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Box 21.6
Causes of optic atrophy
Congenital:
– Dominant and recessive
Optic neuritis
Chronic papilloedema
Toxic:
– Tobacco, lead, alcohol
Optic nerve compression:
– Thyroid
– Tumour
Post- traumatic (direct optic nerve or indirect vascular
damage)
Box 21.7
Causes of the appearance of optic disc swelling
Raised intracranial pressure (papilloedema)
Infiltration:
– Lymphoma
– Sarcoid
Vascular:
– Hypertension (grade 4)
– Central retinal vein occlusion
– Anterior ischaemic optic neuropathy
Pseudopapilloedema:
– Congenital small discs
– High hypermetropia
Optic disc drusen
Figure 21.11 Papilloedema: optic disc swelling caused by raised
intracranial pressure.
power until the centre of the disc is just in focus.
The retina, a short distance from the disc, is then
brought into focus by further reduction of the
lens power. This further reduction indicates the
degree of swelling of the disc (3D is equivalent
to 1 mm of swelling). If papilloedema develops
rapidly, there will be marked engorgement of the
retinal veins with haemorrhages and exudates
on and around the disc, but with papilloedema
of slow onset there may be little or no vascular
change, even though the disc may become very
swollen. The retinal vessels will, however, bend
sharply as they dip down from the swollen disc
to the surrounding retina. The oedema may
extend to the adjacent retina, producing greyishwhite striations near the disc (Paton’s lines), and
a macular fan of hard exudates temporal to the
fovea may develop in some cases.
3. Is the central cup enlarged? The cup is a
physiological central depression formed at the
optic disc as nerve fibres leave the retina to form
the optic nerve. It marks the point where the
retinal vessels enter and leave the eye. It is paler
than the surrounding rim of the disc. The optic
cup- to- disc ratio is estimated by comparing their
ratios vertically. In chronic open- angle glaucoma,
the ratio is increased (>0.3)—optic disc cupping.
When the cup is deep, in advanced glaucoma,
Figure 21.12 Severe papilloedema with retinal haemorrhages.
Figure 21.13 Glaucomatous disc cupping. The cup is oval in the
vertical plane and appears pale. The retinal vessels are displaced
nasally.
retinal vessels disappear as they climb from
the floor to the rim, and reappear as they bend
sharply over the edge of the rim (bayonetting);
in less advanced cases, the cup appears as a
vertical oval extending to the edge of the disc
(Fig. 21.13). In myopic individuals, the disc
and cup appear large, and mimic glaucoma.
Myopes often have a partial ring of pigmentation
or white sclera surrounding the disc, which is
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Figure 21.14 Myelinated nerve fibres: the white area obscures the
disc; this is a normal variant.
easily mistaken for the edge of the cup. In severe
myopia, degenerative chorioretinal changes
may occur in the fundus, which can involve the
macula and impair central vision.
4. Are there any other abnormal features? In
proliferative diabetic retinopathy, new blood
vessels (neovascularization) develop at the optic
disc. Myelinated nerve fibres have a dramatic
white appearance, but they are a unilateral,
harmless and non- progressive congenital
anomaly (Fig. 21.14). They have a characteristic
feathered edge that may obscure the retinal
vessels.
Blood vessels
Four pairs of arterioles and venules form the main
retinal vascular arcades that emerge from the optic
disc: superotemporal (above the macula), inferotemporal (below the macula), superonasal and
inferonasal. Study each in turn. Arterioles are thin,
bright red in colour and with a longitudinal streak
of light reflection. In branch arteriolar occlusion,
a bright yellow (cholesterol) embolus may occasionally be seen (Fig. 21.15). In diabetes or venous
occlusion, the venules are larger, darker and often
dilated or tortuous. Look carefully at arteriolar/
venous crossings: compression and localized dilatation of venules (arteriovenous (AV) nipping)
with arteriolar narrowing (attenuation) is a sign
of hypertension (Box 21.8, Fig. 21.16). Spontaneous arteriolar pulsation is an abnormal finding that
may occur if the intraocular pressure (IOP) is very
high or the central retinal artery pressure very low.
Spontaneous venous pulsation is frequently seen in
normal eyes, but is reduced in papilloedema.
Figure 21.15 Retinal arteriolar emboli. Cholesterol emboli in
the retinal arteries of a patient with atheromatous disease of the
internal carotid artery in the neck.
Box 21.8
Grade 1: arteriolar narrowing (attenuation) and vein
concealment
Grade 2: profound arteriolar attenuation and
venous deflections at crossings (arterior venous
(AV) nipping)
Grade 3: severe attenuation ‘arteriolar copper wiring’,
haemorrhages, cotton wool spots and hard exudates
Grade 4: all of the above, plus very severe attenuation
‘arteriolar silver wiring’ and optic disc swelling
Appearance and classification of hypertensive
retinopathy
Retina and macula
As each main vascular arcade is followed and exam-
ined, the adjacent and peripheral retina can be
systematically assessed. The macula is the central
retinal area bounded by temporal vascular arcades. It
measures approximately five disc diameters across.
Figure 21.16 Hypertensive retinopathy. The arteries are irregular
in calibre and show ‘silver wiring’. Arteriovenous nipping is present.
Characteristic ‘flame- shaped’ haemorrhages and ‘cotton wool’
spots (arrow) can be seen.
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Box 21.9
White
Common retinal abnormalities
Cotton wool spots: white, fluffy, indistinct areas
indicative of retinal ischaemia. This is the accumulation
of axonal proteins in the nerve fibre layer. Causes
include severe hypertension, diabetes and retinal vein
occlusion.
Chorioretinal atrophy: well- defined, ‘punched- out’ lesions
(the white is the sclera). May occur in conjunction with
retinal pigment hypertrophy. Associated with previous
retinal inflammation or injury (including retinal laser).
Yellow
Hard exudates: bright yellow with well- demarcated
edges consisting of lipid deposits that have leaked out of
abnormal blood vessels. Most commonly associated with
microaneurysms in diabetes (Fig. 21.17).
Drusen: small multifocal round yellow features, usually
located in the central macula. Generally smaller and
less bright yellow than hard exudates. Typically bilateral
and relatively symmetrical. Common in elderly people
associated with ‘dry’ age- related macular degeneration.
Red
Microaneurysms: the dots that typify diabetic retinopathy.
They may leak to cause exudates or bleed to cause blot
haemorrhages (Fig. 21.17).
Blot haemorrhages: rounded localized intraretinal
blood, typically caused by diabetic retinopathy, but
other causes include severe hypertension and retinal
vein occlusion.
Deep large haemorrhages: associated with retinal
ischaemia when numerous.
Flame haemorrhages: have a characteristic feathery
shape as the blood is in the nerve fibre layer; may
be present in retinal vein occlusion (Fig. 21.18). Not
typically associated with ischaemia.
Black
Retinal pigment hypertrophy: well- defined black lesions,
often in conjunction with chorioretinal atrophy. May occur
with previous retinal inflammation or injury (including
retinal laser therapy).
Figure 21.17 Diabetic retinopathy. Microaneurysms (tiny red
dots), blot haemorrhages, hard exudates and areas of new vessel
formation (arrow) are characteristic of this condition. In many
patients, hypertensive retinopathy is also present.
Figure 21.18 Branch retinal vein occlusion. There are flameshaped retinal haemorrhages, but the disc is normal.
The fovea at its centre is one disc diameter in size.
The fovea, with its high density of cone photoreceptors, is responsible for fine discriminatory vision. To
find the fovea, locate the optic disc and move the
ophthalmoscope beam temporally (move yourself
towards the nose). Alternatively, ask the patient to
look directly into the light. However, if the pupil is
not dilated, it tends to constrict at this point, and
the patient may recoil because of dazzle (you can
dim the light beam to make it more comfortable).
In young patients, the retina is very reflective and
there is often a small yellow dot in the middle of
the fovea (macula lutea or fovea centralis). Box 21.9
and Figs 21.17 and 21.18 identify common retinal
abnormalities by their colour and appearance.
Slit lamp and intraocular pressure
The slit lamp (Fig. 21.19) provides a stereoscopic, mag-
nified view of the eye and is the key examination tool
for ophthalmologists. Many accident and emergency
departments have a slit lamp, and it can be invaluable
for assessing suspected foreign bodies and corneal abrasions. Some direct ophthalmoscopes are equipped with
a slit- lamp beam, which can be useful. Alternatively,
the anterior orbital structures and globe can be examined with a bright torch and basic magnification, and
the same principles of systematic examination apply.
The slit lamp is composed of a table- mounted binocular microscope column with an adjustable illumination
source that produces a narrow, slit beam of light.
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Figure 21.19 Slit lamp.
The patient is seated with forehead and chin supported. The slit beam illumination and microscope
have a common axis of rotation and coincident
focal lengths, allowing the angle of illumination to
be varied along with its width, length and intensity.
Projected onto the globe, the slit beam illuminates
an optical cross- section of the eye’s transparent
structures, and this can be viewed with magnification varying from 10× to 40× power. An attachment allows the IOP to be measured (tonometry).
The drainage angle can be seen with a special contact lens (gonioscopy) and, with the aid of a handheld lens or a contact lens, the retina can also be
viewed.
The following structures can be examined:
Lid margins, meibomian gland orifices and lashes.
Inflammation of the lid margins (blepharitis) is
one of the most common ophthalmic conditions.
It is related to chalazia, blocked meibomian glands
and infected lash follicles (styes). The puncta, on
the medial aspect of the lids, drain tears into the
canalicular tear drainage pathway. Misdirected
lashes (trichiasis) causing foreign body sensations
occur with chronic lid disease.
Conjunctival surfaces (tarsal, forniceal and bul-
bar). This mucous membrane lines the eyeball
(bulbar conjunctiva) and the inner surface of
the eyelids (tarsal conjunctiva). The conjunctiva
may be pale in anaemia, yellow in jaundice or red
(injected) in conjunctivitis and other inflammatory eye disorders. Directing the patient’s gaze up,
down, left and right ensures that all the bulbar
conjunctiva is viewed. To examine the inferior
tarsal conjunctiva of the lower lid, the lower lid
should be gently everted and the patient asked
to look upwards. To examine the superior tarsal
Figure 21.20 Everting the upper eyelid to expose the tarsal
conjunctiva.
Figure 21.21 Fluorescein used to stain the cornea and tear film.
conjunctiva—for example if a foreign body is
suspected—ask the patient to look downwards
(Fig. 21.20). Grasp the lashes between the fore-
finger and thumb, gently pull down on them and
rotate the eyelid upwards over either the other
thumb or a cotton bud.
Cornea and tear film. The transparent cornea can
be viewed in cross- section. The addition of a drop
of 2% fluorescein reveals defects or foreign bodies
in the corneal epithelium and the tear film can
be assessed (Fig. 21.21). The tear meniscus on the
lower eyelid should be symmetrical and less than
1 mm thick, and the tear break- up time should be
more than 10 seconds. Fluorescein also aids the
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identification of aqueous leakage in a penetrating corneal injury (Seidel’s test). Arcus senilis is
a common crescentic opacity near the periphery
of the cornea. It usually starts at the lower part of
the cornea, extending to form a complete circle.
It is common in old people, but may occur in the
young (arcus juvenilis) in association with type IV
hyperlipoproteinaemia. Corneal sensation should
be tested.
Anterior chamber (filled with aqueous). In iritis, a
cause of red eye, there is inflammation in the aqueous, with flare (protein) and cells (typically leukocytes). In severe iritis, the inflammatory exudate
settles inferiorly to create a white fluid level in the
anterior chamber (hypopyon). Hyphaema has a
similar but red appearance caused by bleeding into
the anterior chamber, usually owing to trauma.
Iris. Note any difference in the colour of the two
eyes (heterochromia), abnormality in the shape
or size of the pupils or signs of iritis. In iritis, the
pupil may be constricted (miosis) or irregular
owing to the formation of adhesions (posterior
synechiae) between the edge of the pupil and
the anterior surface of the lens. Blunt trauma
can cause a dilated (mydriasis) unreactive pupil
with radial ruptures in the iris. An irregular or
teardrop- shaped pupil and a history of a highvelocity foreign body is highly suspicious of a penetrating eye injury where the iris has plugged the
leaking wound. Other abnormalities of the pupils
are described in Chapter 16.
Lens. Cataracts are usually caused by ageing (cen-
tral nuclear sclerosis), but also occur in diabetes
mellitus, after injury and in certain hereditary diseases, for example myotonic dystrophy. Posterior
subcapsular cataract is a common side effect of
corticosteroid therapy. Blunt eye injury may cause
partial dislocation of the lens (subluxation) or
complete dislocation into the vitreous cavity.
Anterior vitreous. This is best examined when the
pupil is dilated. Opacities may be observed, most
easily using a green light. Cells in the vitreous may
be associated with ocular inflammation (vitritis),
trauma (vitreous haemorrhage) or retinal holes/
detachment (retinal pigment).
Measuring intraocular pressure: applanation
tonometry
Intraocular pressures between 10 and 21 mmHg
are considered normal. An increased IOP is a characteristic feature of glaucoma. A diminished IOP
occurs in diabetic coma and in severe dehydration
from any cause. The IOP may be assessed by palpating the eyeball, although only gross variations from
normal can be appreciated. More accurate is applanation tonometry, in which the force required to
flatten (applanate) an area of a sphere (the cornea)
is proportional to the pressure within the sphere
(Fig. 21.22). Topical anaesthetic and fluorescein are
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Figure 21.22 Goldman tonometry.
Dial reading less
than globe pressure
Figure 21.23 Diagrammatic representation of tonometry.
applied to the cornea and a bright cobalt blue filter is
used to illuminate the sterile tonometer head. Contact between the tonometer head and the cornea
creates a thin green circular outline of fluorescein,
and a prism in the head splits this into two semicircles. The tonometer force is adjusted manually until
the semicircles just overlap and then it is read in millimetres of mercury (mmHg; Fig. 21.23).
Dial reading equals
globe pressure
Dial reading greater
than globe pressure
Eyelid, lacrimal and orbital assessment
Eyelids
People of Asian origin have a long, narrow palpebral aperture with an upward and outward obliquity
and a characteristic fold of skin along the upper lid.
The highest point of the aperture is typically at the
junction of its middle and inner thirds. In Down’s
syndrome, the palpebral fissure is also oblique. However, it is also short and wide, with its highest point
at the centre of the lid.
Normally no sclera is visible above the limbus (the
corneoscleral junction). The most common cause of
scleral show is eyelid retraction (Fig. 21.24) owing to
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Figure 21.24 Thyroid eye disease with upper eyelid retraction and
mild exophthalmos (bilateral proptosis).
Figure 21.25 Lower eyelid entropion causing infective keratitis
and corneal opacification.
dysthyroid eye disease, accompanied by other signs,
such as lid lag, in which movement of the upper lid
seems to lag behind that of the eyeball when the
patient looks downwards. In Parkinsonism, there
may be reduced blink frequency. Look for reduced
eyelid closure (lagophthalmos) and levator muscle
function. Ptosis (drooping of the upper lid) may
be congenital or acquired (check old photographs).
In age- related ptosis, owing to levator disinsertion,
levator function is retained and there is a high upper
eyelid skin crease. In ptosis owing to myogenic or
neurogenic lesions, there is reduced levator function
(see Chapter 16). In entropion, there is inversion of
the lid margin with associated malpositioning of the
lashes, which may rub on the cornea (Fig. 21.25);
and, in ectropion, eversion of the eyelid is often associated with watering. The lower lid is susceptible to
skin tumours, particularly basal cell and squamous
cell carcinomas (see Fig. 20.25). Xanthelasmas are
fatty deposits that develop in the upper and lower
eyelids in patients with long- standing hypercholesterolaemia.
Lacrimal gland
Examine the lacrimal gland by pulling up the outer part
of the upper lid while the patient looks downwards
and inwards. Acute inflammation (dacryoadenitis)
Figure 21.26 Schirmer’s test for dry eyes.
causes a tender swollen gland, with oedema of the
upper lid and localized conjunctival injection. Chronic
dacryoadenitis, a painless enlargement of the lacrimal
gland which is frequently bilateral, occurs in sarcoidosis and lymphoproliferative disorders. Tumours of the
lacrimal gland produce a hard swelling of the gland
associated with displacement of the globe. Involvement of the lacrimal gland by any disease process may
cause a dry eye.
Assess the position and size of the puncta (see
above). Painless watering is a feature of obstruction
of the tear drainage pathway, but exclude reflex
tearing and overflow from, for example, a dry eye.
Schirmer’s test uses a standardized strip of filter paper
to detect dry eyes by assessing the extent of wetting
at 5 minutes (Fig. 21.26). Overt nasolacrimal duct
blockage can be excluded if the patient reports fluid
at the back of the throat on probing and syringing
with normal saline (Fig. 21.27).
Orbit
The most common cause of forward displacement of the eyeball—proptosis when unilateral, or
exophthalmos when bilateral—is thyroid eye disease (TED) (see Fig. 19.13). This can cause corneal
exposure and ulceration. Optic nerve damage may
occur despite minimal proptosis. Axial proptosis,
in the primary direction of the eye in forward gaze,
is typical of TED and of tumours in the extraocular muscle cone behind the eye (intraconal mass
lesions) (Fig. 21.28). Non- axial proptosis occurs
in association with space- occupying orbital lesions
outside the muscle cone: for example, lacrimal gland
tumours; these displace the globe forward and inferomedially. Apparent (‘pseudo’) proptosis causes
diagnostic confusion: for example, in ipsilateral eyelid retraction or myopia (where the eye is longer
than normal) or when there is contralateral ptosis or
enophthalmos.
Figure 21.27 Syringe and probing to assess nasolacrimal duct
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function.
Figure 21.28 Axial CT orbits in TED. Left- sided thyroid eye
disease (TED) with exophthalmos and marked hypertrophy
(inflammation) of the medial and lateral rectus muscles on that
side. These muscles in the other eye are also slightly enlarged. The
optic nerve can clearly be seen between the enlarged muscles on
the left side.
Proptosis and enophthalmos can be measured
with the Hertel exophthalmometer (Fig. 21.29).
A difference of more than 2 mm between sides
is abnormal. A proptosis that increases while the
patient performs a Valsalva manoeuvre is suggestive of a venous abnormality. Pulsatile proptosis
with an orbital bruit is a feature of carotid cavernous fistula.
Blunt trauma to the orbit may cause a blowout
fracture of the thin orbital floor. Orbital contents
may prolapse through the fracture, restricting the
movement of the inferior rectus muscle and limiting
upgaze. A full orbital examination should include
palpation, eye movement examination, optic nerve
assessment and testing of the trigeminal nerve for
altered sensation.
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Figure 21.29 Hertel exophthalmometry to quantify proptosis or
exophthalmos.
Examination of the eye in children
The advice given in Chapter 6 on the examination
of children in general is also important when examining children’s eyes. Children may object strongly
to lights and instruments, particularly when they are
wielded by white- coated strangers. Allow the child
to get used to the surroundings while taking a history from the parent, but do not ignore the child.
Constantly observe the child, noting visual behaviour, the position and movements of the eyes and the
general appearance of each eye.
Visual maturation continues after birth and without a focused retinal image, the visual pathways fail
to develop properly, a condition known as amblyopia. Untreated, sight loss from amblyopia becomes
irreversible. It is therefore important to assess visual
acuity in preverbal and young children. Babies should
rapidly fix a large object—for example the examiner’s
face—and follow it. After 6 months, ‘continuous’ and
‘steady’ fixation that is ‘maintained’ (‘CSM’) during
a blink should be demonstrable. If an infant strongly
objects to your covering an eye for even a short time,
consider whether the non- covered eye may not be seeing well. A more sophisticated assessment can be made
using ‘preferential looking’. Cards are presented to the
child with a grating drawn at one end and none at the
other. The child will prefer to look at the image rather
than nothing. Successively smaller spatial gratings are
shown until the child does not see them. The grating
seen can be converted to an approximate Snellen acuity (the vision of a 1- year- old equates to approximately
6/12), although testing each eye independently in this
age group is difficult. From the age of 2 years, a more
accurate estimate of acuity can be made using the Kay
picture- matching test (Fig. 21.30), and from 3 years
the Sheridan- Gardiner letter- matching test. In these
tests, the child, or the child’s parent, holds a card with
a number of pictures or numbers on it. The examiner
holds up an image and asks the child to match this
target to one on the card; the targets vary in size.
Next, the position and movements of the eyes
should be assessed. The least disturbing method
is to observe the corneal light reflex: a light held at
about 1 m should produce a reflection in the centre
of each pupil. If the reflection in one eye is at a different location from that in the other, there may be
a squint (although a wide intercanthal distance may
give this appearance). If a squint is suspected, a cover
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Figure 21.30 Kay picture- matching test to assess the visual
acuity of children 2 years of age and older.
and alternate cover test should be performed. Assess
the ocular movements and the pupillary responses to
light and accommodation. Examine the media and
fundi with the ophthalmoscope through the dilated
pupil. Because of limited cooperation, refraction testing may be limited to the retinoscopy assessment. In
general, only children with refractive errors so severe
that there is a risk of amblyopia require treatment.
Figure 21.31 Dacryocystogram showing restricted flow of
radiopaque dye in the right nasolacrimal duct.
Imaging
Plain X- rays
Plain X- rays have a limited role in the detection of
foreign bodies, but have been largely superseded
by computed tomography (CT) or magnetic resonance imaging (MRI). Ultrasound is used to assess
the globe. A dacryocystogram uses a radiopaque dye
introduced into the lacrimal drainage system to identify sites of lacrimal duct obstruction (Fig. 21.31). It
is particularly useful in the watering eye, when carcinoma is suspected, when repeat surgery is planned
or when trauma has occurred.
Computed tomography and magnetic
resonance imaging
Computed tomography and MRI are used extensively
in the diagnosis of orbital disease. CT is often considered superior because it defines the bony orbit, but
the X- ray dose to the eye and lens is not inconsiderable. CT is the investigation of choice in blunt orbital
trauma and blowout fractures (see above), where finecut coronal spiral images are desirable (Fig. 21.32).
A- and B- mode ultrasound
The A- mode scan is a one- dimensional time- amplitude
study commonly used to assess axial length, which is
an essential measurement for lens implant calculation prior to cataract surgery. The B- mode scan gives
a two- dimensional cross- sectional view of the eye for
the diagnosis of both intraocular and orbital tumours,
Figure 21.32 Coronal CT orbits in blowout fracture. The right
bony orbital floor is fractured and the orbital contents prolapsed.
retinal detachments and intraocular disorders when
the fundal view is impaired (e.g. with vitreous haemorrhage; Fig. 21.33).
Retinal photography and fundus fluorescein
angiography
Retinal photography alone is useful to document
posterior segment abnormalities and allow monitoring, for example, of a choroidal naevus. In conjunction with the intravenous injection of sodium
fluorescein photography it gives a detailed assessment of the retinal and choroidal vasculature
(Figs 21.34 and 21.35). A blue filtered light excites
fluorescence (530 nm) as the dye circulates. Fundus
fluorescein angiography is useful in investigating diabetic retinopathy, age- related macular degeneration
and retinal ischaemia. Minor side effects, including
transient nausea and yellow discolouration of the
skin and urine, are common. Severe anaphylaxis is,
fortunately, very rare.
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Retinal and optic disc tomography
Optical coherence tomography and optic disc
tomography
Becoming increasingly widely used clinically, optical
coherence tomography (OCT) is based on interferometry, typically using near infrared light, to obtain
high- resolution (2–20 μm) cross- sectional images
of the retina (Fig. 21.36) and optic disc. Optical
Figure 21.33 B- mode ultrasound scan showing lens opacity and
vitreous opacities, but no retinal detachment following penetrating
trauma. C, cornea; L, lens; ON, optic nerve; V, vitreous.
coherence tomography angiography (OCTA) is a
newer development that compares motion contrast
imaging with high- resolution volumetric blood
flow information to generate non- invasive retinal
angiography images without injection of fluorescein. OCT has also been developed to generate
images of the anterior segment (ACOCT), for
example showing the angle structures in high resolution. In addition to OCT, a variety of techniques
have been developed to image and assess the shape
of the optic disc that are particularly useful in the
management of glaucoma. These include scanning
laser polarimetry (SLP) and the scanning laser ophthalmoscope (SLO) that assesses nerve fibre thickness indirectly.
Special examination techniques
Refraction and refractive assessment
A refraction test will ascertain the optical power of
an eye, with a view to prescribing glasses or contact
lenses. An objective refraction is performed using
neutralizing lenses in conjunction with a retinoscope. In adults and cooperative children, this is
then refined subjectively by placing neutralizing
Figure 21.34 Fluorescein retinal angiogram of fundus in papilloedema. Note the late- phase leakage of the dye.
Figure 21.35 Fluorescein retinal angiogram of fundus in pseudopapilloedema.
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1)/
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Figure 21.36 Ocular coherence tomography image of a
normal macula. C, choroid; RPE, retinal pigment epithelium; PR,
photoreceptor layer; ICN, inter- connecting neurone layer; NFL, nerve
fibre layer; FD, foveal depression.
)3
Figure 21.38 Indirect ophthalmoscopy, here used intraoperatively.
Note the head light and hand- held 20 D lens.
Indirect ophthalmoscopy
Binocular indirect ophthalmoscopy, using a light
source supported on the examiner’s head and a handheld lens in front of the patient’s eye, allows a much
greater area of the fundus to be visualized (Fig. 21.38).
The retinal periphery is more readily seen.
Figure 21.37 Amsler grid for testing macular function.
lenses in front of the eye and simultaneously assessing visual acuity. Increasingly, the retinoscope is
being replaced by an automated technique (the
autorefractor).
Amsler grid
The Amsler grid is a sensitive test of macular func-
tion. It is composed of a series of vertical and horizontal lines with a central spot for fixation (Fig. 21.37).
The patient is asked to look at this spot and describe
any distortions or missing areas in the grid.
Electrophysiological tests
Visual evoked potentials (VEP), recorded from the
occipital cortex using scalp electrodes while the
patient views an alternating black- and- white chequerboard stimulus, have a role in the diagnosis of
disease of the visual pathway. The principal waveform recorded from the scalp is a positive deflection
occurring about 100 ms after the stimulus (the P100
wave). This is attenuated in amplitude and increased
in latency in disease of the optic nerve, for example
optic neuritis or optic nerve compression. Electroretinograms (ERG) measure the electrical potential
across the eye, recorded with a corneal electrode, with
a reference electrode placed on the forehead. Flash,
flicker or pattern stimuli are used to generate electrical responses from retinal activation. This is useful in
assessing hereditary or acquired retinal degeneration.
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