脳表ヘモジデリン沈着症の診断と治療

Diagnosis and Treatment of Superficial Siderosis

はじめに

　脳表ヘモジデリン沈着症（superficial siderosis：SS）は，感音性難聴，小脳性運動失調，錐体路徴候，認知機能障害などを主徴とし，主に中年以降に発症し，脳表や脊髄表面にヘモジデリンの沈着がみられる原因不明の進行性疾患である¹⁾。MRIの登場以前は，剖検や外科手術により初めて診断されていたが，MRIの登場により生前診断が可能となっている。外傷や出血性疾患後など原因が明らかなもの以外は半数近くが原因不明とされていたが，原因不明のものの多くに脊髄での髄膜貯留がみられ，「硬膜疾患」の1つと考えられるようになってきた。また，小脳，脳幹といった「古典的」な部位にヘモジデリン沈着がみられる例（以下，古典型SS）以外に，大脳皮質のごく一部のみにヘモジデリン沈着がみられる例（以下，限局型SS）が少なからず存在し，脳アミロイド血管症（cerebral amyloid angiopathy：CAA）や，アルツハイマー型認知症との関連も議論されており，疾患概念も変わりつつある。本稿では，これらの最近の知見を含め主として臨床的側面から解説する。

Abstract

　Superficial siderosis (SS) is a rare condition in which hemosiderin is deposited on the pial surface of the brain and/or spinal cord. Hemosiderin deposition is the consequence of recurrent or persistent hemorrhage in the subarachnoid space. There are two types of SS. In "classical"-type SS, hypointense MRI signals are observed in the brainstem and cerebellum with diffuse and symmetrical margins. Causes of hemorrhage in the "classical" type include tumor, vascular abnormality, injury, and dural defect. The source of hemorrhage is not apparent in approximately 50% of patients despite extensive examination. In "localized"-type SS, hypointense MRI signals are localized in the cerebral cortex. The most common causes of hemorrhage in the "localized" type are cerebral amyloid angiopathy and/or Alzheimer's disease. Patients with SS usually present with slowly progressive and irreversible cerebellar ataxia, sensorineural hearing loss, and/or myelopathy due to involvement of the acoustic nerve, cerebellum, and spinal cord. T₂-weighted imaging (WI) or T₂^*WI demonstrates characteristic linear low-intensity signals along the surface of the brain and spinal cord. Treatment of SS involves identification and surgical correction of the bleeding source. Deferiprone, which is a lipid-soluble iron chelator that can penetrate the blood-brain barrier, is reportedly effective at improving the clinical symptoms and deposition of hemosiderin. It is thus a hopeful treatment option for SS.