The term pseudomyotonia (slow relaxation of muscles after voluntary contraction) describes the clinical appearance of myotonia in the absence of myotonic discharges on the electromyography. Pseudomyotonia is most commonly observed as the slow-relaxing or “hung-up” tendon reflexes of hypothyroidism, although other causes are described. Pseudomyotonia is seen in about one-third of patients with Isaacs syndrome, particularly with handgrip, but also after eye and jaw closure; rarely, this can be the first symptom.