asked to compare the volume of sound produced by both positions, and to say where they hear the sound when it is placed against the forehead. If the hearing problem is located in the middle ear (e.g. in glue ear), the sound conducted by the bones of the head, when the fork is placed on the forehead, will be louder than that produced when it is held close to the affected ear (in health the tuning fork will be loudest when placed close to the external ear canal). This is because bone conduction of sound bypasses the blocked middle ear, and will be heard by a healthy inner ear.
If the hearing in the inner ear on one side is compromised, the sound of the tuning fork when placed against the forehead will sound as if it is closer to the unaffected side.
Testing of hearing by means of audiometry
The audiometer is a machine that produces pure tones of sound at varying frequencies and varying intensities. The sound is fed to the patient either through earphones or via a microphone held against the mastoid process (the prominence of bone that sits postero-inferiorly to each ear). In this way air conduction of sound (to the middle ear) can be compared against bone conduction (directly to the inner ear).
The audiogram is a chart of hearing produced as a result of audiogram testing. This chart reveals the threshold of hearing for varying intensities of sound at different frequencies for each ear. Different hearing problems result in characteristic audiograms. For example, the damage to hearing that results from exposure to excessive noise results in impairment of hearing at high frequencies (high-pitched sound).
Speech audiometry is a variant of audiometry in which a recorded word list, rather than pure tones, is presented to each ear.
Testing of the flexibility of the middle ear by means of impedance tympanometry
Impedance tympanometry is a less patient-dependent non-invasive test which assesses objectively the degree to which the eardrum reflects the pure tones with which it is presented by an audiometer. This is a very useful test in assessing the degree to which glue ear is compromising hearing in babies and young children.
Electrophysiological tests (electric response audiometry)
The nerve responses to a sound wave that reaches the inner ear can be tested by means of a similar method to that used to test the nerve responses in the visual pathways. In electric response audiometry, a sound stimulus is applied to the outer ear. The response to this stimulus in the nerves that originate in the inner ear, and which travel via the auditory nerve to the auditory cortex in the brain, can be assessed by means of sensory electrodes placed against the bones of the middle ear, the base of the neck and the skull.
Because this non-invasive technique requires no response from the patient, it can be used in babies and young children, and also, by dint of its objectivity, to provide evidence in cases involving litigation for industrial deafness.
6.1c Diseases of the skin
The skin is able to react rapidly to a very wide range of environmental and internal factors, and in some cases this leads to disease. Known triggers for skin disease include genetic susceptibility, degeneration from aging, infectious agents, physical trauma, allergic reactions, light, heat, cold, psychological stress and self-harm. However, in many cases of skin disease the underlying cause is never clarified. Any one trigger can lead to a diverse range of manifestations of skin disease and it is often not very clear exactly why one person under stress develops itch and another psoriasis, or why antibiotics in one person will cause a painful blistering reaction and in another a rash of red spots.
From a more holistic perspective, it is generally appreciated that the skin, as it is the most superficial organ, might readily express symptoms and signs as an indication of a deeper imbalance. These would be the result of a combination of both internal and external factors, which would begin to explain the diversity of skin diseases.
The aim of this section is to introduce the wide range of ways in which the skin can manifest symptoms, and also the common treatment approaches used in dermatology. This section is split into two parts, which are designed to be studied in separate study sessions.
Generalized diseases in which the skin can be involved include peripheral vascular disease, diabetes mellitus, thyroid disease, Cushing’s syndrome, Addison’s disease, rheumatoid arthritis, chronic liver disease and a wide range of infectious diseases. As all these diseases have already been described in some length in earlier sections, their skin manifestations (with the exception of some of the important infectious skin diseases) are not mentioned further here.
The conditions discussed in this section are:
Part I: Skin infections, acne and scaling conditions
–cellulitis and erysipelas
–boils, abscesses and folliculitis
–scalded skin syndrome
–rashes in generalized viral infections
–herpes simplex infections
–herpes zoster infections
–hand, foot and mouth disease
–tinea (athlete’s foot and ringworm)
–candidal (yeast) infections
–pityriasis (tinea) versicolor
–head, pubic and body lice
–fleas and papular urticaria
Part II: Moles and tumors, connective tissue disorders and other common skin conditions
•Moles and other nevi:
–freckles and melanocytic nevi (common moles)
–stork marks and blue spots in newborns
–Campbell de Morgan red spots
•Skin tumors – benign lumps:
–seborrheic warts (senile keratoses)
–epidermal cysts (sebaoceous cysts), lipomas and milia
•Skin tumors – malignant skin tumors:
–basal cell carcinoma (BCC)
–squamous cell carcinoma (SCC) and Bowen’s disease
–Paget’s disease of the nipple
–lymphoma and the skin
•Connective tissue disorders:
–systemic lupus erythematosus (SLE) and discoid lupus erythematosus (DLE)
–scleroderma and morphea
•Disorders of hair growth:
–excessive hair growth
–abnormal scalp hair loss
•Other common skin conditions:
–urticaria and angio-edema
–prickly heat (miliaria)
–polymorphic light eruption (PLE)
–xanthelasmata and xanthomata
–reactions to drugs.
Part I: Skin infections, acne and scaling conditions
CELLULITIS AND ERYSIPELAS
Cellulitis is a rapidly spreading streptococcal or staphylococcal infection of the subcutaneous and deeper tissues. Erysipelas is a term given to the most superficial forms of cellulitis, but clinically there is no clear distinction between the two conditions. Both manifest as a spreading redness and swelling of the skin and underlying tissues, and are accompanied by fever, malaise and enlarged local lymph nodes.
Cellulitis is not usually a contagious condition and appears to be the result of a breakdown in the usual barrier of the skin to its own microbes. It is a known complication of needle puncture. The risk of infection as a result of acupuncture is minimized by preparation of a clean field, use of sterile single use needles and scrupulous hand washing.6,7
The conventional treatment is with oral or systemic antibiotics (penicillin in most cases).
Information box 6.1c-I
Cellulitis and erysipelas: comments from a Chinese medicine perspective
A Chinese medicine interpretation is that the cellulitis or erysipelas is a manifestation of Wind Heat. The infection is more likely to become apparent if there is Depletion of Blood/Yin and/or Internal Heat.