Infants with an immobile arm may be easily overlooked in primary care settings. Differential diagnoses include injuries, infections, neuropathies, ischemia and rarely, neoplasms. We report the case of a one-year-old boy with weakness in his left arm after minor trauma with a diagnosis of brachial plexus palsy initially. After rehabilitation for 2. months, his weakness progressed to unsteady gait and quadriparesis. MRI revealed a huge solid tumor in the left supraclavicular fossa, which also involved the left brachial plexus, upper thoracic cavity, and left paravertebral space with invasion into the spinal canal. Microscopically, the medium-large polygonal tumor cells had an eccentric eosinophilic cytoplasm and immunostaining showed a loss of nuclear INI1 expression. Array comparative genomic hybridization of the tumor tissue confirmed a segmental deletion at chromosome region 22q11.23 involving the SMARCB1 gene. The final diagnosis was cervical paravertebral malignant rhabdoid tumor with intraspinal epidural and intradural invasion, a rare case of extrarenal extracranial rhabdoid tumor (ERRT). The intraspinal part of the tumor was resected followed by interval-compressed chemotherapy with vincristine-doxorubicin-cyclophosphamide alternating with ifosfamide-etoposide (VDC/IE). The tumor showed very good partial response to four cycles of chemotherapy with gradual recovery of neurological symptoms. ERRT is a very rare and aggressive tumor that mainly occurs in infants and children and may manifest with vague neurological symptoms when it involves the spinal cord and/or peripheral nerves. A neoplasm such as ERRT originating from or involving the brachial plexus should be considered in the differential diagnosis of an immobile arm in infancy.
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