Electrodiagnostic Evaluation Of Motor Neuron Disease

Motor neuron disorders can be thought of as residing on a spectrum, whether upper motor neurons, lower motor neurons, or both are affected. Motor neuron diseases include amyotrophic lateral sclerosis (affects both upper and lower motor neurons), primary lateral sclerosis (affects upper motor neurons), progressive muscular atrophy (affects lower motor neurons ), progressive bulbar palsy (affects lower motor neurons), spinal muscular atrophy (affects lower motor neurons), and post-polio syndrome (affects lower motor neurons). Motor neuron disease is used interchangeably with amyotrophic lateral sclerosis (ALS) as ALS is the most common adult-onset presentation of this disease. ALS is a neurodegenerative disorder leading to weakness of bulbar, thoracic, limb, and abdominal muscles with sparing of sensory function. Death usually occurs within two to five years from respiratory failure. Roughly 85 to 90% of ALS cases are sporadic, with about 10% being of familial origin. According to the 2014 US Census data, the prevalence of ALS was 5 per 100,000 people. Though there is variation in clinical presentation, the majority of the patients present with asymmetric limb weakness (80%) or bulbar dysfunction (20%). Bulbar dysfunction can manifest as dysphagia (trouble swallowing) and dysarthria (trouble speaking). There is progressive spread to other areas of the body with accompanying upper motor neuron and lower motor neuron findings. Upper motor findings include spasticity, hyperactive reflexes, and a positive Babinski sign. Lower motor neuron signs include muscle atrophy, weakness, flaccid paralysis, absent reflexes, fasciculations, and fibrillations. Patients can also display changes in behavior due to frontotemporal dysfunction, and about 15% of patients develop frontotemporal dementia. Some patients may also present with Pseudobulbar affect, which is dysregulation of emotional responses as exhibited by excessive laughter or crying. Many other neurological disorders such as strokes, Alzheimer’s disease, and multiple sclerosis also present with pseudobulbar affect.Once the diagnosis of ALS is suspected, electrodiagnostic testing is needed. Electrodiagnostic testing assesses the integrity of lower motor neurons and is crucial to diagnosing motor neuron disease as oftentimes, neuroimaging, and laboratory studies are normal. Nerve conduction studies (NCS) and needle electromyography (EMG) are important for supporting the diagnosis of ALS and ruling out other potential mimics of the disease. Some disorders that can mimic motor neuron disease are multifocal motor neuropathy with conduction block, chronic inflammatory demyelinating polyradiculoneuropathy, central nervous system tumors, multiple sclerosis, and polyradiculopathy, among others. It is important to rule out such mimics with NCS and needle EMG as the treatment regimens and prognosis differ among the varying disorders. Since the prognosis of ALS is poor, it is imperative to accurately diagnose the disorder to appropriately manage the associated symptoms involved and develop a treatment plan.