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.
Pentorch external examination
The first technique to use on any dog presenting with an ocular problem is an overall external examination with a pentorch 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 pentorch 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 a 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.
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 to 3 centimetres 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 nontapetal 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 pen-torch 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; 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 retroillumination, 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 dioptre the conjunctiva and lid margins can be magnified to facilitate localisation of ectopic cilia in the conjunctiva or distichial lashes on the lid margins.
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 observers 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 2 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 pen torch 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.