A fisherman who could not row

Abstract

In July, 2007, a 28-year-old man came to our institute. Until recently, he had worked as a fi sherman, off the coast of Kerala—where boats traditionally carry 4–5 fi sher men, who must row against strong currents and waves. He had had to leave his job, because, when rowing, he could not keep to the same rhythm as his fellows: he could not extend his elbows rapidly after fl exion. After 5 min, this problem disappeared. Additionally, he could not release his grip quickly, after holding things tightly. His problems had gradually increased over 5 years; he had had to leave his job, and was now working as an assistant in a roadside shop. He had no other medical history of note. General examination showed nothing of note. Neurological examination showed no abnormality of cognitive function, cranial nerves, the somatosensory system, or cerebellum; power and refl exes were normal, and the patient had no muscle atrophy or fasciculations. However, repeated gripping and releasing of the examiner’s fi ngers, tapping over the extensor digitorum communis, and tapping the tongue revealed grip myotonia, percussion myotonia, and tongue myotonia respectively. Blood tests revealed a high concentration of creatine kinase (CK) (2960 IU/L; normal range 21–232 IU/L). Needle electromyography (EMG) showed diff use myotonic discharges (fi gure), but no evidence of myopathy. On cooling of muscle to 20°C, by wrapping the limb in a plastic bag and immersing it in iced water, and after short and long periods of exercise (voluntary contraction of the muscle tested, for 5–10 s and 3–5 min with brief rest breaks), we found no decrease in compound muscle action-potential amplitude. These fi ndings were consistent with true myotonia. But of what cause? The age of onset, high concentration of CK, and normal EMG after exercise were atypical for myotonia congenita. The persistence of symptoms indicated that the patient probably did not have potassium-aggravated myotonia (also known as myotonia fl uctuans). Slit-lamp examin ation showed no cataract, which, together with the absence of muscle weakness, made type 1 myotonic dystrophy unlikely; moreover, genetic testing for this disease gave a negative result. However, routine echo cardiography revealed mode rate pericardial eff usion without tamponade, so we requested thyroid function tests. The thyrotropin con cen tration was very high (>60 mIU/L; normal range 0∙25–5 mIU/L); tri-iodothyronine and thyroxine concentrations were low, at <0∙4 nmol/L (normal 0∙92–2∙33 nmol/L) and 8∙01 nmol/L (normal 60–120 nmol/L) respect ively. We prescribed thyroxine. The pa tient’s symp toms subsided rapidly. At 3-month and 6-month follow-up, we found no clinical or electro physiological evidence of myotonia; concentrations of CK and thyroid hormones were normal. When last seen, in June, 2008, the patient was well, on a thyroxine dose of 200 μg daily, and had returned to work as a fi sher man. Myotonia is the slow relaxation of muscle, after voluntary contraction or percussion. It is painless, but patients often describe muscle stiff ness that improves with repeated use of muscle, the so-called “warm-up phenomenon”. On examination, myotonia may be evident from a delayed release of the hand, in the introductory handshake. EMG shows spontaneous, repetitive discharges with waxing and waning of amplitude and frequency, producing a characteristic “dive-bomber” like sound. Myotonia can be a presenting sign of myotonic dystrophy; non-dystrophic myotonias, in which mytonia is caused by dysfunction of channels for chloride (in myotonia congenita) or sodium (in paramyotonia congenita, potassium-aggravated myotonia, and hyperkalaemic periodic paralysis with myotonia); myopathies (acid maltase defi ciency, polymyositis, myotubular myopathy); or, rarely, denervation of any cause. Syndromes causing myotonia are distinguished by associated clinical features, and by EMG after cooling and exercise. Pseudomyotonia is slow relaxation without myotonic discharge on EMG, and is found in hypothyroidism, when it is characteristically revealed by testing of tendon refl exes. However, hypothyroidism can also cause true myotonia, and can unmask type 2 myotonic dystrophy. Patients with myotonia caused by hypo thyroidism should be given thyroxine; patients in whom myotonic dystrophy is unmasked may benefi t symptom atically from thyroxine, despite persistent myotonic dis charges on EMG. We suspect, on clinical grounds, that our patient had myotonia caused by hypothyroidism; he declined genetic testing for type 2 myotonic dystrophy.