Myotonic Dystrophy

Abstract

A 40-yr-old man presented to our traumatic brain injury clinic for evaluation of traumatic brain injury from repeated falls. He reported a 1-yr history of falling that was progressively worsening in frequency and severity. One fall resulted in a mild traumatic brain injury. He noted a progressive bilateral hand weakness with occasional cramping, which worsened in cold temperature. Physical examination findings included frontal balding, a long, thin face, and temporal and masseter muscle atrophy (Fig. 1). He had difficulty releasing his grasp after a handshake. Percussion of his thenar muscles resulted in sustained thumb flexion and adduction (Fig. 2). Manual muscle examination revealed distal upper and lower limb weakness with normal proximal strength. Cardiac auscultation revealed normal heart sounds with a nondisplaced apex. Diagnostic testing included electromyography, which revealed myotonic discharges, positive sharp waves, fibrillation potentials, and short-duration motor unit potentials with rapid recruitment affecting distal muscles more than proximal muscles. Electrocardiogram showed intraventricular conduction delay.FIGURE 1: Frontal balding and significant temporal and masseter muscle wastingFIGURE 2: Percussion myotonia of the thenar musclesInterventions included physical therapy for gait and transfer training, aerobic conditioning, and bilateral ankle foot orthoses. With these interventions, his gait had improved, with a decreased frequency of falls and mild improvement in distal motor strength. He was referred to the cardiology clinic to evaluate his conduction deficits. Genetic testing confirmed the diagnosis of myotonic dystrophy, which showed approximately 700 cytosine thymine guanine DNA nucleotide repeats (normal range, 5–32 copies). Myotonic muscular dystrophy is the most common form of muscular dystrophy among the white population, with an estimated prevalence of 1 in 8000. Myotonic dystrophy is a multisystem disorder that affects skeletal muscle, smooth muscle, myocardium, brain, and ocular structures. Clinical findings may include facial and distal muscle weakness and wasting, cardiomyopathy with conduction defects, low intelligence or dementia, frontal balding, and cataracts. The hallmark of myotonic dystrophy is myotonia, which is a delay in muscle relaxation. Myotonia may be observed during percussion of the thenar muscles with a reflex hammer, which causes sustained thumb flexion and adduction. In addition, the patient may also have difficulty releasing his hand during a handshake, known as grip myotonia. Myotonic macular dystrophy is usually diagnosed based on history and physical examination findings, with laboratory tests and electromyography to help confirm the diagnosis. Life expectancy is reduced in patients with myotonic dystrophy, due to cardiac arrhythmias or sudden deaths.1,2 The severity of cardiac and skeletal muscle involvement correlates with cytosine thymine guanine nucleotide repeat length.3 Patients with myotonic muscular dystrophy have a significantly increased risk for stumbles and falls when compared with the normal population.4 Gait training and use of orthotics may reduce fall risk. Progressive resistance exercises and aerobic conditioning can increase muscle strength. Early diagnosis is important to limit morbidity and mortality by ensuring appropriate therapeutic interventions, orthotic prescriptions, and cardiac evaluation and management.