Introduction

This chapter 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 speciality of dermatology.

The eye and 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 specialities. Eye diseases are managed within the medical speciality of ophthalmology, and ear diseases are managed within the surgical speciality of otorhinolaryngology (meaning the study of the ear, nose and throat). This speciality embraces diseases that relate to two overlapping systems: the nervous system and the respiratory system.

For clarity, in this section the diseases of the skin, eye and ear are considered in separate chapters as if each is an individual 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 (see Q.6.1a-1). The functions of the skin include:

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-I • A section of the skin, showing how the epidermis rests upon 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 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.

Figure 6.1a-II • The epidermis, illustrating the stratified squamous epithelial nature of this tissue.

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).

The eye and the optic nerve pathways in the brain

The eyes are sensory organs that are specialised 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 fibres carry visual messages to the brain to a control centre 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 fibres reach the thalamus, some of the fibres 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 tumours 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 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.

Figure 6.1a-IV • Cross-section of the eye, showing the three layers to its wall and the lens.

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).

Figure 6.1a-V • The extrinsic muscles that hold the eye in the bony orbit.

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).

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).

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 eyelids (see Q6.1a-12).

Figure 6.1a-VI • The position of the eye in the orbit and the accessory structures of the eye.

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).

Figure 6.1a-VII • The position of the lacrimal gland and the tear duct.

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 centres 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 (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).

Figure 6.1a-IX • The structures of the middle ear.

Figure 6.1a-X • Otoscopic view of the eardrum, showing the handle of the malleus and a shadow formed by the incus.

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).

Figure 6.1a-XI • The structures of the inner ear.

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 of the head (see Q6.1a-14-Q6.1a-19).

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.

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 in a patient 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 should 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 ‘dermoscope’, 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.

Culture and examination of samples taken by swab

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 important technique in the diagnosis of skin disorders. If the problem is small and localised (e.g. a worrying mole), a biopsy can both treat the condition by removing it and generate a sample for examination. This approach, known as ‘excisional biopsy’, ideally is 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 hollow sampling blade. This leaves a tiny hole, which may require a single stitch to stem bleeding.

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.

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.

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.

Examination of the visual fields

The ‘visual field’ is the term used to describe the extent 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 can reveal restrictions that are characteristic of certain diseases, including chronic simple glaucoma and damage to the optic nerve chiasm by pituitary tumours.

Examination of colour vision

Colour blindness can be diagnosed by means of pictures that are made up of many dots of different colours. Colour 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 humours, 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 the neurologist can make a direct, but non-invasive, inspection of part of the nervous system. In addition to visualisation 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 biomicroscope) 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 his or her 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 specialised ‘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 so 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 pressures.

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 specialised 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 humour, 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.

Electrophysiological tests

Specialised 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 conjuctiva can be investigated by means of culture and examination of samples taken by means of swab from the conjunctival secretions.

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 fibre-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 visualisation 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. Visualisation 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 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.

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 vibrator held against the mastoid process (the prominence of bone that sits posteroinferiorly 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