To report a patient with Charcot-Marrie-Tooth type 1F (CMT1F) presenting with indistinguishable clinical, electrodiagnostic, and ultrasonographic features from chronic inflammatory demyelinating polyradiculoneuropathy (CIDP).

The NEFL gene encodes the neurofilament light chain protein, and different mutations of the gene can cause different hereditary neuropathy phenotypes, including primary demyelinating (CMT1F), primary axonal (CMT2E), and dominant intermediate form (DI-CMT type G).

A 56-year-old man with a past medical history of type 2 diabetes mellitus presented with insidious onset and progressive weakness and numbness for about five years. There was no family history of neuropathy. Neurological examination showed asymmetric, and distal more than proximal weakness in the upper and lower limbs. Deep tendon reflexes were reduced or absent throughout. Sensory evaluation revealed diminished sensory responses distally in all modalities. Electrodiagnostic study revealed primary demyelinating polyneuropathy with severe axonal loss, based on abnormal temporal dispersion within the forearm segment of bilateral median nerves, diffusely absent sensory nerve action potentials (SNAPs), and absent tibial and peroneal compound muscle action potentials (CMAPs). Nerve ultrasound showed focal enlargement of the right median nerve and brachial plexus at non-entrapment sites. The CSF study showed albuminocytologic dissociation (protein 154 mg/dL with zero total nucleated cells). The patient met the 2021 EAN/PNS diagnostic criteria of CIDP but did not show any improvement despite receiving three courses of Intravenous immunoglobulin. The sural nerve biopsy showed severe loss of large and small myelinated axons. No significant inflammation, onion bulb formations, or giant axons were identified.  Eventually his genetic test showed a heterozygous pathogenic mutation in the NEFL gene (c.1319C>T (p.Pro 440 Leu)), which can cause CMT1F. 

CMT disease can occasionally be misdiagnosed as CIDP, and associated mutations are most commonly found in PMP22 and MPZ genes. Genetic testing should be considered, especially in treatment refractory CIDP.