MRI in Muscle Channelopathies

Abstract

Myotonia is an involuntary-slowed relaxation after a forceful voluntary muscle contraction which is experienced by the patient as muscle stiffness. Electrical hyperexcitability of the muscle fiber membrane is the basis of myotonia. The stiffness recedes with repeated contractions, a phenomenon called warm-up. Patients in whom muscle stiffness worsens with repetition or with cooling suffer from paradoxical myotonia or so-called paramyotonia. This type of myotonia is associated with episodes of flaccid limb muscle weakness similar to periodic paralysis. Patients with periodic paralysis experience episodic weakness spells with varying intervals of normal muscle function. Electrical inexcitability of the muscle fiber membrane is the basis of periodic paralysis. Two dominant episodic types of weakness with or without myotonia are distinguished by the serum K⁺ level during the attacks of tetraplegia: hyper- and hypokalemic periodic paralysis. Independently of the severity and frequency of the paralytic episodes, many patients develop a chronic progressive myopathy in the forties, an age at which the attacks of weakness decrease. Although channelopathies such as myotonias and periodic paralyses are known for episodic symptoms, in most cases progressive focal or general muscular weakness is present. Routine protocols of proton (¹H) magnetic resonance imaging (MRI) show normal muscle morphology or may demonstrate edematous or lipomatous changes, atrophy or hypertrophy; however, these morphologic changes are not very disease-specific. The following chapter introduces examples of conventional and modern functional imaging methods like ²³Na MRI for evaluation of muscular channelopathies, in which an autosomal-dominant bequeathed defect of muscular Na⁺ channels leads to a pathologic Na⁺ influx that causes intermittent or permanent muscular paresis as well as muscular stiffness. ²³Na MRI by which aspects of muscular pathogenesis such as muscular Na⁺ homeostasis can be visualized and monitored has effectively achieved value in the radiologic management of muscular Na⁺ channel diseases, since ²³Na MRI is able to depict an intracellular muscular sodium accumulation simultaneous to development of muscular paresis. This sodium accumulation correlates well with the grade of paresis and is reproducible.