Corticosteroids p-Aminosalicylic acid
Potassium-depleting diuretics tions in frequency and amplitude seen in patients with other myotonias. In phenotypically similar cases, in which the electrical activity closely resembles that noted in myotonia, the likely diagnosis is myotonia permanans.pl In all these conditions, muscle biopsy shows few abnormalities, although there may be variations in fiber size with fiber hypertrophy and increased central nuclei. In paramyotonia congenita with hyperkalemic periodic paralysis, vacuolated and necrotic fibers may occur. In myotonia congenita, there may be a lack of 2B fibers. Study of family members may be valuable, since clinical or EMG evidence of disease in apparently asymptomatic individuals can clarify the mode of inheritance.
Periodic Paralysis. When periodic paralysis is considered, serial blood tests for potassium, calcium, magnesium, phosphate, and CK should be obtained during an episode of weakness. Each time a blood sample is taken, muscle strength should be tested. Potassium levels should be checked sequentially every 15 to 30 minutes to determine the direction of change when muscle strength is decreasing or improving. The potassium level may be normal during hyperkalemic periodic paralysis and occasionally in hypokalemic periodic paralysis. An electrocardiogram (ECG) may show changes consistent with hypokalemia or hyperkalemia and may also give forewarning of cardiac complications. EMG in these patients shows reduced CMAP proportionate to the degree of weakness. Nerve conduction studies are normal. Neurophysiological tests also serve to exclude other causes of paralysis such as the Guillain-Barre(c) syndrome or myasthenia gravis.
Patients who are seen between attacks require a thorough investigation to exclude a diagnosis of secondary causes of hypokalemia. EMG may be helpful in showing evidence of myotonia that favors a diagnosis of hyperkalemic periodic paralysis. In patients in whom fixed weakness has developed, the EMG may reveal myopathic changes. Even in patients in whom the EMG is initially normal, there may be an exaggerated increment followed by a decline in CMAP with high-frequency 30-Hz stimulation or following repetitive muscle contraction.  Muscle biopsy is often abnormal between attacks in patients with hypokalemic periodic paralysis, demonstrating pathognomonic changes of large central vacuoles and occasional necrotic fibers. In patients with hyperkalemic periodic paralysis, smaller vacuoles may be present, and tubular aggregates may be seen. Provocative testing may be required to confirm the diagnosis. This must be performed under careful supervision including monitoring of the potassium level and the ECG. Hypokalemic challenge can be performed by giving intravenous glucose with or without insulin.[14i Hyperkalemic challenge can be performed by giving repeated doses of oral potassium. ^
Management Myotonias. Because myotonia can be exacerbated by several muscle relaxant drugs and by anticholinesterase drugs, anesthesia for these patients should be planned accordingly. Because potassium administration can exacerbate myotonia, potassium supplements should be given only when necessary and always with caution. The treatment of myotonia congenita relies on membrane-stabilizing drugs such as procainamide and quinine, and these can be used intermittently as necessary. Phenytoin is more useful
for chronic administration and is less likely to produce cardiac side effects. Occasional cases of myotonia are responsive to acetazolamide, and mexilitine is useful for both the myotonia and the weakness associated with paramyotonia congenita.
Periodic Paralysis. Attacks of hypokalemic periodic paralysis may be partially prevented by a low-carbohydrate, low-sodium diet. Acetazolamide prevents paralytic attacks and improves residual weakness between attacks. When hypokalemic periodic paralysis is secondary to thyrotoxicosis, the primary thyroid disease should be treated. In the interim, the paralytic attacks and persistent weakness can be treated very effectively with beta-adrenergic blocking agents. During acute attacks, emergency treatment may be required. In these cases, oral potassium is preferred because its concentration usually declines with parenteral potassium owing to the diluents employed. The ECG should be carefully monitored during attacks and when treating patients because of the risk of cardiac dysrhythmias.
In patients with hyperkalemic periodic paralysis, attacks are usually mild but should be treated to prevent the onset of permanent weakness; thiazide diuretics are effective in these patients. During acute attacks, treatment is seldom needed. Carbohydrate-containing foods and fluid may aggravate the weakness, and potassium-containing foods should be avoided. Inhaled beta-adrenergic agonists such as salbutamol are effective treatments in acute situations but may be contraindicated if the patient has a coexisting cardiac dysrhythmia.
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