外側半規管型クプラ結石症と中枢性頭位めまい症との鑑別

―外側半規管型クプラ結石症の頭位眼振―

抄録

　Apogeotropic positional nystagmus can occur in association with both peripheral and central lesions. Although apogeotropic positional nystagmus in patients with central lesions is rare, such cases should not be missed. The pathophysiology of apogeotropic positional nystagmus caused by peripheral lesions involves the development of cupulolithiasis in the lateral semicircular canal, which is one subtype of benign paroxysmal positional vertigo. In patients with cupulolithiasis, otoconia released from the otolithic membrane settle on the cupula, which responds to gravity. By considering the relationship between the position of the affected cupula of the lateral semicircular canal and the direction of gravity, we can speculate what type of nystagmus will be induced. As long as the head is maintained in a right- or left-ear-down position while the patient is positioned supine, the affected cupula will maintain a constant deviated angle and the nystagmus will be persistent. Therefore, when nystagmus is transient, we should suspect a central lesion. The affected cupula deviates toward the ampullopetal direction and strong nystagmus is induced with the healthy ear down in the supine position; therefore, the affected side is the upper ear when strong nystagmus is seen in the supine position. The affected side is indicated by the direction of nystagmus that occurs when the patient is supine with a midline head position because the cupula deviates toward the ampullopetal direction in the supine position. The affected side should be identical between the above-described methods. When each method reveals a different affected side, we should suspect a central lesion. When leaning or bowing in the sitting position, patients with cupulolithiasis in the lateral semicircular canal show horizontal nystagmus because the affected cupula deviates when leaning and bowing. Therefore, when vertical nystagmus is seen by leaning and/or bowing the head in the sitting position, we should suspect a central lesion.