After studying this chapter, the student should be able to:
Provide an overview of the common neuromuscular disorders organized around their localization along the peripheral neural axis.
Review the etiologies and pathophysiologic mechanisms involved in different neuromuscular disease processes.
Discuss briefly the management of common neuromuscular disorders with a special focus on diagnostic testing and specific treatments.
Neuromuscular disorders are a diverse group of diseases that result from lesions in the nerve cells inside or close to the spinal cord, peripheral nerves, neuromuscular junction, or muscles. The cell bodies of the sensory nerves are located in the dorsal root ganglia just outside the intervertebral foramina (Figure 31–1), while the motor neuron cell bodies lie in the anterior horn of the spinal cord, within the gray matter. The motor unit is a functional entity that includes the anterior horn cell, its nerve axons, and the muscle fibers that are supplied by these axon terminals. Overall, neuromuscular disorders can be classified into disorders of the cell body (neuronopathy), nerve roots (radiculopathy), plexus, peripheral nerves, neuromuscular junction, or muscles.
Schematic illustration of a cord segment with its roots, ganglia, spinal nerves, and rami. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The recent advances in diagnostic methods, specifically genetic testing and the development of consensus criteria for few neuromuscular disorders have led to some evolution in the epidemiologic data for these diseases. Compared to the 1991 first world survey of most inherited neuromuscular disorders, a 2015 comprehensive review of the available prevalence data of 30 neuromuscular diagnoses reported significant increases in some inherited neuromuscular disorders. Increases in prevalence rates were seen in myotonic dystrophy (7.1 to 26.5 per 100,000), Charcot-Marie-Tooth disease (3.1 to 82.3 per 100,000), facioscapulohumeral dystrophy (2.03 to 6.8 per 100,1000), and Becker muscular dystrophy (0.07 to 3.65 per 100,1000). In general, the prevalence rates of neuromuscular disorders were found to range from 0.1 per 100,000 population for oculopharyngeal muscular dystrophy to 60 per 100,000 population for post-polio syndrome. Prevalence of neuromuscular disorders as a group was estimated to be at least similar to that of Parkinson disease worldwide (100 to 300 per 100,000). We still believe that this summed estimate for neuromuscular disorders as a group represents only the tip of the iceberg. Thus, although neuromuscular disorders are rare as individual disease entities, as a group they are not.
DISORDERS OF THE ANTERIOR HORN CELL
Poliomyelitis is a paralytic disease that has historically been responsible for epidemics of weakness, disability, and death. The causative organism of poliomyelitis is the mRNA polio enterovirus that is transmitted by oral–fecal contamination. The virus has a strong predilection for the spinal cord, ...