Terminal latency index in polyneuropathy with IgM paraproteinemia and anti-MAG antibody

Criteria for the diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP) are met by the polyneuropathy associated with immunoglobulin M (IgM) paraproteinemia and anti-myelin-associated glycoprotein (MAG) antibody (MAG-CIDP). However, MAG-CIDP differs from other types of CIDP, mainly in its poorer response to treatment. The utility of terminal latency index (TLI) as an electrophysiological marker for MAG-CIDP has been debated. In this study we confirmed its diagnostic usefulness and evaluated TLI threshold values for motor nerves investigated in routine nerve conduction studies. Median, ulnar, peroneal, and tibial TLIs of 11 subjects with MAG-CIDP, 18 with CIDP, and 76 healthy controls were compared, and threshold values for MAG-CIDP evaluated as the lowest value with a likelihood ratio higher than 10. Mean TLI values and TLIs of all but the peroneal nerve were significantly lower in MAG-CIDP. Median nerve TLI of 0.26 and ulnar nerve TLI of 0.33 were identified as the threshold TLI values for MAG-CIDP.