Objectives: To assess how often mitochondrial myopathy (MMP) mimics amyotrophic lateral sclerosis (ALS) and the phenotypic similarities and differences between these two disorders.
Methods: Records of 123 MMP patients and 59 ALS patients, diagnosed during five years (1996-2000), were retrospectively evaluated.
Results: Re-evaluation revealed that 8 patients, initially diagnosed as ALS had actually MMP (13.6%). Among the MMP patients, 6.5% were initially misdiagnosed as ALS. None of the MMP patients actually had ALS. Common features of ALS and MMP were weakness, wasting, upper motor neurone signs, bulbar abnormalities, and a neurogenic EMG. Features differentiating MMP from ALS were ptosis, sensory disturbances, multi-system involvement, slowly progressive disease course, abnormal lactate stress test, histological and biochemical abnormalities, and mtDNA analysis.
Conclusions: In a small number of cases MMP may clinically and electrophysiologically mimic ALS, particularly in the early stages of the disease. Patients with suspected ALS, but slow progression and multi-organ involvement, should undergo lactate stress testing and muscle biopsy. ALS should be diagnosed only if MMP has been excluded.