Different Distributions of Nerve Demyelination in Chronic Acquired Multifocal Polyneuropathies

Different Distributions of Nerve Demyelination in Chronic Acquired Multifocal Polyneuropathies

Multifocal motor neuropathy (MMN), Lewis-Sumner syndrome (LSS), and chronic inflammatory demyelinating polyradiculoneuropathy with conduction block (CIDP-CB) are representative of acquired multifocal polyneuropathies characterized by conduction block (CB). These conditions share some clinical and electrophysiological features, making differentiation challenging. This study aimed to investigate the demyelinating distribution and selective vulnerability of nerves in MMN, LSS, and CIDP-CB to provide insights for their differentiation.

The study included 15 LSS subjects (107 nerves), 24 MMN subjects (176 nerves), and 17 CIDP-CB subjects (110 nerves). Clinical information was recorded, and nerve conduction studies of the median, ulnar, radial, peroneal, and tibial nerves were evaluated. CB, temporal dispersion (TD), distal motor latency (DML), and F-wave latency were recorded, and nerve conduction velocity (NCV), terminal latency index (TLI), and modified F-wave ratio (MFR) were calculated.

Clinically, MMN and LSS showed male predominance, with 79.2% and 80.0% male subjects, respectively, compared to 64.7% in CIDP-CB. MMN had a longer disease duration (median 24 months) compared to LSS (10 months) and CIDP-CB (6 months). Upper limb onset was more common in MMN (66.7%) and LSS (66.7%) than in CIDP-CB (29.4%). Sensory involvement was less frequent in MMN (25.0%) than in LSS (53.3%) and CIDP-CB (70.6%). Cerebrospinal fluid (CSF) albumin protein levels were highest in CIDP-CB (995.2 mg/dL), followed by LSS (629.9 mg/dL) and MMN (398.5 mg/dL). Anti-GM1 antibodies were most frequently detected in MMN (75.0%), compared to LSS (13.3%) and CIDP-CB (11.8%).

The distribution of CB varied among the three conditions. In CIDP-CB, CB was more likely to occur around the elbow in the ulnar nerve (78.6%) compared to MMN (6.8%). In contrast, CB between the wrist and elbow in the ulnar nerve was more frequent in LSS (39.3%) than in CIDP-CB (10.7%). Tibial nerve CB was most frequently observed in MMN (47.4%), compared to LSS (20.0%) and CIDP-CB (16.0%). No significant differences were found in CB distribution in other nerve segments.

Electrophysiological findings revealed that DML was significantly prolonged in all nerves in CIDP-CB compared to MMN and LSS. The median nerve DML was 4.75 ms in CIDP-CB, compared to 3.67 ms in MMN and 3.4 ms in LSS. The ulnar nerve DML was 3.87 ms in CIDP-CB, compared to 2.75 ms in MMN and 2.71 ms in LSS. The TLI of the ulnar nerve was prolonged in LSS (0.40) compared to MMN (0.35) and CIDP-CB (0.36). When nerves with CB were excluded, upper limb NCV was significantly slower in CIDP-CB. The median nerve NCV was 44.45 m/s in CIDP-CB, compared to 54.8 m/s in MMN and 52.4 m/s in LSS. The ulnar nerve NCV was 51.3 m/s in CIDP-CB, compared to 57.25 m/s in MMN and 52.2 m/s in LSS. Lower limb compound muscle action potential (CMAP) amplitudes were smaller in CIDP-CB compared to MMN and LSS.

The study highlights the distinct distributions of segmental and diffuse demyelination in LSS, MMN, and CIDP-CB. The ulnar nerve was more frequently affected in LSS, particularly in the wrist-to-elbow segment, which is a non-entrapment site. This finding supports the hypothesis that forearm CB in the ulnar nerve is indicative of immune-mediated demyelinating neuropathy, particularly LSS. In contrast, CIDP-CB showed a higher prevalence of CB around the elbow in the ulnar nerve, a common entrapment site, suggesting direct inflammatory attack rather than compression-induced demyelination.

The frequent tibial nerve CB in MMN, despite the condition’s predominant upper limb involvement, suggests that the tibial nerve may be particularly vulnerable in MMN. This finding could aid in differentiating MMN from LSS and CIDP-CB, especially in cases with overlapping clinical features.

The prolonged DML in all nerves in CIDP-CB indicates distal vulnerability, a characteristic feature of CIDP. However, the similar TLI values across most nerves in the three conditions suggest that distal impairments may commensurate with middle segment impairments when CB is present. The slower upper limb NCV in CIDP-CB, even when CB nerves were excluded, further supports the diffuse demyelinating nature of CIDP-CB.

The mechanism underlying the selective vulnerability of nerves in these conditions remains unclear. Autoantibodies targeting paranodal proteins, such as neurofascin, have been implicated in demyelinating neuropathies with CB. The uneven distribution of these targets in nerves may contribute to the selective vulnerability observed in LSS, MMN, and CIDP-CB. Further research is needed to explore the immunological mechanisms driving nerve and segment selectivity in these conditions.

In conclusion, this study provides valuable insights into the different distributions of nerve demyelination in LSS, MMN, and CIDP-CB. The distinct patterns of segmental and diffuse demyelination, particularly in the ulnar and tibial nerves, can aid in differentiating these conditions. The findings underscore the importance of detailed electrophysiological evaluation in the diagnosis and management of chronic acquired multifocal polyneuropathies.

doi.org/10.1097/CM9.0000000000001073

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