Neurodegenerative 'overlap' syndrome: Clinical and pathological features of Parkinson's disease, motor neuron disease, and Alzheimer's disease

Parkinson's disease (PD), Alzheimer's disease (AD), and motor neuron disease (MND) share epidemiological, clinical, and pathological features. Few studies have reported comprehensively on individuals who demonstrate a neurodegenerative 'overlap' syndrome, comprising idiopathic parkinsonism, dementia, and motor neuron dysfunction. We describe clinical, electrophysiological, and pathological features in six patients with neurodegenerative 'overlap' syndrome. All had cardinal features of PD (duration 6-26 years), and any mixture of dementia (slowly advancing), fasciculations, hyperreflexia, Babinski signs and mild atrophy and weakness of distal muscles (slowly progressive). EMG often demonstrated a lack of denervation in conjunction with abnormal MEPs (high thresholds). Patients had either 6FD-PET or pathological studies consistent with PD. Pathological studies also demonstrated moderate numbers of neurofibrillary tangles and plaque formation, typically with sparing of motor neurons in the spinal cord. We conclude that neurodegenerative 'overlap' syndrome may represent forme frustes of traditionally accepted diagnostic categories. Patients with parkinsonism, fasciculations, hyperreflexia and mild atrophy are unlikely to demonstrate active denervation on EMG; their prognosis is better than for classical MND. Neurodegenerative overlap syndrome (clinicopathological mixtures of PD, AD, and MND) may develop in some individuals as a reflection of common etiology, pathogenesis or susceptibility.