Ultrasound biomicroscopy of the equine iridocorneal angle

Abstract

Background: The iridocorneal angle (ICA) is the major pathway of aqueous humour outflow from the anterior chamber of the eye. Ultrasound biomicroscopy (UBM) has been utilised to characterise the morphology of this drainage pathway in numerous species. UBM may allow for early recognition of aqueous humour outflow obstructions in horses, allowing for earlier recognition of risk for glaucoma, a vision-threatening and painful disease. UBM morphology of the normal equine ICA has yet to be described.

Objectives: To determine the ultrasonographic morphology of the equine ICA by UBM in standing sedated horses.

Study design: In vivo experimental study.

Methods: Thirty healthy adult horses underwent UBM of the ICA at four locations (superior, temporal, inferior, nasal) of each eye utilising standing sedation, topical anaesthesia and auriculopalpebral perineural anaesthesia. Anatomic structures were defined on ultrasound images through comparison to published histologic photomicrographs of the equine ICA.

Results: Ultrasound imaging of the ICA at all four locations was easily performed in standing, sedated horses. High-resolution images of the ICA allowed for identification of the pectinate ligament, corneoscleral trabecular meshwork (TM), uveal TM and supraciliary TM.

Main limitations: Pupil size was midrange in all eyes, but was not strictly controlled. Lighting conditions not controlled. Various breeds included.

Conclusion: In vivo UBM of the equine ICA is feasible and provides high-resolution images of the structures of the aqueous humour outflow pathway.

Keywords: UBM; aqueous humour; eye; glaucoma; horse.

Figure 1.

Photograph depicting the Compact Touch STS UBM 50 MHz linear transducer fitted with a ClearScan^® probe cover filled with sterile water applied to the equine eye. The horse is sedated with the head resting on a head stand, topical and auriculopalpebral perineural anesthesia have been applied, and the eyelids are gently manually retracted.

Figure 2.

Representative ultrasound biomicroscopy image of the nasal iridocorneal angle (a) of the eye of a normal adult horse obtained at the location of the yellow line indicated in the clinical image (b). The trabeculae of the pectinate ligament are clearly identifiable, as are the corneoscleral, uveal, and supraciliary trabecular meshwork (TM). Note that at this nasal location the third eyelid is within view but does not interfere with imaging of the ICA. In (b), note that the anterior aspect of the pectinate ligament is visible nasally and temporally but cannot be seen superiorly and inferiorly. The pupil was midrange.

Figure 3.

Representative ultrasound biomicroscopy images of the nasal, superior, temporal, and inferior iridocorneal angle of the normal adult horse. Images from three different horses are included to demonstrate subtle individual differences in morphology. Note that in the superior and inferior images, the corneoscleral junction (limbus) extends further axially than in the nasal and temporal locations. The corpora nigra are seen in the superior and inferior locations. The pupil was midrange.