Axonal Loss and Demyelination: The Twin Pillars of Neuropathic Lesions
In-Depth Examination of Neuropathic Lesions
Neuropathic lesions are disruptions in the peripheral nervous system that can significantly affect motor and sensory function. These lesions are categorized based on the area of the nervous system they affect (e.g., polyneuropathy, plexopathy, radiculopathy, mononeuropathy) and the type of damage incurred (axonal loss vs. demyelination). Understanding these distinctions is critical for diagnosis and treatment planning. Central to understanding these lesions is recognizing two primary types of damage:
- Axonal Loss: Damage to the nerve fiber itself.
- Demyelination: Damage to the myelin sheath that surrounds the nerve fiber.
Both types significantly impact nerve function, but they present distinct patterns in nerve conduction studies (NCSs) and electromyography (EMG).
Axonal Loss Lesions: A Chronological Perspective
Axonal loss refers to direct damage to the nerve fiber, leading to a sequence of changes observable through NCSs and EMG:
1. Immediate Response: Clinical symptoms like weakness and numbness appear post-injury. Initially, NCSs may appear normal because wallerian degeneration hasn’t started.
2. Wallerian Degeneration Timing: This process begins 3–5 days after injury for motor fibers and 6–10 days for sensory fibers. NCSs then show reduced amplitudes, but conduction velocities (CVs) and distal latencies (DLs) are relatively preserved (i.e., CV >75% of lower limit of normal; DL <130% of upper limit of normal).
3. EMG Findings: Early after injury, decreased recruitment of motor unit action potentials (MUAPs) in affected muscles is observed. Fibrillation potentials and positive sharp waves — a sign of denervation — take varying times to develop depending on the muscle’s distance from the lesion:
— Paraspinal muscles: 10–14 days
— Proximal thigh: 2–3 weeks
— Leg: 3–4 weeks
— Distal leg and foot: 5–6 weeks
4. Chronic Stage Reinnervation: MUAPs become longer, higher in amplitude, and polyphasic, indicating muscle fiber regeneration, a process taking several months.
Demyelinating Lesions: Distinctive Features
Demyelination affects the myelin sheath, essential for rapid nerve conduction. Key points include:
1. Conduction Velocity Slowing: The hallmark of demyelination, resulting in prolonged DLs.
2. Conduction Block: Severe demyelination leads to sensory loss and weakness due to motor and sensory fiber blockade.
3. Diagnostic Significance: Conduction block helps differentiate between demyelination due to conditions like Guillain-Barré syndrome or CIDP and inherited neuropathies, which show uniform slowing without blockage.
The Takeaway
Understanding neuropathic lesions through the lens of NCS and EMG is crucial for diagnosis and management. This guide highlights the importance of recognizing the distinct patterns of axonal loss and demyelination, providing a foundation for interpreting complex nerve studies.
📒 Compiled by — Sigrid Chen, Rehabilitation Medicine Resident Physician, Occupational Therapist, Personal Trainer of the American College of Sports Medicine.