Extensive Cervicothoracic Syringomyelia Associated With Borderline Tonsillar Descent: A Case Report of Posterior Fossa Decompression

Chiari malformation type I is a structural disorder characterised by herniation of the cerebellar tonsils through the foramen magnum, frequently associated with syringomyelia.Although a descent of at least 5 mm is conventionally required for diagnosis, lesser degrees may be clinically significant when cerebrospinal fluid dynamics at the craniocervical junction are disturbed.A 40-year-old female patient presented with cervical and left shoulder pain persisting for two months, without motor or sensory deficit.Cervical magnetic resonance imaging revealed a crowded posterior fossa, tonsillar herniation of only 4.5 mm below the basion-opisthion line, reduced subarachnoid spaces at the craniocervical junction, and a 130 mm hydrosyringomyelia cavity extending from the odontoid process to T2, 9×12 mm at its widest, with cord expansion and perisyrinx T2-weighted hyperintensity suggestive of chronic spinal cord injury.Surgery was indicated by the extent of the syrinx and the risk of irreversible cord injury, rather than pain alone.The patient underwent posterior fossa decompression consisting of suboccipital craniectomy, C1 posterior arch excision, and duraplasty.The early postoperative course was uneventful; a minor superficial wound problem was managed conservatively during follow-up.At 18 months her pain had improved, the neurological examination was unchanged, and serial imaging showed sustained decompression with syrinx regression.This case suggests that craniocervical morphology and cerebrospinal fluid flow, rather than the millimetric extent of tonsillar descent alone, should guide assessment and the decision to operate.