Is There Demyelination in Critical Illness Myopathy (CIM)?
No, demyelination is not a primary feature of critical illness myopathy (CIM)—this condition is characterized by axonal injury and myopathic changes, not demyelination. 1, 2
Pathophysiological Characteristics of CIM
Critical illness polyneuromyopathy (CIPNM) is fundamentally an acute axonal sensory-motor polyneuropathy, not a demyelinating disorder. 3 The key pathological features include:
- Axonal degeneration affecting peripheral nerves, which distinguishes this from demyelinating polyneuropathies like Guillain-Barré syndrome 1, 3
- Myonecrosis and muscle fiber changes, including extensive type 2 fiber atrophy, disarray of sarcomere architecture, and selective loss of myosin 2
- Preserved myelin sheaths in nerve conduction studies, with the primary abnormality being reduced compound motor action potential (CMAP) amplitudes rather than prolonged latencies 2
Electrophysiological Distinction from Demyelinating Disorders
The electrophysiological profile of CIM clearly differentiates it from demyelinating conditions:
- Normal or near-normal nerve conduction latencies (6-8 milliseconds for phrenic nerve/diaphragm in adults), which would be prolonged in demyelinating polyneuropathies 1
- Severely reduced CMAP amplitudes indicating axonal loss rather than demyelination 2
- Phrenic nerve/diaphragm latencies are abnormally slow in demyelinating polyneuropathies like Guillain-Barré syndrome, but this is not the pattern seen in CIPNM 1
Clinical Implications
Understanding that CIM is an axonal/myopathic process rather than a demyelinating one has important clinical implications:
- The prognosis differs significantly—axonal injury typically requires longer recovery periods with potential for incomplete recovery, whereas demyelinating disorders may show faster recovery once remyelination occurs 1, 2
- Severe axonal critical care neuropathies result in prolonged periods of convalescence and incomplete recovery 1
- Electrophysiological studies can distinguish between the axonal pattern of critical illness polyneuropathy and the myopathic patterns, both of which lack demyelination 2
Common Diagnostic Pitfall
A critical pitfall is confusing CIPNM with Guillain-Barré syndrome or other demyelinating polyneuropathies based solely on clinical weakness. 1 The key distinguishing features are:
- Preserved sensory function in CIPNM versus sensory involvement in many demyelinating neuropathies 2
- Context of critical illness with sepsis, prolonged mechanical ventilation, and exposure to neuromuscular blocking agents or corticosteroids 1
- Electrophysiological findings showing axonal loss (reduced CMAP amplitudes) with normal conduction velocities, rather than the slowed conduction velocities characteristic of demyelination 1, 2