Congenital Hypoplasia of the C1 Posterior Arch Masquerading as Multiple Sclerosis: A Case Report

Hypoplasia of the C1 posterior arch is a rare congenital anomaly, classified using the Currarino system into subtypes A through E depending on the extent and nature of arch deficiency. The majority of cases are incidental radiological findings; symptomatic presentations causing cervical myelopathy are exceptionally uncommon and not well represented in the surgical literature. In such cases, the absence of established management guidelines renders clinical decision-making particularly challenging. We present a case of a 45-year-old male surgeon who presented with a one-year history of episodic bilateral upper and lower limb weakness. Initial investigations, including brain and cervical spine MRI, were interpreted as consistent with a diagnosis of multiple sclerosis, for which treatment was commenced. Following a symptom-free interval after a one-month course of corticosteroids, the patient experienced a recurrence at eight months, prompting dynamic MRI of the cervical spine, which revealed a type C hypoplastic C1 posterior arch causing dynamic spinal cord compression during hyperextension. The diagnosis was revised accordingly. Supplementary investigations confirmed atlantoaxial stability and normal vertebral artery anatomy bilaterally. The patient provided consent and underwent posterior surgical decompression of the entire posterior arch, without instrumented fusion. Intraoperative neurophysiological monitoring guided the extent of resection; decompression began on the fibrous left-sided arch, extending to the level of the vertebral artery groove, followed by the bony right-sided arch, sequenced to preserve facet joint integrity. The patient achieved complete symptom resolution postoperatively. This case highlights the diagnostic challenge posed by symptomatic C1 arch hypoplasia, which may mimic demyelinating disease, and demonstrates that surgical decompression, guided by intraoperative neurophysiological monitoring and tailored to arch morphology, can achieve excellent neurological outcomes without the need for instrumented stabilisation.