Light Cupula Theory in Persistent Type of Geotropic Direction-changing Positional Nystagmus

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  • 持続型方向交代性下向性頭位眼振の病態
  • リンショウ ジゾクガタ ホウコウ コウタイセイ ゲコウセイ トウイ ガンシン ノ ビョウタイ

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Geotropic direction-changing positional nystagmus is considered to be caused by canalolithiasis of the horizontal semicircular canal. However, horizontal nystagmus, which persists without decreasing over time, can not be explained by canalolithiasis. The behavior of nystagmus as a function of the head position in space was studied in four patients without any evidence of neurologic disease and with persistent geotropic direction-changing positional nystagmus. In the supine position, permanent horizontal nystagmus toward the intact side was observed. In neutral position 1, with the head turned 15 to 45 degrees to the affected side, the nystagmus decreased and eventually stopped. With greater head-turn, nystagmus toward the earth lasted for more than one minute. These phenomena were considered to indicate that the cupula was deflected by the buoyancy of attached light debris which is hypodense compared with the endolymph. In the prone position, persistent horizontal-torsional nystagmus to the affected side was detected. In neutral position 2, with the head turned 15 to 45 degrees to the affected side, the nystagmus stopped. On the basis of these findings, the pathophysiology of the persistent type of geotropic direction-changing positional nystagmus is considered to be a result of light debris cupulolithiasis of the horizontal semicircular canal.

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