Foix-Alajouanine syndrome (FAS), also known as congestive myelopathy due to spinal vascular malformations, presents with paraplegia, sensory disturbance of lower limbs, and dysfunction of the bladder and rectum. Although FAS is characterized by a subacute onset of neurological symptoms that may wax and wane over a few years, the progression mechanism remains unclear. We report a case of FAS due to an angiographically occult arteriovenous fistula (AVF) that was diagnosed by an open surgical biopsy and intraoperative indocyanine green (ICG) angiography. The patient was a 74-year-old female who presented with a one-year history of gradually progressive gait disturbance, weakness, and decreased sensation in her legs associated with bladder and rectum dysfunction. MRI showed intramedullary T1 hypointensity, T2 hyperintensity at level Th4-12, and intramedullary enhancing with a Gd-DTPA lesion at level Th8-12. A true-FISP image of the MRI revealed an abnormal tortuous vessel in the dorsal spinal subarachnoid space, but digital subtraction angiography of the spine at the C1-L5 level showed no abnormality. The patient also underwent Th8-12 laminectomy for open biopsy. ICG angiography revealed blood flow stagnation in an abnormally enlarged posterior spinal vein. Histological findings indicated necrotizing myelopathy and stenosis with wall thickening of the posterior spinal vein. We hypothesized that the progression mechanism in the present case may have resulted from histological changes in the draining veins of an AVF. Intraoperative ICG angiography may be a valuable method, not only for diagnosing AVFs but also for determining the existence and pathological characteristics of FAS.