Motor Predominant Neuropathy is a Large Fiber Neuropathy
Motor predominant neuropathy primarily affects large nerve fibers, as it involves the motor axons which are myelinated large fibers. 1
Characteristics of Motor Predominant Neuropathy
- Motor predominant neuropathy affects large myelinated motor fibers, causing weakness, atrophy, and reduced deep tendon reflexes 2
- Clinical presentations typically include distal weakness, muscle atrophy, and hyporeflexia without significant sensory involvement 2
- In severe cases, patients may present with distinct physical deformities such as hollow foot and stork legs 1
- Motor impairment is often associated with reduction or absence of deep tendon reflexes, distal weakness, atrophy of small feet muscles, tremor, and cramps 1
Differentiating Large vs Small Fiber Neuropathies
Large Fiber Neuropathy
- Affects myelinated Aα and Aβ fibers (large diameter) 1
- Motor predominant neuropathies fall into this category as they affect large myelinated motor axons 2, 3
- In sensory involvement, affects proprioception, vibration sense, and light touch 1, 4
- Electrophysiological studies show abnormalities in nerve conduction studies due to involvement of myelinated fibers 1
- Manifests with weakness, muscle atrophy, and loss of deep tendon reflexes 2
Small Fiber Neuropathy
- Affects unmyelinated C and thinly myelinated Aδ fibers 1, 5
- Primarily involves nerve terminals responsible for temperature and pain perception 1
- Presents with painful sensations like burning feet/hands and lancinating pain 1
- Clinical examination reveals decreased pain perception and temperature sensation 1
- May include autonomic symptoms (postural hypotension, bladder disturbances, reduced heart rate variability) 1, 5
- Often has normal nerve conduction studies as these primarily test large fiber function 5
Diagnostic Approaches
- Conventional neurophysiological methods (EMG with nerve conduction studies) are useful for diagnosing motor predominant neuropathies as they assess large fiber function 1
- Motor neuropathies show reduction of compound muscle action potential (CMAP) amplitude correlating with axonal damage in EMG 1
- For motor predominant neuropathy, electrophysiological studies reveal motor conduction blocks at non-compressible sites 3
- In multifocal motor neuropathy, a specific type of motor predominant neuropathy, peripheral nerve MRI and neuromuscular ultrasound often show non-uniform enlargement of nerve roots, plexuses, or peripheral nerve segments 3
Clinical Implications
- Recognizing motor predominant neuropathy as a large fiber neuropathy is crucial for appropriate diagnostic workup 2, 3
- Motor neuropathies can sometimes be difficult to differentiate from motor neuron diseases like ALS, making accurate classification essential 2
- Some immune-mediated motor neuropathies (like multifocal motor neuropathy) are treatable with immunotherapy, highlighting the importance of correct identification 2, 3
- Relying solely on sensory symptoms or pain characteristics may lead to misdiagnosis, as motor predominant neuropathies primarily affect strength and muscle function rather than sensation 2
Common Pitfalls
- Assuming all neuropathies with motor symptoms have sensory involvement 2
- Misdiagnosing motor predominant neuropathy as a motor neuron disease like ALS 2
- Failing to recognize that some motor neuropathies may have minimal sensory symptoms that don't define the condition 3
- Overlooking treatable causes of motor predominant neuropathy, such as immune-mediated conditions 2