CMT is a heterogenous group of inherited sensorimotor polyneuropathies for which the current treatment is supportive. Interestingly, there is a subset of patients described in the literature, who develop a stepwise decline in function and are found to have coinciding chronic immune demyelinating polyradiculoneuropathy (CIDP). It is postulated that some CMT patients may be more susceptible to immune-mediated demyelination due to existing deficits in myelin structure, triggering a heightened immune response. This can create a diagnostic dilemma, especially in patients with more advanced CMT, in whom nerve conduction studies may not provide differentiation between the two conditions.

We describe two cases of patients with slowly progressive CMT1A who developed a subacute decline of sensory and motor function, accompanied by new objective worsening on exam. Repeat EMG/NCS showed interval development of conduction block at non-compressive sites.  Neuromuscular ultrasound revealed focal enlargement at these non-compressible sites. Both patients were diagnosed with superimposed CIDP and improved back to their prior baseline with treatment using intravenous immunoglobulin.

Acute or subacute stepwise decline, proximal weakness and asymmetry in patients with demyelinating CMT should raise concern for development of an acquired demyelinating neuropathy that may respond to immunotherapy. The diagnosis of CIDP can be supported by albuminocytologic dissociation in cerebrospinal fluid, electrodiagnostic features on NCS and features on peripheral nerve ultrasound, inflammatory changes on nerve biopsy and response to treatment. Peripheral nerve ultrasound is non-invasive and can be of diagnostic utility in cases where the electrodiagnostic findings of acquired demyelination are masked by chronic hereditary changes.