Description of ciliary body anatomy and identification of sites for transscleral cyclophotocoagulation in the equine eye

Objective To determine the most appropriate anatomical location for diode l aser probe placement to accurately photoablate the equine ciliary body usin g a contact, transscleral approach. Design Original research. Procedures Forty-two freshly enucleated adult equine eyes were evaluated. T he horizontal, medial vertical, central vertical, and lateral vertical leng ths of the cornea were measured from limbus to limbus. Needles were inserte d perpendicular to the sclera at specific distances posterior to the extern al limbus at the 1, 2, 4, 5, 6, 7, 8, 10, 11 and 12 o'clock positions. The per cent frequency that needles penetrated internal anatomical regions (len s, lens zonules, pars plicata, anterior, middle, and posterior pars plana, ora ciliaris retinae, or retina), when inserted at 2, 4, 6, and 8 min poste rior to the limbus, were calculated for all eyes combined, for right vs. le ft, for males vs. females, and for three age groups. The internal distance from the limbus to the anterior pars plana, and from the anterior pars plan a to the ora ciliaris retinae were also measured in six of the horses (12 e yes). Mean distances and standard deviations were computed for all paramete rs. Results Average corneal sizes and standard deviations were: 30.24 mm +/- 1. 53 (horizontal); 24.69 mm +/- 1.52 (central vertical); 22.79 mm +/- 1.49 (m edial vertical); and 19.79 mm +/- 1.55 (lateral vertical). Internal distanc es of the pars plicatas ranged from 5.33 mm +/- 0.49 to 10.67 mm +/- 1.15. Internal distances of the pars planas ranged from 0.33 mm +/- 0.49 to 3.17 mm +/- 0.39. High probabilities of penetrating the pars plicata correspond to positions 4 min posterior to the external limbus dorsotemporal 10, 11 o' clock (OD), and 1, 2 o'clock (OS), dorsonasal 1 o'clock (OD) and 11 o'clock (OS), and ventrotemporal 5-7 o'clock (OU). Low probabilities of penetratin g the pars plicata correspond to positions ventronasal 4 o'clock (OD), 8 o' clock (OS) and dorsonasal 2 o'clock (OD), 10 o'clock (OS) at 4 and 6 mm pos terior to the external limbus as well as ventrotemporal 4 o'clock (OS), 8 o 'clock (OD) at 6 min posterior to the external limbus. Conclusions Transscleral cyclophotocoagulation may be a viable alternative to medical therapy for control of intraocular pressure in horses with glauc oma. Overall, the most accurate anatomical position on the sclera for cyclo photocoagulation of the equine eye is 4-6 min posterior to the limbus, avoi ding the nasal quadrants. Accurate transscleral cyclophotocoagulation shoul d optimize the therapeutic outcome and minimize potential side-effects such as retinal detachment and cataract formation.