Medial rectus electromyographic abnormalities in Duane syndrome.

We report on four patients from the botulinum toxin clinic at Moorfields Eye Hospital with Duane syndrome, who exhibited paradoxical medial rectus activity. EMGs were performed with a standard toxin needle and were qualitative in nature. Current physiodynamic theories as to the etiology of Duane syndrome are based on an aberrant innervation of the lateral rectus. Paradoxical electromyographic activity of the medial rectus may occur in Duane type I syndrome. A possible explanation for this finding is that the medial rectus may receive aberrant innervation but this may not be physiodynamically significant. We postulate that peripheral innervational anomalies may be secondary to, or coexist with, a brain stem anomaly. If an extraocular muscle possesses a dual innervation, then electromyographic abnormalities, without physiodynamic significance, may occur if the recording electrode comes into contact with muscle fibers innervated by a nerve that supplies a small segment of muscle.