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Myotonias are rare disorders characterized by difficulties in skeletal muscle relaxation. Either dominant or recessive modes of inheritance are possible. Underlying gene mutations cause defects in the ion channels of the muscle membranes. Previously undiagnosed myotonias may occur among military conscripts. We report here eight such patients with enhanced symptoms of myotonia during their military service. Six patients had myotonia congenita, one had myotonic dystrophy, and one paramyotonia congenita. In myotonia congenita, serum creatine kinase and aldolase levels correlated with the recommended service fitness classification. Because some anesthetic agents may have unfavorable side effects in myotonia, both patients and anesthesiologists need to be aware of the diagnosis. The awareness of military surgeons regarding the possibility of myotonia is necessary to provide a correct diagnosis and to establish the service fitness of these patients.

Introduction

Myotonia means difficulty in the relaxation of skeletal muscle after voluntary contraction. It is present with initial activation and usually abates after repeated muscle activity (warm-up phenomenon). The diagnosis is based on a careful neurological examination demonstrating myotonic features of muscles and, usually, skeletal muscle hypertrophy. Electroneuronomyography (ENMG) reveals typical repetitive discharges of varying amplitude and frequency.1 There is considerable overlap between the various clinical subtypes, and the severity of the clinical findings varies considerably.2 Muscle biopsy findings are nonspecific in differential diagnoses of myotonias.1 Several gene defects affecting ion channels in muscle membranes may lead to myotonia. Direct identifications of gene mutations causing the disorder are frequently available for further classification.3 Myotonia occurs in several specific disorders; three of them, pertinent to this study, are described below (for a more extensive description, see, for example, Online Mendelian Inheritance in Man, at http://www.ncbi.nlm.nih.gov/omim).

In myotonia congenita, myotonia is most clearly demonstrated by an inability or delay in release of the hand grip. Muscle strength is usually normal at rest, but myotonia may cause functional difficulties in proximal muscles, for example while climbing stairs. Myotonia may lead to muscular hypertrophy, particularly in the limbs and trunk. Percussion myotonia, a local postpercussion contraction followed by abnormally slow muscle relaxation, occurs in approximately one-half of the patients. The symptoms are usually not aggravated by exposure to cold. Myotonia congenita is caused by mutations of the gene on chromosome 7 coding the proteins of the chloride channel of the skeletal muscle membrane. Both dominant (Thomsen's myotonia) and recessive (Becker's myotonia) modes of inheritance are possible.3

Paramyotonia congenita is a dominantly inherited myotonia aggravated by exercise and exposure to cold. It is often associated with muscle weakness or flaccid paresis after myotonia1 and with lability of serum potassium levels.4 The disease is nonprogressive; no atrophy or hypertrophy of the muscles is observed. Paramyotonia congenita results from mutations of a gene on chromosome 17 coding proteins of a natrium channel.3

Myotonic dystrophy is an autosomal dominant disorder characterized by myotonia and muscular dystrophy starting from the distal muscles of the extremities and the face, head, and neck. Cataracts, hypogonadism, frontal balding, and electrocardiographic changes usually occur as well. The symptoms typically manifest in middle age but may start during the second decade of life.1 A gene mutation on either chromosome 19 (type 1) or chromosome 3 (type 2) results in sodium and chloride channel defects. Cognitive performance declines in proportion to increased size of the gene defect and decreased age of onset of symptoms.5 Amplification of the gene defect is frequently observed after parent-to-child transmission.6

The drills during basic military training include exercises requiring rapid onset of movement from rest, without any possibility for individual stretching or warm-up. This may make myotonia particularly distinct. Moreover, myotonia is frequently increased by cold, hunger, fatigue, and emotional upset. All of these may occur during military service. Augmentation of myotonic symptoms during conscript service has been described.7

The prevalence of myotonia congenita in Northern Finland has recently been estimated to be 7.3 cases per 100,000. The disease affects male subjects more often than female subjects. The mean age of onset of symptoms is 11 years, but the mean time needed for diagnosis after the onset of symptoms is 18 years.8 Therefore, military conscripts may have undiagnosed myotonias. To facilitate detection of these patients, this report describes the symptoms and signs of myotonias diagnosed among conscripts and military regulars at Central Military Hospital during 1996-2002.

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