Ophthalmologists, whatever their gender, might
be seen as just grown-up boys with toys! Slit
lamp biomicroscope, head-mounted indirect ophthalmoscope, Finhoff transilluminator and more; it
seems that you have to have a lot of cash and a fair bit of
expertise to be a dyed-in-the-wool ophthalmic expert.
Yet the man who taught me just about everything I know
about the subject, Dr Keith Barnett, carried around a direct
ophthalmoscope in his back pocket and made the vast
majority of diagnoses with that and that alone. Now the
vast majority of veterinary surgeries will have these simple
facilities for basic ophthalmological examination, yet very
few veterinary surgeons use the direct ophthalmoscope to
best effect.
The key to ophthalmology is first being able to obtain
an adequate view of all parts of the eye and secondly
being able to describe what is seen. We will cover the first
important topic of adequate visualisation in this article.
A prerequisite of good ophthalmological examination
is adequate immobilisation of the animal. For the vast
majority of examinations, holding the animal’s muzzle
lightly with one hand and the ophthalmoscope with the
other is sufficient,
but in some cases
a restraining hand
from an assistant
to avoid the animal
moving backwards is
invaluable.
Pen-torch
external
examination
The first technique
to use on any dog
presenting with an
ocular problem is
an overall external
examination with a pen-torch in a light and then moderately
darkened room. Gross abnormalities of shape, colour or
position of the globe and adnexa will be obvious.
Use of the pen-torch should present no problems, but one
useful tip is to move the light around in a circular motion in
front of the eye: this avoids mistaking reflections from the
cornea or lens as opacities, for while a genuine lesion will
remain still, reflections move with the light beam.
Direct and consensual light reflexes should be assessed.
A crisp, clear reflection from the ocular surface shows a
devent tear film and a healthy epithelium, but a broken-up
reflection denotes either a defective tear film or an eroded
corneal epithelium or both.
This is also the time to observe eye movements and
obvious sight deficits. Sometimes a low-power head
loupe can prove useful to obtain higher magnification
if examination of the eyelid margin for distichia, or the
lacrimal puncta is required although this is often left until
direct ophthalmoscopy is used.
Distant direct ophthalmoscopy
Before taking a close examination of the eye with the direct
ophthalmoscope, visualisation of the tapetal reflex from
a distance of around two feet should be achieved. The
ophthalmoscope is set at 0 dioptres and the bright tapetal
reflex is seen when the observer’s eye is in line with the
optical axis of the animal’s eye.
This has a number of purposes. First it allows the
animal to become accustomed to examination with the
ophthalmoscope. Second it allows assessment of any
opacities in the ocular media such as cataracts which
reduce the reflex. Third, it allows note to be taken of
increased tapetal reflex such as would be seen in advanced retinal atrophy. Fourthly, it can be a very useful method for
assessing mild anisocoria, since the size of both pupils can
be easily compared as reflected light passes through them.
Direct ophthalmoscopy
This is the most common use of the ophthalmoscope, but
also the area which is found most difficult by those using it
for the first time. Each operator will have a preference for
the order of use of the instrument and that described here
is merely a personal preference.
1. The posterior segment
With the ophthalmoscope set on
0 dioptres, the instrument is brought close to the observer’s
eye and then 2-3cm from the animal’s globe. It is wise to
become accustomed to using both eyes so that nose-to-nose contact can be minimised by using the right eye to
look at the right eye of the animal and left for left.
The direct ophthalmoscope gives a highly-magnified view
of the fundus, which can be disconcerting especially if the
animal moves its eye a great deal. The easiest landmark to
visualise is the retinal vessel running vertically from the
disc. Having located this vessel, any adjustment in dioptres
can be made to compensate for myopia or hypermetropia
on the part of the animal.
The observer’s eye should be relaxed to accommodate for
the far distance. Then a systematic survey of the optic disc
and the four quadrants of temporal and nasal tapetal and
non-tapetal fundus can be made. This is much facilitated by
the use of mydriatics such as tropicamide, although many
prefer to leave this until the iris has been observed because
of the possibility of missing small lesions such as persistent
pupillary membranes with a widely-dilated pupil. Changes
in tapetal reflectivity, in pigment distribution or in vessel
appearance should be sought. The position of lesions not at the focal plane of the eye such as optic disc colobomata
and retinal detachments can be estimated by changing
the dioptric power of the ophthalmoscope until they are
in focus. Similarly, abnormalities in the vitreous can be
identified and placed.
2. The anterior segment
The structures between and
including the lens and the posterior cornea form the anterior segment of
the eye and can be
visualised best when
the dioptric power of
the lens is changed to
+10. Because of the
transparency of the
lens, it is often easier
to start by focusing
on the pupil edge and
then observing the
front and back of the
lens by moving one’s
head forward and back
slightly.
The different parts of the lens can be identified in several
ways. By moving the head slightly from side to side, or
watching while the animal’s eye moves, parallax can be
utilised to show whether an opacity is at the posterior or
anterior part of the lens.
Another way of differentiating the front from the back of
the lens is by using the underwear rule – that is to say ‘Y
fronts’: the anterior subcapsular suture lines make the form
of a Y while the posterior suture lines form an inverted Y.
Opacities of the lens should be classified as to form
and position. One important point to make is that
nuclear sclerosis, the grey pseudo-opacity seen on
pentorch examination of the eye of older dogs should be
differentiated from true cataract by distant direct and direct
ophthalmoscopy as the tapetal reflex is not obscured by
this change in refractivity of the lens nucleus and inner
cortex.
Abnormalities of the iris should be noted at this point,
including persistent remnants of the embryological
pupillary membrane system, changes in pigmentation or
vasculature. At this point any flare or cloudiness of the
aqueous reducing visibility of the iris detail should be noted.
The estimation of cell number in the aqueous is only
really practical on slit lamp examination, but gross
increases in cells or fibrin will be noticeable especially if the
aqueous is viewed by retro-illumination, that is to say with
light reflected back from the tapetum.
3. The cornea and adnexa
When the ophthalmoscope is
set at +20 dioptres, it acts basically as a simple powerful
magnifying glass but requires close apposition of the
observer and the animal.
Focusing on the transparent cornea can be difficult
for the beginner, but two helpful tips are first to practice
focusing on one’s hand to get a feel of how close one needs
to be, and secondly to begin the corneal examination at
the limbus, where the pigment and vessels of the cornea-scleral junction provide an easy landmark upon which to
focus.
As with the lens, the most difficult task is to localise a
lesion within the otherwise transparent cornea. Here a slit
beam or pencil beam can be very helpful if available on the ophthalmoscope. By directing this beam at an angle, an
optical cross-section of the cornea can be obtained and the
lesion localised in the beam.
As with the lens, the situation is much improved if
a slit lamp biomicroscope is available. By using the
ophthalmoscope at the same diopter, the conjunctiva and
lid margins can be magnified to facilitate localisation of
ectopic cilia in the conjunctiva or distichial lashes on the lid
margins.
Indirect ophthalmoscopy
The direct technique, as its name suggests, produces a
direct, real and thus upright but highly-magnified image of
the retina for the observer using just one lens close to the
observer’s eye.
The indirect method uses a lens close to the animal’s eye,
but with the observer at arm’s length, producing a virtual
image, inverted but much less magnified which can be seen
with the naked eye or with a loupe.
A plastic 20 dioptre lens, around two inches wide,
which can be purchased online for a few pounds is ideal
while achromatic mounted glass lens is ideal but costly.
The uniocular method utilising a cheap lens and a direct
ophthalmoscope or pentorch is a relatively easy technique,
though it requires an assistant to hold the dog’s head.
For best results, the pupil should be dilated. To look in
the left eye, the lens is held at around arm’s length in the
right hand, resting on the dog’s muzzle and orbital rim
with the lens close to the eye. The direct ophthalmoscope
or another suitable focal light source is held adjacent to
the left eye and moved so that the tapetal reflex is seen
through the lens. The lens is then moved slightly away from
the animal’s eye until the image of the retina fills the field of
view.
At this point, a large amount of the fundus will be seen.
To visualise the edges of the tapetal fundus, the head and
ophthalmoscope are moved, keeping the lens still. Because
of the inverted image, to view the left fundus the head must
be moved left or to view the inferior fundus the head must
be moved down.
This takes a few minutes of practice, since it is the
opposite of the direct method, but the advantages of such a
wide view of the fundus are tremendous.
A more expensive, but much easier type of indirect
ophthalmoscopy uses a binocular head loupe with coaxial
mains illumination. This has the advantage that both hands
are free to hold the head and the lens and moreover that
the image is three-dimensional so that colobomas and
retinal detachments are very easily seen.
The mains halogen illumination means that in cases
where cataractous changes or vitritis obscure the view
with a direct ophthalmoscope, the retina can still be
seen with the binocular indirect method. Another type of
indirect ophthalmoscope has an integral first lens, but
only monocular vision. This has been designed to have a
moderately-magnified upright image, but again has the
disadvantage of expense.
