Summary

Background: Ocular hypotony is a common unexplained feature of myotonic dystrophy type 1. Spuriously low applanation tonometric readings can be caused by thin corneas, flat corneal curvature and corneal edema.

Objectives: To determine whether structure abnormalities of the cornea cause spuriously low readings in applanation tonometry.

Methods: We utilized a TMS-2N corneal topographer, a NonconRobo SP-6000 Specular microscope and a Corneo-Gage Plus 1A Pachymeter to examine seven patients with DM1[1] and eight healthy controls. Intraocular pressure, central corneal thickness, and endothelial cell density were measured, and simulated keratometry readings were made. Cornea guttata and irregularity of corneal topography patterns were also sought.

Results: The mean intraocular pressure was 9.86 ± 1.29 mmHg for all patients (intraocular operated and non‑operated eyes) and 12.88 ± 1.89 mmHg for the controls (P = 0.000021, two-tailed t-test). Central corneal thickness was 530.57 ± 35.30 micron for all patients and 535.00 ± 39.62 micron for the controls (P = 0.75, two-tailed t-test). Endothelial cell density was 3164 ± 761 cells/mm² for all patients and 3148 ± 395 cells/mm² for the controls (P = 0.94, two-tailed t-test). Simulated keratometry readings were similar in both groups when the operated eyes were excluded. Cornea guttata and irregularity of corneal topography patterns were also noted in the study group.

Conclusions: Corneal thickness, corneal curvature and corneal hydration were within normal limits and thus were not the cause for the low applanation tonometry reading in DM1. The presence of cornea guttata and irregularity of corneal topography patterns in DM1 warrants further investigation.