A 34-year-old woman had a year-long history of right lower extremity weakness. Her problems were dated to a fall several months prior to this assessment, followed by subsequent falls and difficulties lifting her right foot when walking. She had no speech or swallowing difficulties, urinary or bowel complaints, numbness, or upper extremity complaints.

Our patient was a 34-year-old woman with gradually progressive painless muscle weakness and atrophy, initially affecting her right leg and then spreading to the left. She had no sensory loss or bladder or bowel disturbances. Examination demonstrated a lower motor neuron pattern of asymmetric lower extremity weakness without any upper motor neuron signs. Imaging of the spine, cerebrospinal fluid (CSF) analysis, and routine laboratory studies were normal.

The electrophysiologic studies indicated relatively normal motor nerve conduction velocities. The needle examination showed chronic and active neurogenic changes in multiple muscles. Together, these findings indicated a lower motor neuron syndrome that could be due to anterior horn cell disease, lumbosacral polyradiculopathy, or multifocal motor neuropathy.

Variants of motor neuron disease include amyotrophic lateral sclerosis (ALS), primary lateral sclerosis, progressive muscular atrophy (PMA), and progressive bulbar palsy (Table 11.1). ALS is the most common degenerative motor neuron disease of adulthood. It is recognized by the presence of both upper motor neuron (UMN) and lower motor neuron (LMN) signs in multiple spinal segments. Among patients with findings restricted to the LMN, PMA is in general a slower form of motor neuron disease associated with abnormalities of the survivor motor neuron gene. Patients with pure LMN syndromes with predominant bulbar weakness may also have X-linked bulbospinal neuropathy (Kennedy disease) or multifocal motor neuropathy (MMN). Patients with MMN have asymmetric focal weakness of the distal limbs and have marked conduction block on nerve conduction studies, and approximately half of them demonstrate elevated levels of anti-GM1 antibodies. Patients with pure LMN syndrome should also be screened for a possible monoclonal gammopathy. As with any focal LMN disorder, particularly involving paraparesis, meticulous imaging of the spine is mandatory to exclude structural lesions (tumor, spinal vascular malformation, syrinx, etc.). Inflammatory disorders of the roots, such as an immune-mediated polyradiculopathies or infectious polyradiculopathies, should be considered and evaluated with CSF studies. Infiltrating processes such as sarcoidosis or fungal or carcinomatous meningitis might also affect multiple motor nerve roots and lead to a progressive clinical course.