2 A thorough ophthalmic examination can provide a rapid and accurate diagnosis for many ophthalmic diseases, because most ocular structures can be visualized either directly or indirectly. Furthermore, the eye lends itself to numerous simple and efficient diagnostic procedures, many of which can be performed during a routine examination. This chapter demonstrates examination and diagnostic techniques. Most of these procedures are noninvasive, and a thorough understanding of them can facilitate the identification and diagnosis of many ocular disorders. The basic equipment necessary to perform a proper ocular examination includes a bright, focal light source (a Finoff transilluminator is ideal), Schirmer tear test strips, ocular stains (vital dyes), topical anesthetic, mydriatic agent, eyewash, sterile culture swabs, forceps, surgical blades, glass slides, cannulas for nasolacrimal duct irrigations, and an ophthalmoscope for examination of the fundus. Magnification is very helpful to identify small or subtle lesions. The eye examination must be complete, organized, and strategic. Certain tests or observations must precede others to avoid interference or spurious conclusions. If indications for microbiologic sampling are present, samples are taken on moistened sterile swabs before instillation of any diagnostic drugs (stains, mydriatics, local anesthetics) as they can contain preservatives that prevent microbial growth. The Schirmer tear test must be performed before excessive ocular manipulation and instillation of any ophthalmic solutions or ointments, otherwise the result will be an inadequate reflection of tear production. The pupillary light reflexes should be evaluated before mydriatics or miotics are used. Similarly, measurement of intraocular pressure (IOP) should precede instillation of mydriatics. A thorough examination assesses all ocular and periocular structures, outside to inside and front to back. Accurate recording of examination will permit assessment of progress (Figure 2.1). Figure 2.1 (A) Essential equipment for ophthalmic examination. (B) An example of an examination form for recording findings. The ophthalmic examination begins with an indirect assessment of vision (menace response, visual placing, maze testing) and comfort, and should be performed prior to sedation and nerve blocks, if these are necessary. The distant evaluation assesses the size, position, and direction of the globe and its movements. Any ocular discharge and asymmetry should be noted. It is important to examine each eye successively and to assess ocular and adnexal structures for symmetry. The distant examination should be performed from different angles when facing the patient as subtle changes in globe position can become apparent when viewing, say, from above the animal’s head. Evaluation of ocular movements can be achieved by turning the animal’s head from side to side. Normal saccadic and optokinetic movements are noted as the eyes move back and forth in synchronicity, with the fast phase occurring in the direction of head movement. Complete external examination should include palpation of the orbit and retropulsion of both globes (Figure 2.2). Figure 2.2 (A) Pupillary light reflexes should be assessed with a bright light (here, a Finoff transilluminator is being used) in both bright and dim lighting conditions. (B) Retropulsion of the globes should be equal on each side and nonpainful to the patient. (C) Assessing the patient for asymmetry is an important part of the ophthalmic examination. In this case, a retrobulbar mass has resulted in relative exophthalmos and strabismus. Radiation therapy has resulted in cataract formation and whitening of the hair coat. (D) Characterization of any ocular discharge can help with diagnosis and staging of severity and chronicity of ocular conditions. Akinesia of the palpebral or auriculopalpebral (branch of palpebral) nerves facilitates examination of the eye in large animals, particularly equids. There are three main points at which the auriculopalpebral or palpebral nerves can be blocked in the horse. The first is just anterior to the base of the ear where the auriculopalpebral nerve emerges from the parotid salivary gland and becomes subcutaneous on the lateral aspect of the coronoid process. Here local anesthetic can be injected into the depression just caudal to the ramus of the mandible at the ventral edge of the temporal portion of the zygomatic arch. The rostral auricular artery and vein should be avoided. The second is just lateral to the highest point of the caudal zygomatic arch where the palpebral nerve can be “strummed” under the skin over the dorsal border of the bone. The third is where the palpebral nerve lies on the zygomatic arch caudal to the bony process of the frontal bone (Figure 2.3). Figure 2.3 Landmarks for the auriculopalpebral motor block to facilitate a complete ophthalmic examination in a horse. The nasolacrimal system has both secretory and drainage functions. Evaluation of this apparatus should note any tearing or hypofunction, as well as the endpoint of drainage. Excessive tearing can be caused by partial or complete obstruction of the drainage apparatus and increased lacrimation by ocular irritation and uveitis. The quality and quantity of the tear secretion is indirectly assessed by observation of the normal preocular tear film, which consists of three main components. The middle, or aqueous, layer is produced primarily by the lacrimal and nictitans glands in mammals, and deficiencies can be identified via the Schirmer tear test (Figure 2.4). The folded end of the Schirmer tear test strip is inserted in the lower conjunctival fornix, in contact with the cornea, near the junction of the middle and temporal thirds of the eyelids where it should remain for 1 minute. Tears are measured from the fold of the strip in millimeters per minute, immediately following removal. In common domestic species, tear production greater than 15 mm/minute in the absence of disease is considered adequate. The inner, mucin layer of the tear film is produced primarily by conjunctival goblet cells and the outer, lipid layer is produced by the meibomian glands in the eyelids. Deficiencies of the outer and inner layers result in qualitative dry eye. These can be assessed by tear film break up time (rapid evaporation) which utilizes topical fluorescein, rose Bengal, or lissamine green stains to delineate foci of mucin absence.
The Ophthalmic Examination and Diagnostics
External Examination
Nerve Blocks
Tear Film Assessment
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