Diseases of the skin, the eye and the ear

6.1 Diseases of the skin, the eye and the ear



Chapter 6.1a The physiology of the skin, the eye and the ear



Learning points





The physiology of the skin (the integumentary system)



The functions of the skin


The skin is often described as the largest organ of the body. It may not be obvious immediately that the skin, like many of the deep organs, has a range of complex functions image(see Q.6.1a-1). 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 give each person a unique smell that contributes to the non-verbal communication between individuals.


Absorption: although this function is believed conventionally 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, which is considered part of the musculoskeletal system, is the silvery fibrous connective layer that underlies the subcutaneous fat and separates it from underlying muscles and bones. This is the tissue that is seen when the skin of animals is 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-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 continually occurs 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 colour. 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 help keep skin infections at bay.


The hair follicles are specialised tubules of germinative epithelium that generate the cells and the protein keratin, which will form a 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 fibres in the dermis. When the muscle contracts, the hair becomes erect and ‘stands on end’.


The nails are also composed of keratinised 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 colour (see Q6.1a-2-Q6.1a-4).image



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 best 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 which 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 minimised 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 imageeffect (see Q6.1a-5 and Q6.1a-6).




The physiology of the eye




The structure of the eye


The eye has a very specialised 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 colour. The sclera becomes transparent anteriorly; this clear area is called the ‘cornea’. The next layer, the choroid, is dark in colour. 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 humour’ (in this situation the term ‘humour’ 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) humour’.


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 fibres 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 colour 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 specialised sensory nerve cell bodies called ‘rods’ and ‘cones’. These contain a light- and colour-sensitive pigment which 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 colour).


The cones are sensitive to different colours of light, but do not respond to light of low intensity. This is why it is not possible to distinguish colours in a darkened room.


The blind spot (optic disc) is the point at which all these nerve fibres 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 most dense concentration of sensory cell bodies and is the region that responds to light coming from the centre of the field of vision. It is at the macula where fine detail and bright clear colours 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 (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 Q6.1a-7-Q6.1a-10)image.



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 be 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 a near object and thin (stretched) to focus 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 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. Secondly, 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 centre of each retina. Without convergence the viewer would experience double vision.


In distance vision (objects more than 6 metres 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 are required.


Each eye receives slightly different images, and the differences are more pronounced for close objects. The visual centre of the occipital lobe of the cerebrum is important in 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’ (see Q6.1a-11)image.



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 which 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 fibres 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 eyelidsimage (see Q6.1a-12).



The eyebrows, eyelids and conjunctiva are adapted to provide mechanical protection for the eye. The lacrimal gland is a specialised organ for producing the tears. Tears are constantly produced by this gland (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 (see Q6.1a-13)image.





The physiology of the ear




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.



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 (Figure 6.1a-IX). The largest ossicle, the malleus, can be seen through the eardrum when the eardrum is visualised by means of the hand-held otoscope (Figure 6.1a-X).




From the perspective of the development of the ear in the embryo, the link that the middle ear has with the respiratory system becomes more clear. 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 that they contain is in direct communication with the air 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 (Figure 6.1a-XI).



Within the cochlea, the spiral organ of Corti lies along its length and is fully immersed in endolymph fluid. 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 centres 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 which 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 imageof the head (see Q6.1a-14-Q6.1a-19).




imageSelf-test 6.1a The physiology of the skin, the eye and the ear





Answers




1.





2. The three layers of the eye are the sclera, the choroid and the retina.


The sclera forms the cornea anteriorly. This is the transparent window of the eye.


The choroid forms the ciliary body and the iris anteriorly. The ciliary body suspends the lens between the anterior and posterior chambers of the eye. By contraction and relaxation, it can vary the accommodation of the eye. The iris provides a variable aperture to control the amount of light that enters the eye.


The retina is discontinued anteriorly, stopping just behind the ciliary body.


3. The three parts to the ear are the external ear, the middle ear and the inner ear.


Sound waves are funnelled into the external ear by means of the pinna. The sound travels down the short external auditory meatus to the eardrum. Vibration of the eardrum is transmitted and amplified by the three ossicles in the middle ear to the membrane of the oval window. Vibration of the oval window sets up fluid waves in the perilymph of the cochlea. This, in turn, causes vibration of the endolymph, which bathes the organ of Corti. The organ of Corti contains sensory nerve cells, which transform these fluid movements into sensory messages, which are then sent down the vestibulocochlear nerve.


Movements of the head cause fluid movements in the perilymph and endolymph of the semicircular canals. These movements stimulate sensory nerve endings, which also respond by sending sensory messages down the vestibulocochlear nerve.



Chapter 6.1b The investigation of the skin, the eye and the ear



Learning points




Introduction


The diseases of the skin, eye and ear are managed within the distinct specialties of dermatology, ophthalmology and otorhinolaryngology (ear, nose and throat), respectively.


Investigation of disorders of the skin includes:



Investigation of disorders of the eye includes:



Investigation of disorders of the ear includes:



These investigations are considered below briefly in turn.



Investigation of skin disorders







Patch testing for allergies


Patch testing is used to test for the 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 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 a 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) which manifests in dermatitis/eczema (redness, localised swelling and itch) after 2 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, oedema 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.





Investigation of eye disorders


Often, eye disorders are first picked up during examination by an optician. The optician routinely tests for clear, unrestricted sight by means of tests for visual acuity, the integrity of the visual fields, colour vision and the eye movements. The optician will also examine the internal structure of the eye by means of a hand-held ophthalmoscope.


A hospital ophthalmologist will also perform these investigations, but also has access to other powerful tools of examination, including the slit lamp, tonometry, 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 humour, 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 is, the less clearly its features can be seen. This is because 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 optician. 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 he or she can read with confidence. This is then compared with what is known to be a normal and healthy level of acuity.


The optician 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 problem is due totally to a problem in the focusing of the image by the lens in the eye, glasses or contact lens should be able to offer the patient a return to normal visual acuity. If, however, the problem is due, for example, to cataract or retinal damage, glasses may not be able to improve vision very much.













Investigation of ear disorders




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 Yintang). 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.





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



imageSelf-test 6.1b The investigation of the skin, the eye and the ear





Answers




1. The optician should have already tested the visual acuity in each eye by means of the Snellen chart, the visual fields by means of confrontation with a moving object, and the eye movements. Evidently, reduced visual acuity was found affecting the eye in which the optician suspects cataract formation. Ophthalmological examination will then have revealed a cloudy obscurity in the lens, which will have prevented clear visualisation of the retina.


These tests might have been repeated at the appointment in the ophthalmology department (eye clinic), but the additional test that the patient should expect is slit lamp examination. First, his pupils will be dilated by means of mydriatic drops, and then he will be asked to sit in front of the biomicroscope in a darkened room. The examining doctor will probably also want to assess the intraocular pressure by means of tonometry, as well as examine the cataract. If surgery to the cataract is considered, an ultrasound scan of the eye may be arranged to assess the structure of the eye posterior to the cataract.


2. In the first instance, the child is likely to be investigated by the GP or health visitor. With young children, simple distraction tests can be used to assess hearing loss. The child is distracted by means of an entertaining event while sounds are made or words whispered behind each ear in turn. A child with healthy hearing will normally turn from the entertainment to investigate the origin of the noise.


With an older child, a tuning fork assessment may give useful information, and in the case of glue ear confirm that the deafness is conductive (i.e. the location of the problem is in the middle ear).


Examination with an auriscope (otoscope) might reveal the characteristic dull appearance of the eardrum in a case of glue ear.


A more detailed assessment can be made after referral to an audiologist, who can generate an audiogram for each ear and perform impedance tympanometry.


3. The most important aspect of dermatological assessment is pattern recognition. The experienced dermatologist might have a good idea of the diagnosis in this case simply by detailed inspection of the rash and its pattern of distribution.


If fungal infection or scabies infestation is suspected, skin scrapings might be taken for microbiological analysis, and a Wood’s lamp might be used to view the rash. If a bacterial or viral infection is a possibility (unlikely in this sort of distribution of rash), swabs might be taken of any fluid exuding from the rash. If the rash appears to be eczema, patch testing might be used to investigate whether or not the patient is allergic to any one of the common environmental allergens known to precipitate contact dermatitis.


Although unlikely to be performed in a first appointment, skin biopsy might be used to exclude the possibility of conditions such as psoriasis or pemphigus (a blistering condition).

Oct 3, 2016 | Posted by in MANUAL THERAPIST | Comments Off on Diseases of the skin, the eye and the ear
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