Introduction We aimed to report a rare case of chronic inflammatory demyelinating polyneuropathy (CIDP) presenting as focal lower limb amyotrophy.

Case A 65 year old man presented with a 12 month history of painless, left anterior thigh wasting. Examination revealed marked left quadriceps and mild left hamstring and tibial wasting (Image 1). He had MRC grade 4/5 left hip flexion and 3/5 knee extension weakness, with normal power elsewhere. His left knee jerk was absent. Remaining reflexes were retained. There was reduced left L5 pinprick and light touch sensation. His neurological exam was otherwise normal. Blood tests, including creatine kinase, blood glucose, and autoimmune panel, were normal. Anti-neuronal, anti-ganglioside and anti-MAG antibodies were negative. Trace IgM lambda para-proteinaemia was detected. Cerebrospinal fluid was unremarkable [protein 0.37 g/L (n<0.45), glucose 4.1 mmol/L (2.5–4.5), leukocytes 0×10⁶/L]. Nerve conduction studies showed normal lower limb motor velocities with reduced left peroneal motor amplitudes and mildly prolonged sural sensory latencies bilaterally. Electromyography showed chronic denervation of bilateral L3-S1 innervated muscles without fibrillation potentials. Posterior tibial somatosensory evoked potential studies demonstrated absent lumbar responses and prolonged cortical latencies. Muscle biopsy confirmed chronic denervation without evidence of myopathy or vasculitis. The patient declined sural nerve biopsy. MRI revealed bilateral multifocal lumbosacral plexopathy with increased T2 signal and thickening of multiple nerves, including femoral, lateral femoral cutaneous and L5 nerve roots (Image 2). CT chest, abdomen and pelvis, and whole body PET scan were normal. High-dose oral prednisone and induction intravenous immunoglobulin (IVIG) therapy followed by 4-weekly maintenance IVIG and low-dose prednisolone were commenced for a presumed diagnosis of CIDP.

Conclusion In this patient, the presence of multifocal nerve thickening on MRI was consistent with an inflammatory neuropathy. The normal PET scan and CSF studies made other diagnoses, such as neuro-lymphoma, less likely. CIDP should be considered in atypical presentations such as focal amyotrophy.