Familial motor neuron disease with prominent onion-bulb-like structures and axonal swelling restricted to the spinal ventral root: autopsy findings in two siblings.

We report autopsy cases of two siblings who developed muscular atrophy and dementia, clinically considered to be familial motor neuron disease (MND). They presented with motor neuron signs predominantly in the distal limbs without sensory impairment. At autopsy, severe neuronal loss in the anterior horn consistent with MND was found, but histopathological hallmarks like Bunina bodies and skein-like inclusions were absent. Surprisingly, numerous huge axonal swellings (about 30 microm in diameter) and onion-bulb-like structures were found in the spinal ventral roots. These changes were not observed in spinal dorsal roots or peripheral nerves. However, obvious segmental demyelination of the ventral root was not found. In addition, neurofibrillary tangles (NFTs) and neuritic plaques were present in the frontal cortex, temporal cortex and hippocampus, and to a lesser degree, in the amygdala, substantia nigra and thalamus. Our two cases are a hitherto unreported type of MND, which shows focal giant axonopathy and prominent formation of onion-bulb-like structures due to Schwann cell proliferation restricted to the spinal ventral roots.