Neurovascular compressionによる耳鳴・眩暈・温度眼振反応消失の症例

TINNITUS, VERTIGO AND LOSS OF CALORIC RESPONSE DUE TO NEUROVASCULAR COMPRESSION

　抄録　第8脳神経に対するmicrovascular decompression （MVD）を施行した症例を報告した。49歳男性で，7年来の聴力低下を伴わない耳鳴，眩暈を訴え，caloric testの反応消失があり，神経放射線学的に小脳橋角部腫瘍を疑われた。後頭手下開頭により，前下小脳動脈による第8脳神経へのneurovascularcompression （NVC）が発見され，MVDを行った。術後，直ちに耳鳴，眩暈は消失し，聴力低下等の合併症はなかった。第8脳神経のglia-schwann移行部は，第5，第7脳神経に比べ，脳幹近くのroot entryzoneよりもずっと末梢側にあるため，第8脳神経のNVCは，内耳孔近傍でおこりうる。今後，難治性耳鳴，眩暈で，保存的治療が奏功しない場合には，MVDが適応となる可能性がある。第8脳神経に対するMVDの症例数は，世界でもまだ少いが，手術適応を決める必要条件として，耳鳴，眩暈が患行の社会生活上，大きな負担となっていること，すべての保存的治療が失敗していること，耳鼻科医により，中耳内耳疾患が除外されていることなどが挙げられる。

The authors report a case of neurovascular comp-ression of the eighth cranial nerve in a 49-year-old businessman. The patient was admitted to the University of Tokyo Hospital because of progres-sive vertigo and tinnitus on the right without hearing loss over the seven years prior. There were no other symptoms. The general examina-tion was normal. He was neurologically intact except for loss of caloric response. Audiometric studies and brain stem response were normal.

The findings of routine hematology, biochemis-try, and serology were within normal limits. To-mogram showed that right internal auditory meatus was wider than the left by 2 mm. Computed to-mography with metrizamide demonstrated a fil-ling defect in the right cerebellopontine angle.

We decided to proceed with exploratory ope-ration with the tentative diagncsis of a left cereb-ello-pontine angle mass, perhaps neurinoma eno-laque meningioma, or epidermoid tumor. Left retromastoid craniectomy with microsurgical exp-loration of the cerebellopontine angle revealed not a tumor, but a loop of the anterior interior cerebellar artery (AICA) compressing the eighth cranial nerve close to the porus acousticus. A piece of muscle was inserted between the eighth cranial nerve and the AICA.

His postoperative course was uneventfull with complete relief of symptoms and without impair-ment of hearing.

In patients with hemifacial spasm and trigeminal neuralgia, neurovascular compression (NVC) has been found at the root entry zone of the facial or trigeminal nerves close to the brain stem at the "junction zone" on the glia and schwann sheath of these nerves. But in acoustic and vestibular nerves, the "junction zone" is not at the root entry zone, but is close to the porus acoustics, and our operative findings are compatible with the concept of NVC of cranial nerves presented by Gardner and Jannetta.

Microvascular decompression (MVD) for tinnitus and vertigo has been performed by a limited number of neurosurgeons and so far the total number of cases is small. Their operative results have not been as good as for cases of hemifacial spasm or trigeminal neuralgia, and the indication of MVD for tinnitus and vertigo is not yet clearly defined. Compared with hemifacial spasm or trige-minal neuralgia, tinnitus and vertigo are less spe-cific symptoms, and they can occur in many dis-eases (otitis media, Meniere's disease, syphylis, acoustic tumors, cardiovascular diseases, etc.). Al-though our case showed NVC can be a cause of those symptoms, it is quite important to rule out otologic and systemic diseases before contemplat-ing MVD.

Until definite indication can be established, the minimal requirement for MVD for tinnitus and vertigo should be as follows :

1) Otological and systemic diseases should be ruled out.

2) Tinnitus or vertigo should be an outstanding symptom in the patient's life.

3) All conservative treatments should have fail-ed.