Discussion

Introduction.

As an optical system, the eye is divided into zones, each with unique anatomical and physiological features. The anterior chamber is bordered by the corneal endothelium, scleral shelf, the trabecular meshwork of the iridocorneal drainage angle and the anterior face of the iris. The posterior chamber represents that space between iris and lens. The volume of the anterior chamber is 3.04 plusminus 1.27 ml (Gilger et al. 2005) and is maintained by aqueous, an ultrafiltrate of plasma. A balance exists between the production of aqueous from the epithelium of the ciliary body processes and its outflow, via both the conventional and unconventional routes. Examination of the anterior chamber is facilitated by magnification, controlled illumination (such as a bright slit of light), tonometry, ultrasonography and infrared photography. It is helpful to adopt a methodical approach to examining the anterior chamber and its borders and the following landmarks should be considered:

Endothelium
- The innermost layer of the cornea whose purpose is to dehydrate the corneal stroma via an active pumping mechanism. Dysfunction results in corneal oedema.
- Inflammatory deposits can be observed in cases of uveitis (where they are termed keratic precipitates) and in cases of endothelialitis.
- Embryological remnants may be observed attaching to the endothelium.

Iridocorneal drainage angle
- The curvature of the equine cornea allows for the viewing of the trabecular meshwork, which represents the opening to the conventional outflow tract from the eye, both medially and laterally.
- Trabecular avulsions (often as a result of blunt trauma) and inflammatory deposits (uveitis) can be observed in this region.
- There is anecdotal evidence that primary glaucoma exists (albeit rarely) in the horse and that malformation/ degeneration of the drainage angle may therefore contribute
- Normal variations in iris colour should be recognised in conjunction with varying coat colouration.
- Zonal colour differences can also exist in the heterochromic iris and between the pupillary and ciliary zones of the pigmented iris.
- Zones of iris hypoplasia can be observed, typically in the 12 o'clock position in colour dilute irides (Buyukmihci et al. 1992)
- The anterior face of the iris has a refined variation in its detail and colouration which can be lost with chronic or recurrent inflammation.
- Remnants of the pupillary membrane are common and are located at the junction of the pupillary and ciliary zones of the iris. Posterior synechiae in contrast, originate from the iris margin.
- The corpora nigra are located at the pupil margin, primarily in the centro-dorsal position. Cystic dilatation of the corpora nigra can obscure the pupil and cysts liberated from their epithelial attachments typically come to rest in the ventral portion of the anterior chamber. Iris cysts are typically spheroid in appearance and can transilluminate (allow the passage of incident light). Where clarification is required, ultrasound allows for the differentiation of a cystic structure from a solid mass.
- Many iris cysts appear to have no effect on functional vision but some cysts are significant in terms of vision and behavioural issues (Gilger et al. 1997; Gemensky-Metzler et al. 2004). Some iris cysts will burst and come to lie either on the anterior lens capsule or the posterior surface of the cornea.
- The pigmented epithelium of the posterior iris can typically be seen at the pupillary margin. This margin can be seen to curl anteriorly (ectropion uveae) in cases of recurrent or chronic uveitis.

Aqueous
- Being in effect an electrolyte solution, does not reflect light and is amorphous in appearance.
- Breakdown of the blood-ocular barrier (uveitis) can result in the accumulation of protein and inflammatory cells (flare, hypopyon), fibrin and red blood cells (hyphaema) in the anterior chamber. The anterior chamber appears to have a high capacity for clearing cellular debris. Fibrin is also amenable to natural degradation and a risk benefit evaluation should be made regarding the intra-cameral injection of tissue plasminogen activator (TPA) in the management of anterior chamber clots.
- Tumours such medullloepitheliomas (Bistner 1974) and melanomas (Latimer and Wyman 1983) can extend into the anterior chamber.

General considerations of the anterior chamber
- The corneal endothelium has a limited capacity to withstand insults and has poor regenerative capabilities.
- Glaucoma, as a disturbance of aqueous dynamics, can be challenging to document by tonometry and is oftentimes poorly responsive to treatment.
- Active uveitis should be treated aggressively according to the cause. Treatment for primary uveitis should ideally extend for several weeks beyond the resolution of clinical signs.
- Tumours involving the anterior chamber tend to be benign and slow growing but typically threaten the survival of the eye.
- Where hyphaema obscures examination of the posterior segment, visual potential should be assessed by means of the dazzle response and consensual pupillary light reflex testing. Ocular ultrasound is invaluable in documenting catastrophic anatomical derangements (such as lens luxations and retinal detachments) in these cases.
- Indicators of previous uveitis should alert the clinician to the possibility of recurrent disease.

The lens
The crystalline lens is biconvex with an axial length varying from
11 to 13.5 mm with a diameter of 20 mm (Martin and Anderson
1981; Lavach 1990; Severin 1996). The central nucleus represents the oldest region within the lens, with cortical fibres decreasing in age as they approach the periphery. Lens fibres are contained within the lens capsule. Disorders of the lens include cataracts, lens luxations and lens colobomas. Refractive interfaces can be observed in the normal equine lens particularly between the nucleus and cortex as well as in a lamellar ('onion ring') distribution throughout the entire lens. Nuclear sclerosis represents a normal ageing change of the lens and is often more apparent ultrasonographically than optically.
A cataract is, by definition, any opacity of the lens and its capsule. A lens opacity prevents the complete passage of light through the affected lens fibres and can be easily detected by distant direct ophthalmoscopy. As a general rule, the location of an opacity can be located using parallax with central opacities showing no movement of parallax with globe movements, whilst anterior opacities show 'with gaze' movement and posterior lens opacities 'contra gaze' movements. The use of slit lamp biomicroscopy is the preferred technique for evaluating the lens anatomy although the indirect ophthalmoscope can be used to examine the lens with some magnification.

General considerations for the crystalline lens
- In general, incomplete cataracts do not appear to adversely affect functional vision (a definition of which is the ability of an individual to accomplish a task, despite imperfect vision using strategies and other sensory modalities) although they preclude perfect vision.
- Cataracts not associated with recurrent uveitis tend to progress slowly if at all (Matthews 2000).
- Lens luxations are typically associated with chronic uveitis and glaucoma. As a general rule, they are not amenable to removal (Brooks 2009).
- High frequency ultrasound allows for the detection of alterations in lens size, position and integrity of the posterior lens capsule. The normal lens is largely anechoic on ultrasound examination.
- Positive visual outcomes are achievable in the treatment of cataracts by phacoemulsification