Megerian Cliff A
Department of Otolaryngology-Head Neck Surgery, University Hospitals of Cleveland, Case Western Reserve University, 11100 Euclid Avenue, Cleveland, OH 44106, USA.
Laryngoscope. 2005 Sep;115(9):1525-35. doi: 10.1097/01.mlg.0000167804.82950.9e.
OBJECTIVE/HYPOTHESIS: Endolymphatic hydrops (ELH) is an important histopathological hallmark of Ménière's disease. Experimental data from human temporal bones as well as animal models of the disorder have generally failed to determine the mechanism by which ELH or related pathology causes hearing loss. Hair cell and spiral ganglion cell counts in both human and animal case studies have not, for the most part, shown severe enough deterioration to explain associated severe sensorineural hearing loss. However a limited number of detailed ultrastructural studies have demonstrated significant reductions in dendritic innervation densities, raising the possibility that neurotoxicity plays an important role in the pathology of Ménière's disease (MD) as well as experimental endolymphatic hydrops (ELH). This study tests the hypothesis that neurotoxicity is an important primary mediator of injury to the hydropic ear and is reflected in measurable deterioration of the cochlear nerve in the animal model of ELH. This study also explores the previously presented hypothesis that cochlear injury in ELH is mediated through the actions of nitric oxide (NO) by evaluating whether hearing loss or various measures of cochlear damage can be ameliorated by administration of an agent that limits excess production of NO.
Part one of the project involves the surgical induction of endolymphatic hydrops and correlation of long term hearing loss with histological parameters of ELH severity as well as cochlear nerve and eighth cranial nerve diameter measurements. In part two, aminoguanidine is administered orally to a separate set of hydropic animals in an attempt to limit cochlear injury presumably mediated by NO.
Guinea pigs are subjected to surgical induction of unilateral endolymphatic hydrops after establishing baseline ABR thresholds at 2, 4, 8, 16, and 32 kHz. Threshold shifts are established prior to sacrifice at 4 to 6 months and temporal bones processed for light microscopy. Measurements of cochlear nerve and eighth cranial nerve maximal diameters as well as average maximal diameters are carried out and correlated to hearing loss and a semi-quantitative measure of hydrops severity. The identical experiments are carried out in animals treated with aminoguanidine, an inhibitor of inducible nitric oxide synthase.
: The mean maximal diameter (n = 14) of the hydropic cochlear nerve was significantly reduced (432.14 +/- 43.18 vs. 479.28 +/- 49.22 microns, P = .0025) as compared to the control nerve. This was also seen in measures of the eighth cranial nerve (855.71 +/- 108.82 vs. 929 +/- 81.53 microns, P = 0.0003). Correlation studies failed to show correlation between hydrops severity and a cochlear nerve deterioration index (r = -0.0614, P = .8348). Similarly, hearing loss severity failed to correlate with cochlear nerve deterioration (r = 0.1300, P = .6577). There was a significant correlation between hearing loss and hydrops severity (r = 0.6148, P = .0193). Aminoguanidine treated animals (n = 5) also sustained nerve deterioration to the same degree as non-treated animals and there appeared to be no protective effect (at the dosage administered) against ELH related hearing loss, hydrops formation, or nerve deterioration.
ELH results in significant deterioration of cochlear nerve and eighth cranial nerve maximal diameters in the guinea pig model. These findings are in accord with previous studies which detected ultrastructural evidence of dendritic damage and indicate that neural injury is of sufficient severity to result in light microscopic evidence of cochlear nerve and eighth cranial nerve deterioration. These data support the concept that the principle pathological insult in ELH is a form of neurotoxicity, especially in light of previous studies which indicate relative preservation of hair cells at similar points in time. The lack of correlation between the severity of hydrops and nerve deterioration suggests that nerve deterioration is independent of hydrops severity.
目的/假设：内淋巴积水（ELH）是梅尼埃病的一个重要组织病理学特征。来自人类颞骨以及该疾病动物模型的实验数据通常未能确定ELH或相关病理学导致听力损失的机制。在人类和动物案例研究中，毛细胞和螺旋神经节细胞计数在大多数情况下并未显示出严重到足以解释相关严重感音神经性听力损失的退化。然而，少数详细的超微结构研究表明树突支配密度显著降低，这增加了神经毒性在梅尼埃病（MD）以及实验性内淋巴积水（ELH）病理学中起重要作用的可能性。本研究检验了以下假设：神经毒性是积水耳损伤的一个重要主要介质，并且在ELH动物模型中可通过可测量的耳蜗神经退化反映出来。本研究还通过评估给予一种限制一氧化氮（NO）过量产生的药物是否可以改善听力损失或各种耳蜗损伤指标，来探索先前提出的假设，即ELH中的耳蜗损伤是由NO的作用介导的。
该项目的第一部分涉及手术诱导内淋巴积水，并将长期听力损失与ELH严重程度的组织学参数以及耳蜗神经和第八颅神经直径测量结果进行关联。在第二部分中，对另一组积水动物口服氨基胍，试图限制可能由NO介导的耳蜗损伤。
在确定2、4、8、16和32kHz的基线听性脑干反应（ABR）阈值后，对豚鼠进行单侧内淋巴积水的手术诱导。在4至6个月处死后确定阈值变化，并对颞骨进行光镜检查。进行耳蜗神经和第八颅神经最大直径以及平均最大直径的测量，并将其与听力损失和积水严重程度的半定量测量结果相关联。在用氨基胍（一种诱导型一氧化氮合酶抑制剂）治疗的动物中进行相同的实验。
与对照神经相比，积水耳蜗神经的平均最大直径（n = 14）显著减小（432.14±43.18对479.28±49.22微米，P = 0.0025）。在第八颅神经测量中也观察到这种情况（855.71±108.82对929±81.53微米，P = 0.0003）。相关性研究未能显示积水严重程度与耳蜗神经退化指数之间的相关性（r = -0.0614，P = 0.8348）。同样，听力损失严重程度与耳蜗神经退化也没有相关性（r = 0.1300，P = 0.6577）。听力损失与积水严重程度之间存在显著相关性（r = 0.6148，P = 0.0193）。用氨基胍治疗的动物（n = 5）的神经退化程度与未治疗的动物相同，并且（在所给予的剂量下）似乎对ELH相关的听力损失、积水形成或神经退化没有保护作用。
在豚鼠模型中ELH导致耳蜗神经和第八颅神经最大直径显著退化。这些发现与先前检测到树突损伤超微结构证据的研究一致，并表明神经损伤严重到足以导致耳蜗神经和第八颅神经退化的光镜证据。这些数据支持了ELH中主要病理损伤是一种神经毒性形式的概念，特别是鉴于先前的研究表明在相似时间点毛细胞相对保存。积水严重程度与神经退化之间缺乏相关性表明神经退化与积水严重程度无关。
