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R. Gerometta, D. Escobar, O. A. Candia; Pressure Transmission From Anterior Chamber to Optic Nerve in the Cow Eye. Invest. Ophthalmol. Vis. Sci. 2010;51(13):983.
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To determine if IOP is transmitted to the retina perpendicular to its surface thus compressing the whole surface field of ganglion cells, transmitted via the suprachoroidal space (SCS) to the optic nerve (ON) strangling it, or both.
Freshly isolated bovine eyes were used. A 27-ga needle connected to a syringe and to a pressure transducer with a meter (PT&M), was introduced into the anterior chamber (AC). IOP was maintained at ≈ 15 mmHg by periodically injecting small amounts of fluid into the AC to compensate for leaks. Then, 2 protocols were implemented. A) a needle with another PT&M was inserted into the vitreous at various distances from the retina to see if changes in IOP were transmitted to the vitreous body. B) a 27 ga needle connected to a graduated pipette filled with fluid was introduced into the SCS posterior to the eye’s equator at various distances from the ON cup. The level of fluid in the open pipette was set to coincide with the pressure (P) in the AC. Then, the pipette was moved vertically to determine if this change in P in the SCS was transmitted to the AC, and to see whether there was a fluid flow from/to the pipette based on the difference in P between the pipette level and AC's IOP.
P applied to the AC was not transmitted to the vitreous body except at large values (over 50 mmHg) that displaced the lens and compressed the vitreous. Changes in pipette fluid level corresponding to values between 5 - 30 mmHg were transmitted to the AC until the 2 pressures were within 5% of each other, suggesting a communication and fluid flow between pipette and AC compartments. Based on the volume change in the pipette, the time elapsed for the change and the ΔP between compartments, an equivalent facility between AC and point of puncture in the SCS of ≈ 0.2-0.6 µL/min · mmHg was determined.
In freshly isolated cow eyes with a needle in the SCS open to the atmosphere, a transmission of P and fluid flow from/to the AC was observed. In the in situ eye, the P in the AC would be transmitted and applied to the more external axons of the ON as they leave the eye. This finding aids in the interpretation on how measured IOP is applied to ON axons.
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