The location of sites and effect of semiconductor diode trans-scleral cyclophotocoagulation on the buphthalmic equine globe

Abstract

Purpose: To determine appropriate location and energy settings for trans-scleral cyclophotocoagulation (TSCPC) for buphthalmic equine globes.

Animals: Eleven horses with a buphthalmic eye blinded by glaucoma presented for enucleation.

Methods: Globe and corneal dimensions were measured via ultrasonography and calipers, and TSCPC was performed under general anesthesia immediately prior to enucleation. Part 1: In nine globes, sixty sites were lasered 4 mm posterior to the limbus in the dorsotemporal and ventrotemporal quadrants at settings of 1500 milliwatts and 1500 ms. Globes were processed and sectioned sagitally over the temporal aspect in two blocks, each with five histologic sections examined by light microscopy. A digital imaging system was used to determine the location and length of the pars plicata on one slide from each block. Part 2: Based on results in Part 1, two globes were measured and lasered using the same time and energy settings at the following distances posterior to the limbus: 8 mm dorsally, 6 mm dorsotemporally, 5 mm ventrotemporally, and 5 mm ventrally.

Results: Globe and corneal dimensions exceeded normal values in all globes. Part 1: In all nine globes, photocoagulation affected the anterior ciliary processes and iris base and in 8/9 coagulation of the pectinate ligaments was noted. Part 2: In both globes, coagulation was confined to the pars plicata.

Conclusions: The previously recommended TSCPC sites are located too far anteriorly for a buphthalmic globe. Buphthalmic equine globes should have TSCPC performed at the following distances posterior to the limbus: 6-8 mm dorsally, 5-6 mm dorsotemporally, 4-5 mm ventrotemporally, and 4-5 mm ventrally.

Keywords: Buphthalmic; equine; glaucoma; histology; semiconductor diode laser; trans-scleral cyclophotocoagulation.

Figure 1

Diagram of an equine globe. The red dots demonstrate the location of sites of laser application 4 mm posterior to the limbus. The vertical black lines indicate the histologic blocks examined (block 1 is more median and block 2 is more temporal). Five histologic sections were processed from each block. D=dorsal, T=temporal, V=ventral, N=nasal. Image from Cavens et. al. The long-term effects of semiconductor diode laser transscleral cyclophotocoagulation on the normal equine eye and intraocular pressure. Vet Ophthalmol 2012;15(6):369-375.

Figure 2

Diagram of the internal anatomy of an equine globe. The purple indicates the ciliary body. The ridged purple tissue indicates the pars plicata. The non-textured purple tissue indicates the pars plana. The oval white dotted line indicates the location of the external limbus of the cornea. The lens is depicted in the center. T=temporal, N=nasal. The green lines and associated number values indicate the estimated appropriate distances from the external limbus for laser probe application during TSCPC in a buphthalmic equine globe.

Figure 3

Diagram of an equine globe. The red dots demonstrate the location of sites of laser application in a buphthalmic globe. The labels in each quadrant (clockwise from the “D”: dorsal, dorsotemporal, ventrotemporal and ventral) identify the recommended distance posterior to the limbus for laser application. D=dorsal, T=temporal, V=ventral, N=nasal, mm=millimeters.

Figure 4

Photomicrograph of a diode laser induced lesion of the pars plicata region of the ciliary body. Photocoagulation of the sclera and stroma as well as the ciliary processes is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. Disruption of the ciliary pigmented and non-pigmented epithium has occurred. Hematoxylin and eosin, 200X.

Figure 5

Photomicrograph of a diode laser induced lesion of the iris base, pectinate ligaments and iridocorneal angle. Photocoagulation of the stroma is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. The termination of Descemet’s membrane inserting on the pectinate ligaments can be seen on the bottom left. Hematoxylin and eosin, 40X.

Figure 6

Photomicrograph of a diode laser induced lesion of the pars plicata region of the ciliary body. Photocoagulation of the sclera and stroma as well as the ciliary processes is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment streaming. Note the disruption of the non-pigmented epithelium and complete coagulation of the stromal blood vessels and the adjacent vascular congestion. Hematoxylin and eosin, 40X.

Figure 7

a-b. Photomicrographs of the pectinate ligaments of the superior and inferior iridocorneal angle in a 21-year-old Quarter horse mare. a) Photocoagulation in the superior pectinate ligaments is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. b) The pectinate ligaments in the inferior iridocorneal angle have not been affected by laser application. Hematoxylin and eosin, 200X.

Figure 7

a-b. Photomicrographs of the pectinate ligaments of the superior and inferior iridocorneal angle in a 21-year-old Quarter horse mare. a) Photocoagulation in the superior pectinate ligaments is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. b) The pectinate ligaments in the inferior iridocorneal angle have not been affected by laser application. Hematoxylin and eosin, 200X.

Figure 8

a,b. Aperio scanned photomicrographs of the globe of a 20-year-old Haflinger gelding. Measurements were taken from the histologic external limbus to the most anterior aspect of the pars plicata in representative sections from the more median (a) and the more temporal blocks (b). A) In this median section, the corpora nigra at the edge of the dorsal iris leaflet and the ventral location of the optic nerve provide orientation. Dorsally the distance from limbus to pars plicata (7.155 mm) is longer than the same measurement ventrally (6.486 mm); B) In this more temporal section, the limbal to pars plicata measurement (4.005 mm) is shorter than the measurements in the median section.

Figure 8

a,b. Aperio scanned photomicrographs of the globe of a 20-year-old Haflinger gelding. Measurements were taken from the histologic external limbus to the most anterior aspect of the pars plicata in representative sections from the more median (a) and the more temporal blocks (b). A) In this median section, the corpora nigra at the edge of the dorsal iris leaflet and the ventral location of the optic nerve provide orientation. Dorsally the distance from limbus to pars plicata (7.155 mm) is longer than the same measurement ventrally (6.486 mm); B) In this more temporal section, the limbal to pars plicata measurement (4.005 mm) is shorter than the measurements in the median section.

Figure 9

Distance from the dorsal limbus to the posterior margin of the pars plicata versus axial globe length by ultrasonographic measurement. A best fit line indicates roughly linear correlation between globe size and length of the ciliary body in a buphthalmic equine globe.

Figure 10

Distance from the dorsal limbus to the posterior margin of the pars plicata versus horizontal corneal diameter. A best fit line indicates roughly linear correlation between horizontal corneal diameter and length of the ciliary body in a buphthalmic equine globe.

Figure 11

a,b. a) Photomicrograph of the unaffected inferior pectinate ligaments, iris base, and iridocorneal angle at the bottom left in a Phase 2 globe lasered at the more posterior sites. Photocoagulation of the adjacent ciliary processes on the upper right is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. Hematoxylin and eosin, 40X. b) Photomicrograph of the superior pars plicata immediately posterior to the large vessels of the iris base. Photocoagulation of the sclera and ciliary body stroma and processes is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. Hematoxylin and eosin, 200X.

Figure 11

a,b. a) Photomicrograph of the unaffected inferior pectinate ligaments, iris base, and iridocorneal angle at the bottom left in a Phase 2 globe lasered at the more posterior sites. Photocoagulation of the adjacent ciliary processes on the upper right is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. Hematoxylin and eosin, 40X. b) Photomicrograph of the superior pars plicata immediately posterior to the large vessels of the iris base. Photocoagulation of the sclera and ciliary body stroma and processes is evidenced by basophilic color change, loss of collagen fibrillar architecture and pigment dispersion and streaming. Hematoxylin and eosin, 200X.