Hydraulic conductivity of fixed retinal tissue after sequential excimer laser ablation: barriers limiting fluid distribution and implications for cystoid macular edema.

OBJECTIVES: To measure the hydraulic conductivity (HC) of human retina and to determine the presence and location of high-resistance barriers to fluid movement through the retina.
METHODS: Forty-one pairs of human eyes were investigated using an HC chamber. Once baseline HC had been determined, the effect of ablating through varying thickness of retina from the vitreous or photoreceptor surface using an excimer laser (193 nm) was investigated. Tissue samples were then processed for histological investigation.
RESULTS: The HC of fixed intact human retina was 2.54 x 10(-10) m/s per pascal at 539 Pa (range, 0.6 x 10(-10) to 3.3 x 10(-10) m/s per pascal; SD, 0.6 x 10(-10) m/s per pascal [1 mm Hg equals 133 Pa]). Ablation from either surface resulted in little change in HC until a critical depth was reached, at which point there was an order of magnitude increase. The critical depth was approximately 170 microm from the inner limiting membrane when ablating from the vitreous surface and 70 microm from the inner limiting membrane when ablating from the photoreceptor surface. Histological specimens showed that these barriers were the synaptic portion of the outer plexiform layer, and the inner plexiform layer, respectively.
CONCLUSIONS: The 2 high-resistance barriers to fluid flow through the retina are the synaptic portion of the outer plexiform layer, and the inner plexiform layer. CLINICAL RELEVANCE: These observations help to explain the distribution of cystoid macular edema seen in histological studies and with optical coherence tomography.