Anterior Horn Cell and Spinal Cord Disorders
Anterior horn cell and spinal cord disease
Amyotrophic lateral sclerosis (ALS) is the prototypical anterior horn cell disorder: progressive, painless weakness combining upper motor neuron (UMN) and lower motor neuron (LMN) signs, with normal sensory examination and no sphincter involvement until late. Revised El Escorial/Awaji criteria require LMN degeneration in at least two regions plus UMN signs in at least one, or LMN in one region with UMN in two; fasciculation potentials plus chronic reinnervation changes can substitute for active denervation in the Awaji modification. Look for split-hand weakness (thenar wasting with relative sparing of the first dorsal interosseous and abductor digiti minimi), bulbar onset with tongue fasciculations, and pseudobulbar affect. Sensory nerve action potentials remain normal, a key discriminator from cervical spondylotic myelopathy.
Mimics matter: multifocal motor neuropathy has conduction block, asymmetric distal arm weakness, and responds to IVIG; spinal muscular atrophy (SMN1 homozygous deletion) causes symmetric proximal weakness with tongue fasciculations and preserved sensation; Kennedy disease (X-linked CAG repeat) adds gynecomastia, tremor, and elevated creatine kinase. Poliomyelitis and post-polio syndrome produce asymmetric LMN deficits without UMN signs.
For spinal cord localization, distinguish intramedullary (syrinx, ependymoma, B12 deficiency) from extramedullary compression. Intramedullary lesions often produce dissociated sensory loss and sacral sparing; extramedullary lesions cause radicular pain and early spasticity. Anterior spinal artery infarction spares dorsal columns, while tabes dorsalis and B12 deficiency impair proprioception and vibration.
Rule of thumb: if sensory potentials are abnormal in a suspected ALS case, question the diagnosis and look for a mimic.
Trap: fasciculations alone are nonspecific and do not establish ALS.