Anterior Horn Cell Disorders and Motor Neuron Disease
Anterior horn cell disorders are primarily caused by motor neuron diseases, with amyotrophic lateral sclerosis (ALS) being the most common type, representing approximately 85% of all cases. 1
Classification and Relationship
- Motor neuron diseases (MNDs) are a group of neurodegenerative disorders characterized by degeneration of upper motor neurons, lower motor neurons, or both 1
- Anterior horn cell disorders specifically affect the cell bodies of lower motor neurons located in the anterior horn of the spinal cord, representing a central nervous system disorder with peripheral manifestations 2
- The anterior horn cell diseases, with the exception of polio, are primarily progressive degenerative diseases of the motor neurons 3
Types of Motor Neuron Diseases Affecting Anterior Horn Cells
- Amyotrophic Lateral Sclerosis (ALS): Characterized by degeneration of both upper and lower motor neurons in the brain and spinal cord along the corticospinal tracts 1
- Progressive Muscular Atrophy (PMA): Involves degeneration of only the lower motor neurons, leading to progressive muscle weakness and atrophy without upper motor neuron signs 1
- Spinal Muscular Atrophy (SMA): Types I to III in children and type IV in adults 3
- Progressive Bulbar Palsy (PBP): A variant that primarily affects the bulbar muscles first 1
- Kennedy's disease: A genetic form of motor neuron disease 3
- Some manifestations of Pompe disease can involve anterior horn cells 2
Diagnostic Features
- Electrodiagnostic studies are crucial for diagnosing anterior horn cell disorders 2
- EMG findings characteristically show:
- Denervation with fibrillation potentials
- Positive sharp waves
- Fasciculations
- Complex repetitive discharges 3
- Motor nerve conduction studies may be normal or reveal low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 3
- Sensory nerve conduction studies are typically normal (except in Kennedy's disease) 3
- Needle electromyography (EMG) can determine the presence of denervation as evidence of anterior horn cell involvement 4
Imaging Findings
- MRI of the head without IV contrast is recommended as the optimal initial imaging modality for patients with suspected motor neuron disease 2, 1
- Spine MRI may show abnormal T2/STIR signal in the anterior horns ("snake eyes" appearance), corresponding to lateral corticospinal tracts 2, 1
- Brain MRI may show abnormal T2/FLAIR signal in the corticospinal tracts, particularly in the posterior limb of the internal capsule and cerebral peduncles 2
Clinical Implications and Management
- Regular motor and functional assessments are recommended at 3-6 month intervals for children under five years, and annually in older children and adults 5
- The American Academy of Medical Genetics and Genomics recommends submaximal, functional, and aerobic exercise for progressive muscle diseases, while avoiding excessive resistive and eccentric exercise 5
- Proper classification guides appropriate diagnostic workup, including specific imaging studies, electrophysiological testing, and potentially genetic testing 2
Important Distinctions
- Anterior horn cell disorders must be distinguished from peripheral neuropathies, which involve pathology of the peripheral nerves outside the spinal cord 2
- Peripheral neuropathies typically present with both sensory and motor symptoms, often in a length-dependent pattern, while anterior horn cell disorders primarily affect motor function 2
- To diagnose a widespread disorder of motor neurons, abnormalities must be present in multiple muscles with different nerve root and peripheral nerve innervation in multiple limbs 3