隱藏性聽力損失

作者:張慧珊/亞洲大學聽力暨語言治療學系助理教授
臺北市立聯合醫院和平婦幼院區耳鼻喉科聽力師
中華民國聽力師公會全國聯合會監事
台灣聽力語言學會理事
台中市聽力師公會理事

一.隱藏性聽力損失的定義

大多成人是因內耳毛細胞或聽神經纖維受損而產生的感音神經性聽力損失，毛細胞與聽神經纖維一旦產生損傷，以當今的技術都無法再生[1]。近年來的研究指出，即使暴露噪音沒有出現急性或慢性的毛細胞損傷，但內毛細胞與耳蝸神經元之間的突觸可能損傷高達50%，相同的原發性耳蝸突觸損傷也發生在老化的耳朵中[2, 3]。雖然耳蝸突觸損傷是立即發生的，但常規的組織學檢查中卻無法看見突觸，且後續的螺旋神經節細胞損失也需要幾個月到幾年的時間才會出現聽力損失，因此耳蝸突觸病(cochlear synaptopathy)一直處於”隱藏”的狀態[4]。

此外，因最易受噪音暴露損害的是高閾值（high threshold）、低自發放電率（low spontaneous rates）的聽神經纖維[2, 5]；此纖維被認為與偵測大音量聲音較為相關，尤其是在連續的背景噪音下，此時中、高自發放電率纖維的放電率已達飽和[4, 6]，而偵測安靜環境的小音量聲音多為低閾值、高自發放電率的聽神經纖維[4, 6]所負責，因此耳蝸突觸病不會使行為聽力閾值與電生理閾值變差[7, 8]，除非耳蝸突觸病已非常嚴重。

二.隱藏性聽力損失者的聆聽困難

後續許多研究指出耳蝸突觸病可能是正常聽力者有聆聽困難的主因。大約12% - 15%的正常聽力者可能患有隱藏性聽力損失[9]，包括老年人[2, 3, 10, 11]，他們在噪音的環境中，會有聽覺困難、語音辨識困難、耳鳴與聽覺過敏，也被稱為”隱藏性聽力損失(hidden hearing loss, HHL)” [12-14]。

三.隱藏性聽力損失的診斷方式

　　到目前為止，臨床上的診斷方法仍眾說紛紜，因此尚無一致且有效的臨床聽力檢查方式。以下為可能可檢測出隱藏性聽力損失的臨床聽力檢查方法：

行為聽力檢查－噪音情境中的純音聽力檢查

　　在正常聽力年長者的研究中，寬頻噪音情境中的純音聽力檢查發現年長者在低頻與高頻結果較年輕者表現差[15]，尤其在高頻與低頻較中頻率具更顯著性。

電生理聽力檢查

變頻耳聲傳射聽力檢查法（Distortion Product Otoacoustic Emission, DPOAE）：根據研究顯示，隱藏性聽力損失者的DPOAE反應波振幅可能較小[16]。
聽性腦幹反應檢查（ABR）/耳蝸電圖（ECochG）：暴露於大音量噪音者，即使聽力閾值正常，但超聽閾音量之刺激音所誘發的聽性腦幹反應與耳蝸電圖(ECochG)之第一波/動作電位振幅變小[17-19]，即耳蝸電圖的總和電位(SP)與動作電位(AP)的振幅比值變大與聽性腦幹反應第一波與第五波的振幅比值變小，因此超聽閾音量的第一波/動作電位的振幅變化，可能會是評估和偵測耳蝸突觸病的敏感、無創之臨床聽力檢查指標。

四.結語

目前尚未有一致且有效的臨床聽力檢查方法來診斷隱藏性聽力損失，但已知隱藏性聽力損失大多因暴露噪音有關，因此建議常處於噪音情境者或隱藏性聽力損失者，可介入進行諮商與衛教，告知長期暴露噪音所帶來之無法逆轉的聽覺損傷，也需適時使用噪音防護器來保護耳朵，並定期追蹤聽力，以避免造成更大的損傷。

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