Sinking skin flap syndrome masquerading as Chiari malformation: failed syringomyelia treatment rescued by cranioplasty. Illustrative case.

backgroundSyringomyelia is most often associated with Chiari malformation, for which posterior fossa decompression is the standard treatment. Treatment failure should raise suspicion for alternative mechanisms, particularly in patients with a history of supratentorial craniectomy. In such cases, sinking skin flap syndrome (SSFS) may alter intracranial-extracranial pressure gradients, impair CSF dynamics, and result in an acquired Chiari malformation with secondary syrinx formation. OBSERVATIONS: A 64-year-old man with a remote supratentorial decompressive craniectomy for intracerebral hemorrhage presented with progressive headache, neck pain, and bilateral arm paresthesia. MRI revealed tonsillar herniation with syringomyelia. Posterior fossa decompression led to initial symptom relief and reduction of the syrinx. Three months later, he developed new bilateral leg sensory deficits, and follow-up MRI showed syrinx recurrence despite adequate decompression and no arachnoid adhesions. Because of prior cranioplasty-related infection, he hesitated to undergo further surgery, but ultimately cranioplasty was performed. Postoperatively, symptoms improved, with sustained clinical recovery and syrinx regression confirmed on serial MRI at 12 and 24 months. LESSONS: Persistent or recurrent syringomyelia after Chiari decompression should prompt evaluation for acquired causes. SSFS may mimic primary Chiari malformation; in such cases, cranioplasty can restore CSF dynamics and achieve durable syrinx resolution. https://thejns.org/doi/10.3171/CASE25763.