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Prolonged distal compound muscle action potential duration (DCMAPD) can be helpful in diagnosing demyelinating neuropathies.[1, 2] However, it is not known whether this measure is specific for identifying demyelination when motor axonal loss is present. It is important to avoid misdiagnosing an axonal neuropathy as one having demyelinating features.

We reviewed retrospectively electrophysiological records of 80 patients with chronic axonal neuropathies with low compound muscle action potentials (CMAPs), 9 of whom had been included in our previous study.[1] All had a clinical diagnosis of sensorimotor or sensory polyneuropathy. None fulfilled European Federation of Neurological Societies/Peripheral Nerve Society (EFNS/PNS) clinical criteria for typical chronic inflammatory demyelinating polyneuropathy (CIDP) or EFNS/PNS electrodiagnostic criteria for probable or definite CIDP, excluding consideration of DCMAPD.[3] Motor conduction velocity for a single lower limb nerve of <30 m/s was present in 5/80 (6.25%). None had upper limb motor conduction velocities <40 m/s. None had prolongation of distal motor latencies >150% of the upper limit of normal, although mild prolongation (120% to 150% of the upper limit of normal) was present for 1 lower limb nerve in 8/80 (10%). None had nerve entrapments. This study was part of a retrospective analysis of neurophysiological findings in patients with neuropathy for which Ethics Committee approval was not required, as per our institutions.

All subjects had undergone study of at least 3 motor nerves, including at least 1 lower limb nerve. All were selected because of at least 1 reduced lower limb CMAP amplitude (fibular CMAP<1.5 mV or tibial CMAP<3 mV) and low amplitude or absent lower limb sensory potentials. Electrophysiology had been performed using percutaneous supramaximal stimulation and surface electrode recording at limb temperatures ≥32°C, with low frequency filter settings of 2–3 Hz.

There were 48 men and 32 women. Mean age was 66.3 years. Mean fibular CMAP amplitude was 0.71 mV (range: 0.1–1.4 mV; SD [standard deviation]: 0.4) and mean tibial CMAP amplitude was 1.15 mV (range: 0.1–2.9 mV; SD: 0.87). Mean DCMAPD was 5.52 ms (SD: 1.31) for the fibular nerve and 5.92 ms (SD: 1.49) for the tibial nerve. DCMAPD prolongation indicative of demyelinating neuropathy[1] (>9.1 ms) was present in 1/38 fibular nerves, yielding a specificity of 97.4% for demyelinating versus axonal neuropathies with low fibular CMAPs. Similarly only 2/61 tibial nerves showed significant DCAMPD prolongation[1] (>8.7 ms), indicating a specificity of 96.7% for demyelinating versus axonal neuropathy with low tibial CMAPs. Thus, 3/80 patients had demyelinating-range DCMAPD prolongation for any fibular or tibial nerve study with reduced CMAP amplitude, corresponding to a global specificity of 96.25%. None of these 3 had lower limb motor conduction velocities <30 m/s.

These findings further justify the use of DCMAPD prolongation as a marker of demyelination at the cutoffs derived with the filter settings used.[1] Although we included consecutive subjects with axonal neuropathy, some of whom had mild/isolated demyelinating features, this did not adversely affect the specificity of DCMAPD prolongation. Although cutoffs for other demyelinating parameters depend on the degree of CMAP reduction,[3] this does not appear to be necessary for DCMAPD prolongation. However, more studies are required to confirm this, particularly, with severely reduced CMAP amplitudes due to axonopathy or motor neuronopathy.

  • Yusuf A. Rajabally, MD1,2

  • Darren Martin-Lamb, MSc2

  • Guillaume Nicolas, MD3,4

  • 1Neuromuscular Clinic, Department of Neurology, University Hospitals of Leicester, United Kingdom

  • 2Neuromuscular Clinic, Department of Neurophysiology, University Hospitals of Leicester, United Kingdom

  • 3Centre Hospitalier Universitaire d'Angers, Angers, France

  • 4Hôpital Raymond-Poincaré (AP-HP), Université Versailles-Saint-Quentin-en-Yvelines, France