Effect of age on outflow resistance washout during anterior chamber perfusion in rhesus and cynomolgus monkeys

When aqueous humour outflow resistance is measured by two-level constant pressure perfusion in non-human primate eyes, a progressive decrease in outflow resistance, known as the 'washout effect' occurs with time. The effect of age on total outflow resistance washout (the reciprocal of outflow facility (OF)) was determined in rhesus and cynomolgus monkeys. In cynomolgus monkeys, the effect of time between exchange of the anterior chamber (AC) contents and post-exchange OF measurements on outflow resistance was also examined. Total OF was determined at baseline in one eye of 35 rhesus monkeys aged 4-29 yrs, and in 27 cynomolgus monkeys, aged 3-17 yrs, at baseline and after 10 min AC exchange with 1 or 2 ml Bárány's perfusand or in Bárány's containing 0.01-1% dimethyl sulfoxide (DMSO) or Tris base. Resistance washout did not differ with age at baseline in rhesus or cynomolgus monkeys. Similarly, no changes were found when comparing post-exchange resistance washout vs. age in cynomolgus monkeys that had undergone AC exchange with Bárány's perfusand only or Bárány's containing 0.01-1% DMSO or Tris base. Rate of resistance washout decreased with increased length of time between exchange of the AC contents and post-exchange outflow facility readings (−0.0004±0.0092 mmHg min−1 μl−1 yr−1; p=0.016). Several explanations for these findings are plausible.