Motor neuron degeneration in a 20-week male fetus: spinal muscular atrophy type 0.

Background: Neuropathological changes in degenerating motor neurons are well documented in the term neonate with spinal muscular atrophy, but not at midgestation.

Methods: Postmortem neuropathological examination was performed in a 20-week male fetus with a hypoplastic left cardiac anomaly.

Results: Selective degeneration of spinal and hypoglossal motor neurons was an incidental finding. Degenerating motor neurons were not immunoreactive with neuronal nuclear antigen (NeuN) or neuron-specific enolase (NSE), as were the normal motor neurons. Synaptophysin reactivity was reduced around the soma of degenerating normal motor neurons. Ubiquitin and tau were expressed in degenerating motor neurons. Gliosis, inflammation and microglial activation were lacking in the ventral horns of the spinal cord. Laryngeal striated muscle was unaltered for age. No cerebral malformations or hypoxic-ischaemic changes were found.

Conclusion: This case represents an early motor neuronal degeneration and corresponds to the recently described "type 0" spinal muscular atrophy. Lack of contractures is attributed to the early fetal age, since most muscular growth occurs in the second half of gestation.