Juvenile progressive bulbar palsy. Clinical and electrodiagnostic findings.

• Differentiation of juvenile progressive bulbar palsy from bulbar myasthenia gravis is difficult. Characteristics of both may include ocular involvement, fluctuant course, abnormal fatigability, and normal acetylcholine receptor (AChR) antibody titers. Electrodiagnostic evaluation may demonstrate moment-to-moment variability in motor unit action potential amplitude, fibrillation potentials, and decremental motor evoked responses. Increased jitter with blocking may be the most prominent electrodiagnostic abnormality in either disorder, even in asymptomatic extremity muscles. Complete paralysis of facial muscles with electrical silence on needle electromyography, low-amplitude facial evoked responses without a decrement to repetitive stimulation, increased jitter and fiber density in asymptomatic extremity muscles, and normal AChR antibody levels suggested juvenile progressive bulbar palsy in two patients initially thought to have bulbar myasthenia. Early differentiation of these disorders is important because of therapeutic, genetic, and prognostic implications.