Introduction/Aims Progressive axonal loss in multifocal motor neuropathy (MMN) is often assessed with nerve conduction studies (NCS), by recording maximum compound muscle action potentials (CMAPs). However, reinnervation maintains the CMAP amplitude until a significant portion of the motor unit (MU) pool is lost. Therefore, we performed more informative CMAP scans to study MU characteristics in a large cohort of patients with MMN. Methods We derived the maximum CMAP amplitude (CMAPmax), an MU number estimate (MUNE), and the largest MU amplitude stimulus current required to elicit 5%, 50%, and 95% of CMAPmax (S5, S50, S95) and relative ranges ([S95 − S5] × 100 / S50) from the scans. These metrics were compared with clinical, laboratory, and NCS results. Results Forty MMN patients and 24 healthy controls were included in the study. CMAPmax and MUNE were reduced in MMN patients (both P < .001). Largest MU amplitude as a percentage of CMAPmax was increased in MMN patients (P < .001). Disease duration and treatment duration were not associated with MUNE. Relative range was larger in patients with anti‐GM1 antibodies than in those without anti‐GM1 antibodies (P = .016) and controls (P < .001). The largest MU amplitudes were larger in patients without anti‐GM1 antibodies than in patients with anti‐GM1 antibodies (P = .037) and controls (P = .044). Discussion We found that MU loss is common in MMN and accompanied by enlarged MUs. Presence of anti‐GM1 antibodies was associated with increased relative range of MU thresholds and reduction in largest MU amplitude. Our findings indicate that CMAP scans complement routine NCS, and may have potential for practical monitoring of treatment efficacy and disease progression.

No abstract

Background and purposeThe lack of reliable early biomarkers still causes substantial diagnostic delays in amyotrophic lateral sclerosis (ALS). The aim was to assess the diagnostic accuracy of a novel electrophysiological protocol in patients with suspected motor neuron disease (MND).MethodsConsecutive patients with suspected MND were prospectively recruited at our tertiary referral centre for MND in Utrecht, The Netherlands. Procedures were performed in accordance with the Standards for Reporting of Diagnostic Accuracy. In addition to the standard diagnostic workup, an electrophysiological protocol of compound muscle action potential (CMAP) scans and nerve excitability tests was performed on patients' thenar muscles. The combined diagnostic yield of nerve excitability and CMAP scan based motor unit number estimation was compared to the Awaji and Gold Coast criteria and their added value was determined.ResultsIn all, 153 ALS or progressive muscular atrophy patients, 63 disease controls and 43 healthy controls were included. Our electrophysiological protocol had high diagnostic accuracy (area under the curve [AUC] 0.85, 95% confidence interval [95% CI] 0.80–0.90), even in muscles with undetectable axon loss (AUC 0.78, 95% CI 0.70–0.85) and in bulbar‐onset patients (AUC 0.85, 95% CI 0.73–0.95). Twenty‐four of 33 (73%) ALS patients who could not be diagnosed during the same visit were correctly identified, as well as 8/13 (62%) ALS patients not meeting the Gold Coast criteria and 49/59 (83%) ALS patients not meeting the Awaji criteria during this first visit.ConclusionsOur practical and non‐invasive electrophysiological protocol may improve early diagnosis in clinically challenging patients with suspected ALS. Routine incorporation may boost early diagnosis, enhance patient selection and generate baseline measures for clinical trials.