When Flow Voids Are Missed: Delayed Diagnosis of Foix–Alajouanine Syndrome—A Case Report

Introduction: Foix–Alajouanine syndrome (FAS) is a rare, progressive congestive ischaemic myelopathy caused by a spinal dural arteriovenous fistula (SDAVF). Because of its non-specific clinical presentation and subtle imaging features, FAS is frequently misdiagnosed; in particular, perimedullary flow voids—its characteristic imaging hallmark—are readily overlooked on initial MRI, leading to diagnostic delay and potentially harmful interventions. Case Presentation: We report the case of a 66-year-old man who developed progressive myelopathy following revision knee arthroplasty performed under spinal anaesthesia. Initial MRI at another institution demonstrated extensive intramedullary T2 hyperintensity, interpreted as spinal cord infarction, while perimedullary flow voids were present but overlooked. Lumbar puncture and high-dose corticosteroid therapy were followed by marked neurological deterioration. At our centre, re-evaluation of a 3T MRI revealed serpentine perimedullary flow voids, and digital subtraction angiography confirmed an SDAVF at the Th5–Th6 level. Outcome: Transarterial endovascular embolisation with Onyx resulted in neurological stabilisation and partial recovery, with reduced pain and improved bladder control at follow-up. Conclusions: This case highlights the importance of recognising perimedullary flow voids, which may otherwise lead to delayed diagnosis of SDAVF. In patients with unexplained longitudinally extensive myelopathy, particularly when the clinical course is atypical or worsens after lumbar puncture or corticosteroid therapy, the MRI should be carefully reviewed for flow voids and dedicated spinal vascular imaging considered.