PURPOSE: To evaluate the prevalence of prism adaptation response in Japanese patients with intermittent exotropia (X [T]) using the prism adaptation test and to assess whether patients with selected types of X [T] benefit from surgical outcome to which prism adaptation response may contribute.

METHODS: In a prospective study, 128 consecutive patients with X [T] between 1990 and 1995 were enrolled. The prism adaptation test was conducted by neutralizing the angle of deviation for 2 to 3 hours. Patients who showed an increase in exodeviation by 10▵ or more with the prism adaptation test were defined as having a prism adaptation response. For classification of the pattern of X [T], we chose a value of 15▵ as the difference between the distance and near measurements.

RESULTS: The percentage of patients in whom the prism adaptation response was observed at near fixation was significantly larger than those at distance fixation [35 (27%) patients versus 10 (8%) patients, P < .05]. Of 35 patients shown to have a prism adaptation response at near fixation, 21 patients (83%) had the basic type of exotropia. Fourteen patients (17%) with the basic type were changed to convergence insufficiency type because of an increase in near deviation and were defined as pseudo basic type. Patients with pseudo basic type had a significantly better surgical outcome compared with that of true basic type, whereas in the convergence insufficiency type, no definite tendency was found between the two subtypes, true and pseudo types.

CONCLUSION: Patients with the pseudo basic type of X [T] in whom a prism adaptation response was demonstrated had a more favorable surgical outcome.