An acquired, sporadic disorder characterized by
intermittent episodes of muscle weakness alternating with periods of normal
muscular function. Occurring during hyperthyroidism and thyrotoxicosis.
During attack, hypokalemia is present. May be precipitated by a low plasma
concentration of insulin.
Hashitoxic Periodic Paralysis
Most common in Chinese and Japanese. The incidence is
estimated at 1.9 to 8.8% of hyperthyroid patients. The disorder is rare
in non-Asian populations and only 5% of cases occur in women. A few cases
have been reported in African Americans, and this
disorder may be more frequent in black patients.
Exact pathophysiology remains to be defined. The
following factors appear to be important in the pathogenesis: (a)
hyperthyroidism increases Na/K pump ATPase activity, (b) increased platelet
Na/K pump ATPase activity has been demonstrated in patients with thyrotoxic
periodic paralysis, as compared with thyrotoxic patients, (c) thyroxine
sensitizes beta-adrenergic receptors to the effects of catecholamines (Na/K
pump ATPase activity is increased), (d) insulin increases Na/K ATPase
activity, which may explain the relationship of acute episodes of paralysis
to carbohydrate ingestion. These factors result in an intracellular shift of
potassium and reduction of the extracellular potassium, causing membrane
hyperpolarization. Some of the changes are present in patients who do not
develop thyrotoxic periodic paralysis, therefore a role for Na/K
ATPase-independent potassium influx has been suggested and the role of
Ca2+ gated ion channels is under investigation. The sarcolemmal
membrane is thought to be the site of the primary defect. Hypophosphatemia
is occasionally found in conjunction with thyrotoxic periodic paralysis and
may be synergistic in producing paralysis.
Demonstration of clinical or subclinical hyperthyroidism
associated with muscle weakness or paralysis.
Thyrotoxic Periodic Paralysis is uncommon in
childhood, with approximately 80% of cases presenting after the age of 20
years. Symptoms include myalgia, fatigue, muscle weakness, and symptoms of
hyperthyroidism, although these are often subtle. There may be a history of
weakness related to exercise and carbohydrate or alcohol intake. Proximal
muscles are more affected than distal muscles, and lower limbs more than
upper limbs. Examination commonly reveals a goiter and flaccid paralysis in
the affected muscles. Respiratory and bulbar muscles are rarely involved.
Cardiac dysrhythmias (atrial fibrillation/ flutter, supraventricular
tachycardia ventricular extrasystoles) are common. Electrocardiographic
changes reflect the hypokalemia (U waves, small T wave, ST depression, long
QT interval). Definitive treatment is correction of the hyperthyroidism.
Beta2-antagonism with propranolol may prevent paralysis in the face of
hypokalemia. Potassium supplementation may be used in the setting of acute
paralysis; however, it should be noted that body potassium stores are normal
and that hyperkalemia may occur as paralysis resolves.
Elective surgery is absolutely
contraindicated until the patient is rendered euthyroid; paralysis will not
occur in the euthyroid state. Examine for evidence of muscle weakness and
decrease reflexes. An ECG is mandatory to exclude dysrhythmia and ECG signs
of hypokalemia. Laboratory investigations, measure and correct serum
electrolytes (Na+, K...