Ocular rigidity and intraocular pressure in keratoconus.

Eighty-five patients with keratoconus and 20 normal controls were studied. Ocular rigidity, calculated using the Friedenwald nomogram, correlated with corneal thinning (p less than 0.001) and area of cone (p = 0.05). Schiøtz pressures with increasing weights fell more than normal in Grades 0 to 3 keratoconus, but returned to normal following corneal grafting, as did ocular rigidity. Goldmann applanation pressures were low over the apex of the cone, varying most in eyes with lowest ocular rigidity and with greatest corneal thinning, but also in large sagging cones which had only minimal corneal thinning. Pressures by Mackay-Marg tonometry were normal in keratoconus. The axial length of eyes with keratoconus even after corneal grafting was longer than normal and correlated with myopia. It is concluded that abnormalities of ocular rigidity, Schiøtz and Goldmann pressures in keratoconus are determined by abnormal corneal rigidity. If corneal rigidity is low or corneal thinning is present, Goldmann applanation pressures are falsely low and Mackay-Marg tonometry more closely approximates the true intraocular pressure. The cone develops by stretching with limited loss of tissue and the lower rigidity and reduced apical stress decrease the likelihood of rupture.