Effect of general anesthesia and muscle relaxants on keratometry measurements using a handheld keratometer

Purpose: Keratometry measurements are often obtained under general anesthesia in the supine position in difficult patients and pediatric procedures. This study investigates the effect of general anesthesia and muscle relaxants on keratometry readings using a handheld keratometer. Methods: Fifty patients (with no history of intraocular surgery or corneal pathology) undergoing general anesthesia were prospectively enrolled. Keratometry readings were obtained using the Nidek KM-500 handheld keratometer (Nidek, Inc., Fremont, CA). in three settings: when the patient was awake in the upright and supine positions, and after general anesthesia. Readings were averaged in each eye and compared among the three settings; patients were also subgrouped by whether muscle relaxants were administered at induction. Intraclass correlation coefficients were calculated and Bland-Altman analysis was performed. Results: Keratometry readings were comparable between the upright and supine positions before anesthesia in all groups. In the muscle relaxant group, keratometry readings were flatter after anesthesia and this was statistically significant for right eyes (P = .02), but not for left eyes (P = .16). In the group with no muscle relaxant, no significant differences were noted. Intraclass correlation coefficients of the differences were high (≥ 0.97) for all eyes in both groups and Bland-Altman plots showed most of the differences to be within the limits of agreement. Conclusions: Keratometry readings using the handheld keratometer obtained under general anesthesia were as reliable as readings obtained in the awake state, regardless of posture; administration of muscle relaxants at induction may produce flatter keratometry readings.