I read with great interest the recent article on metabolic disease in adults by Talente and colleagues [1] and the accompanying editorial by Segal and Roth [2]. Laboratory testing for inborn errors of metabolism has been successfully extended to adults through cooperation between pediatric and adult clinical and laboratory services in academic medical centers.

Our laboratory does about 1500 tests annually for metabolic disorders, such as lysosomal storage diseases, aminoacidopathies, and metabolic muscle disease in all age groups. Our core neuromuscular group bridges several departments, including neurology, pediatrics, rehabilitation medicine, and pathology, and every week reviews regionally and nationally referred cases.

During the past 2 years, 125 patients were diagnosed with specific metabolic muscle diseases; 53 of these were 18 years of age or older. Their diagnoses are shown in Table 1. Their clinical features included muscle cramps, pain, weakness, and exercise intolerance. Common histopathologic features included lipid storage (eight patients), glycogen storage (seven patients), abnormal mitochondria (six patients), inclusions or vacuoles (six patients), and ragged red fibers (four patients). Many patients had not been appropriately studied, even though they reported neuromuscular, electrocardiographic, and oculomotor abnormalities known to be associated with metabolic disorders; only 22% were evaluated for a family history of metabolic disease.

Our special interest in carnitine palmitoyl transferase deficiency may be the reason it is the most prevalent disorder among our adult patients. However, this deficiency has been reported to be the most common lipid myopathy to affect skeletal muscle [3]. Although most of the listed disorders are autosomal recessive, the more heterogeneous mitochondrial respiratory chain defects can be autosomal dominant (mitochondrial depletion disorders) or maternally inherited mitochondrial gene defects. More importantly for aging adults, some respiratory chain defects can occur sporadically due in part to the effect of a high mutation rate in mitochondrial DNA [4].

Many metabolic disorders of muscle in adults have an insidious course with clinical manifestations triggered by exertion, fasting, infection, or extremes in temperature, making the determination of the true incidence of these disorders difficult. Therefore, physicians should be aware not only of the common features of muscle disease in affected patients but also of certain high-risk groups such as patients with idiopathic cardiomyopathy, encephalopathy, or suspected but unproven malignant hyperthermia [5].