Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

Direct view/measurement of newly formed aqueous humor Use of markers in aqueous humor (radioactive, fluorescein, para‐aminohippuric acid). Measure the decay rate of intracamerally injected isotopes. Fluorophotometry, a noninvasive method, is primarily used today II Procedures to investigate the exit of aqueous humor Ocular perfusion to lower IOP Perilimbic suction cup Tonography (conventional outflow/pressure sensitive) Use of markers (fluorescein, nitrotetrazolium, latex spheres, radioactive tracers). Both conventional and uveoscleral outflow routes are measured III Methods to measure the episcleral venous pressure Partial to complete collapse of the episcleral veins to affect alteration in the blood flow Torsion balance Pressure chamber (filled with air or saline) Air jet Ocular compression Direct cannulation and measurement by transducer

To determine AH outflow invasively, perfusion of the anterior chamber of in vivo and ex vivo eyes has been performed in numerous species. The constant pressure perfusion technique is the most frequently used. It involves maintaining a constant level of IOP with periodic, intermittent, or continuous minivolumes of perfusate. In the perfusion decay test, either a preselected volume of perfusate is injected or a preselected IOP is achieved. Once the perfusate has been injected, the time for the IOP to regain the baseline or preexisting measurement is obtained. In many ways, the perfusion techniques are similar to the noninvasive tonography methods (and yield similar results).

Table 2.9 Comparative volumes of the chambers and select structures of the eye.

Species	Anterior chamber (ml)	Posterior chamber (ml)	Lens volume (ml)	Vitreous volume (ml)
Human	0.2	0.06	0.2	3.9
Rabbit	0.3	0.06	0.2	1.5
Pig	0.3	—	—	3.0
Dog	0.8	0.2	0.5	3.2
Cat	0.8	0.3	0.3	2.8
Cow	1.7	1.5	2.2	20.9
Horse	2.4	1.6	3.1	28.2

      The percentages reported for normal uveoscleral outflow range from 30% to 65% in nonhuman primates, 15% in dogs, 13% in rabbits, 4–14% in humans, and 3% in cats. The horse appears to have an extensive uveoscleral outflow system, but the volume and percentage of the total outflow system have not been reported. Often uveoscleral outflow is now calculated as the difference between applanation tonography and the results from fluorophotometry.

      Uveoscleral outflow pathway has been demonstrated using observable tracers measuring from 10.0 nm to 1.0 μm in diameter. As one would anticipate, the smaller‐diameter (i.e., pore) tracers penetrate into the different tissues to greater extents. After perfusion at different IOPs and for different time intervals, the eyes (especially the root of the iris, entire ciliary body, suprachoroidal space, and choroid, even as far posterior as the optic nerve) are examined by light microscopy, scanning electron microscopy, and transmission electron microscopy for these markers. These same methods have also demonstrated the ability of the trabecular endothelium and wandering macrophages to phagocytize particulate material within the outflow pathways. An alternative method to estimate the amount of uveoscleral outflow (either as μl or %) is by using radioactive isotopes injected into the anterior chamber; the time, amount of the isotope, or both are standardized. At the conclusion of perfusion, either the ocular tissues are dissected into the different sections and analyzed for radioactivity or the entire globe is sectioned and the radioactivity of each area is measured by scintillation counters.

      Ocular Rigidity

      Another key concept in the measurement of IOP is ocular rigidity (k), or the resistance offered by the fibrous tunics of the eye (i.e., sclera and cornea) to a change in intraocular volume. Ocular rigidity may also be defined as the change in IOP per incremental change in the intraocular volume; this resistance manifests as a change in IOP. Ocular rigidity is determined by Schiotz indentation tonometry, and it estimates the change in volume (open manometer system) when the instrument is placed on the cornea as well as after injections of exact volumes or preselected elevations in IOP. With applanation tonometry, ocular rigidity is not a factor! This logarithmic relationship between IOP and volume of the globe is

Table 2.10 IOPs in select animal species.

	IOP results
Species	Mean ± SD	Tonometer	Investigator
Alligator	23.7 ± 2.1	TonoPen	Whittaker et al. (1995)
Cat	22.6 ± 4.0	Mackay‐Marg	Miller et al. (1991b)
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