Marieke van Rosmalen

General discussion 147 8 Laboratory findings or electrophysiology to monitor disease course might be more appropriate options to explore. For example, serum neurofilament light chain (sNfL) has been studied as a biomarker in various neurological disorders, such as multiple sclerosis, SMA and ALS. 97 sNfL is released into the cerebrospinal fluid and into the blood as a subunit of a cytoskeletal protein in case of neuroaxonal damage. Two exploratory studies have been performed on the prognostic value of sNfL in disease course of patients with CIDP and MMN. The first study included patients with various acquired polyneuropathies (n = 25, from which 12 patients with CIDP and 3 with MMN) and found increased levels of sNfL in patients with a polyneuropathy, correlating to disease activity measured with the overall neuropathy limitations scale. 98,99 As this study included various polyneuropathies the results could not be translated to a prognostic value for CIDP and MMN. A recent study that included patients with CIDP (n = 80) and healthy controls correlated sNfL levels to NCS results and found a correlation between sNfL and axonal damage. 100 Unfortunately, not all patients in this study underwent NCS and the found correlation was not significant when corrected for age. They also found that patients with an active disease (i.e. patients who did not respond to treatment and patients who relapsed) had higher sNfL levels compared to patients with stable disease (i.e. patients who responded to treatment and patients who were successfully withdrawn) which is promising for future studies. Future studies could include more time points of serum collection, repeated NCS and neurological examination, and eventually the use of a validated scale for impairment such as the inflammatory Rasch-built overall disability scale. 101 Another quantitative technique that might be of interest for monitoring disease course in CIDP and MMN is the compound muscle action potential (CMAP) scan. A CMAP scan plots the response size of motor units against stimulus intensity which results in a visual assessment of one entire muscle. 102 A CMAP scan can be performed in the large muscles of the arms and legs as well as in the small muscles of the hands and feet. This makes this technique of interest for patients with CIDP and MMN. The CMAP scan is rarely used in electrodiagnosis in current clinical practice but is increasingly used in scientific research. For example, the CMAP scan has been explored in patients with ALS (n = 10) and PMA (n = 3) as a potential biomarker to monitor disease progression. 103 They correlated a CMAP scan-based progression score to motor unit number estimation, which provides an estimate of the number of functional motor units, and found a significant correlation. They concluded that the CMAP scan is able to quantify disease progression in muscles affected by MND, but unfortunately did not report correlations with clinical data. Recently, the first CMAP scan study results in patients with CIDP (n = 16) have been reported in a prospective study. 104 This study showed that some CMAP scan parameters of the abductor pollicis brevis (i.e. step number and step percentage) decreased after 6 months of treatment, which was negatively correlated to pinch power, handgrip strength and upper MRC sum scores. Future studies should validate these results and should include more CMAP scan examinations during treatment (e.g. before and after a dose of immunoglobulins).