Evidence for the occurrence of myotonia in the extraocular musculature in patients with dystrophia myotonica

Abstract

Dystrophia myotonica (DM) is a multisystem disorder which has been reported to affect eye movements. There is, however, disagreement in the literature as to whether the observed slowing of saccadic eye movements is due to a central abnormality or due to abnormalities of the extraocular muscles themselves. A ‘warm-up’ effect is well known to occur in the peripheral muscles in DM. This effect involves a progressive decrease in the degree of myotonia observed with each successive relaxation during a period of repetitive contraction and relaxation of a given muscle. The authors have studied seven patients using two different paradigms to provide evidence that the ‘warm-up’ phenomenon can be demonstrated in extraocular musculature in patients with DM. Such an effect is not seen in controls. The observation of a ‘warm-up’ phenomenon suggests that the disorder of saccadic eye movements is likely to be due to an abnormality of the extraocular muscles themselves rather than a central mechanism.

Key Words:

• dystrophia myotonica
• saccades
• peak velocity
• ‘warm-up’ phenomenon