Episodic weakness and vacuolar myopathy in hypokalemic periodic paralysis

We report a 50-year-old woman who presented with a 20 year history of gradually progressive lower extremity weakness, characterized by knee buckling with occasional falls and foot dragging. She also experienced difficulty in lifting her arms above her shoulders. The primary periodic paralyses are rare disorders caused by dysfunctional ion channels in skeletal muscle. The hypokalemic type is generally an autosomal dominant condition, due to missense mutations in the alpha subunits of the skeletal muscle L-type calcium channel genes, CACN1AS, or the skeletal muscle sodium channel gene, SCN4A. The affected patients typically present with episodic weakness. For our patient, the consumption of foods high in carbohydrates seemed to precipitate the episodes of weakness. Her family history was significant for six blood relatives, including three sons and three relatives on the paternal side, who had experienced similar symptoms. A biopsy of the left rectus femoralis muscle showed vacuolar myopathic changes in the scattered muscle fibers, accompanied by occasional degenerating and regenerating muscle fibers. There was no evidence of inflammation on the biopsy. The vacuoles were often associated with increased acid phosphatase staining. An electron microscopic examination showed that the vacuolar changes were due to T-tubule dilation, a characteristic of hypokalemic periodic paralysis. Other metabolic etiologies of vacuolar myopathy, such as acid phosphatase (lysosomal) associated acid maltase deficiency (a glycogen storage disease), need to be considered in the differential diagnosis.