Cat model for intraocular pressure reduction after transscleral Nd:YAG cyclophotocoagulation

Development of a rabbit model of persistent hypotony without ciliary body injury

In order to study the pathophysiological alterations of the ciliary body (CB) during persistent hypotony, it is necessary to develop an animal model without CB injury. In this study, we successfully established a modified model of persistent hypotony without CB injury in New Zealand rabbits. A 23-gauge pars plana vitrectomy (PPV) was performed and a trocar-formed fistula was allowed to remain in situ, to produce a continuous outflow of intraocular fluid. Both eyes underwent PPV with normal intraocular pressure (IOP); eyes with no surgical intervention were used as controls. The IOP was monitored and used to evaluate the reliability of the model. Secondary changes of hypotony were evaluated by slit-lamp biomicroscopy and B scans while morphological changes of the CB were observed by haematoxylin and eosin staining. The mean IOP in the hypotony groups were consistently lower than 6 mmHg. Furthermore, there were no significant differences in IOP between the PPV control group and normal eyes. Collectively, our data indicate that this model successfully simulates the secondary changes of hypotony, including a reduction in corneal size, corneal oedema, anterior chamber inflammation, morphological alterations of the CB, cataract, retinal detachment, and choroidal detachment. The morphological structure of the CB tissue changed dramatically after persistent hypotony, indicating that normal IOP may be required in order to maintain normal function in the CB. This model of persistent hypotony potentially represents a valuable tool for future studies aiming to investigate the pathophysiological mechanisms underlying CB dysfunction and other secondary changes that occur during hypotony.

Feline glaucoma--a comprehensive review

Cats with glaucoma typically present late in the course of disease. It is likely that glaucoma in cats is under-diagnosed due to its insidious onset and gradual progression, as well as limitations of some commonly used tonometers in this species. Treatment of glaucoma in feline patients presents a clinical challenge, particularly as glaucoma is often secondary to other disease processes in cats. In this review, we consider the clinical features, pathophysiology, and classification of the feline glaucomas and provide current evidence to direct selection of appropriate treatment strategies for feline glaucoma patients.

Histologic effect of semiconductor diode laser transscleral cyclophotocoagulation on the normal equine eye

OBJECTIVE

To determine the acute histologic effects of semiconductor diode laser transscleral cyclophotocoagulation (TSCP) on the normal equine eye.

ANIMAL STUDIED

Part 1: eight eyes of four horses. Part 2: 10 eyes of five horses.

MATERIALS AND METHODS

Part 1: TSCP was performed on four eyes at 4 mm and four eyes at 6 mm posterior to the limbus with 15 sites treated in four quadrants at 1800 mW for 1500 ms. The globes were sectioned transversely or sagitally to examine all quadrants and histologic sections were taken every 1 mm for the entire globe. Part 2: Based on the results from Part 1, TSCP was performed at 20 sites 4 mm posterior to the dorsotemporal limbus with a constant energy varying from 0.75 to 4 J/site. Histologic sections were taken every 1 mm for a total of 10 sections per eye and 20 sections per energy level group.

RESULTS

Part 1: At 4 mm posterior to the limbus, coagulation of the nonpigmented epithelium (NPE) of the pars plicata was observed in the temporal (14%) and dorsal quadrants (12%). Retinal detachment was observed in the nasal quadrant (12%). Hemorrhage was common in the nasal (19%) and temporal (12%) quadrants. At 6 mm posterior to the limbus, coagulation of the NPE of the pars plicata was observed in the dorsal (14%), ventral (16%), nasal (2%), and temporal (2%) quadrants. Retinal detachment was observed in the dorsal (8%), ventral (18%), nasal (20%) and temporal (2%) quadrants. Part 2: Settings of 0.75 J/site were ineffective; 1.5, 2.25 and 3 J/site damaged the pars plicata without disruption of anatomy; and 4 J/site caused disruption of normal architecture.

CONCLUSIONS

The most appropriate site for equine TSCP appears to be 4 mm posterior to the dorso- and ventrotemporal limbus avoiding the 3 and 9 o'clock positions and using an initial energy setting of 2.25 J/site. This results in effective damage to the pars plicata while minimizing surgical complications such as retinal detachment and hemorrhage.

Ex vivo evaluation of Tono-Pen and pneumotonometry in cat eyes

PURPOSE

To evaluate the validity and intraobserver reliability of intraocular pressure (IOP) measurements with both pneumotonometry and the Tono-Pen in a closed ex vivo system in cat eyes.

METHODS

IOP was increased step by step in 5 enucleated cat eyes, while taking IOP measurements with the Tono-Pen and pneumotonometry. The outcomes were compared to readings of a digital manometer simultaneously measuring the actual pressure in the anterior chamber.

RESULTS

Pneumotonometry overestimated IOP below 15 mm Hg and underestimated pressures above 20 mm Hg. Tono-Pen tonometry considerably underestimated IOP over the whole spectrum in all of the eyes tested. The pneumotonometer was identified as the more valid and reliable instrument for cat eyes.

CONCLUSION

Both tonometers are clinically useful tools to assess IOP for glaucoma studies using a cat animal model. However, one has to consider underestimation of IOP in the upper ranges. A correction formula can be used to calculate the actual IOP.

Effects of cyclophotocoagulation with a neodymium:yttrium-aluminum-garnet laser on corneal sensitivity, intraocular pressure, aqueous tear production, and corneal nerve morphology in eyes of dogs

OBJECTIVE

To determine effects of cyclophotocoagulation via administration of 100 J with a neodymium:yttrium aluminum garnet (Nd:YAG) laser on corneal touch threshold (CTT), intraocular pressure (IOP), aqueous tear production, and corneal nerve morphology in eyes of dogs.

ANIMALS

15 dogs.

PROCEDURE

Noncontact Nd:YAG laser was transsclerally applied (10 applications; 25 W for 0.1 seconds for each application to each of 4 quadrants) to the ciliary body of the left eye of 15 dogs; the right eye was the control eye. Corneal integrity, CTT, tear production as measured by the Schirmer tear test (STT), and IOP were evaluated for 14 days following laser treatment. On day 14, dogs were euthanatized, eyes harvested, and corneas stained with gold chloride. Major nerve bundles were analyzed by use of a drawing tube attached to a light microscope, and maximum diameters were measured by use of image analysis software.

RESULTS

All laser-treated eyes had significantly higher CTT values, compared with control eyes. Six of 15 laser-treated eyes developed ulcerative keratitis. On most days, IOP was significantly lower in laser-treated eyes in both morning and evening. Laser-treated eyes had a significant decrease of approximately 1 nerve bundle/corneal quadrant. Values for STT or nerve bundle diameters did not differ significantly.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of 100 J with a Nd:YAG laser effectively reduced IOP while increasing CTT and caused a significant decrease in number, but not diameter, of major corneal nerve bundles. Nerve damage and corneal hypoesthesia are etiologic factors in ulcerative keratitis following Nd:YAG cyclophotocoagulation.

Lasers in ophthalmology

Laser technology continues to progress with the addition of new lasers, new delivery systems, and new applications. The introduction of lasers to veterinary ophthalmology has radically changed the level of care that we can provide to our patients. The development of the diode laser has particularly had an impact on veterinary ophthalmology. The diode's affordability, portability, and broad applications for veterinary patients have allowed laser surgery to become a routine part of veterinary ophthalmology practice. Educating the public and veterinary community in available laser techniques will generate improved ophthalmic care and provide more data on which to build future applications.

Surgery for glaucoma

Most cases of glaucoma in small animals ultimately require surgical treatment for long-term control of intraocular pressure. Surgical procedures that have the potential to preserve vision in acute cases are categorized into those that reduce aqueous production (cyclodestructive techniques). Salvage procedures for irreversibly blind eyes include enucleation, implantation of an intraocular prosthesis, and pharmacologic ciliary body ablation. The indications, surgical technique, and complications of these procedures are discussed in this article.