Methods: Compound muscle action potentials (CMAP) duration of 3 patients with POEMS syndrome and 2 with hyaline vascular type CD without clonal plasma cell dyscrasia were retrospectively analysed.

Results: Median and ulnar nerves distal CMAP duration were prolonged in all patients irrespective of plasma cell dyscrasia or M protein. All lower limbs distal CMAP responses were absent. Greatest distal CMAP duration prolongation was observed in median nerves for POEMS syndrome (17.0 ms, 158% upper limit normal) and in ulnar nerves for CD (9.8 ms, 47% upper limit normal). Distal/proximal CMAP duration ratio of <0.7 were seen in 33% of median and ulnar nerves studied among POEMS syndrome. Among nerves with ratio >0.7, all had distal CMAP duration prolongation (Range 7%-158% of upper limit normal).

Conclusions: Abnormal distal CMAP dispersion is not uncommon in POEMS syndrome and CD without clonal plasma cell dyscrasia or M protein. POEMS syndrome has greater distal CMAP duration in median and ulnar nerves, particularly in median nerve that can reach up to 150% of upper limit normal, compared to <50% in CD.