A study of aqueous humor dynamics in keratoconus

Clinical observations suggest that patients with keratoconus have lower intraocular pressures, on average, than normal subjects. Our purpose was to determine whether differences in aqueous production and outflow facility could account for differences in intraocular pressure between a group of patients with keratoconus and a group of normal, age-matched control subjects. Aqueous humor dynamics were determined by the use of fluorophotometry in one eye of seven patients with keratoconus and ten age-matched normal subjects. Intraocular pressure was measured by applanation tonometry. Keratoconus patients had a statistically significant lower mean intraocular pressure than normal control subjects (11.3 ± 1.6 mmHg vs. 16.6 ± 2.8 mmHg, P = 0.0004). The difference in mean intraocular pressure remained significant even after correcting for possible errors in applanation tonometry due to thin corneal stroma. There was no difference in mean aqueous humor flow rates in the keratoconus patients as compared to controls (2.29 ± 0.53 μl min^-1 vs. 2.21 ± 0.48 μl min^-1, P = 0.73). The mean apparent outflow facility was 0.21 ± 0.07 μl min^-1 mmHg^-1 for keratoconus patients compared to 0.14 ± 0.03 μl min^-1 mmHg^-1 for controls (P = 0.02). Lower mean intraocular pressure in keratoconus patients appears to be due to increased outflow facility as compared to normal subjects.