Introduction: Antiphospholipid syndrome is an autoimmune disease that causes thrombosis and is diagnosed in the laboratory with elevation of lupus anticoagulant ratio.

Case report: A 28-year-old male with no relevant medical history presented with right foot drop. Brain magnetic resonance imaging revealed a left frontal cortical lesion suggestive of vasculitis or low-grade glioma. Physical examination showed distal plegia of the right lower limb, preventing ambulation. Neurology and Rapid Diagnostic Unit of Internal Medicine ruled out underlying causes. Negative tests included blood work on three occasions with serologies and tumor markers, all types of lupus anticoagulant, complete cerebrospinal fluid analysis on three occasions (including PCR for microorganisms and flow cytometry), chest-abdomen CT scan, Positron Emission Tomography (PET), PET-dopamine, and cerebral arteriography. A follow-up MRI at three weeks revealed new lesions, leading to a biopsy with normal brain parenchyma on pathology. The patient showed radiological progression of lesions. A second opinion was sought, recommending a new biopsy. Pre-surgical MRI highlighted resolution of the lesions without treatment. Ultimately, lupus anticoagulant (dRVVT-S) was detected in two tests with a ratio of 1.25 (normal: ≤ 1.2), leading to a diagnosis of antiphospholipid syndrome. After initiating anticoagulant therapy with acenocoumarol, the patient experienced symptom improvement with independent walking.

Discussion: Antiphospholipid syndrome can manifest as an intracranial space-occupying lesion and sudden neurological focalization with lupus anticoagulant near the normal limit.