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Why Hearing Today May Be Gone Tomorrow

2006-05-28 22:37   来源:Andrew Derrington       我要纠错 | 打印 | 收藏 | | |

   

    The human ear is an incredible piece of engineering and exquisitely sensitive. Calculations show that the quietest sound we can hear vibrates the eardrum by less than the diameter of a hydrogen atom. [2] Before those vibrations are analysed by the brain they pass through several stages of mechanical and neural processing that select the sounds we want to hear, adjust their level for comfort and intelligibility, and turn down the volume of distracting sounds.

    [3] But for 8.5m people in the UK something goes wrong with one of the stages. Different parts of the processing chain are vulnerable at different ages. Some of the causes are avoidable and many are treatable. Treatments for problems that occur early in the processing chain are more straightforward and more effective.

    [4] The easiest problem to treat is the one that occurs earliest in the chain. Obstruction of the ear canal by wax causes a relatively mild hearing loss. The loss is negligible unless the canal is completely blocked, which is rare, and is easily restored by cleaning out the wax.

    [5] Things get more complicated behind the eardrum, in the airfilled middle-ear cavity.

    [6] Middle-ear problems are common, treatable and the subject of intense debate about who should be treated and how. Anything that impairs transmission across the middle ear——even if it is only the pressure changes in an aircraft cabin, or blockage of the eustachian tube by a cold——causes hearing loss.

    [7] At the centre of the debate over treatment is the common childhood condition known in the medical profession as otitis media with effusion or OME.

    [8] This is usually caused by an infection of the middle ear, often in the after-math of a cold, in which the middle ear cavity fills up with a liquid effusion. The effusion can be thick and sticky, giving the condition its colloquial name of "glue ear". The liquid causes variable degrees of hearing loss by impairing the transmission of sound through to the inner ear.

    [9] Glue ear can be treated surgically by inserting a tiny tube——a grommet——in the eardrum. This allows the glue to drain away, and ventilates the middle ear, which reduces the effusion of glue. There is evidence that removing the adenoids at the same time as inserting the grommet also helps by reducing the probability of future infections.

    [10] Treatment of glue ear is controversial. The condition affects millions of children between the ages of one and four, at the time they are learning to speak.

    [11] Evidence that it impairs the development of language and other cognitive functions led to a huge enthusiasm for grommet operations——which are the most common surgical operation in children——in the 1980s. Since 1992, doubt about whether the benefits of treatment would always justify the risk of surgery has swung the pendulum the other way.

    [12] There are two problems that complicate the assessment, according to Mark Haggard, director of the Medical Research Council Institute for Hearing Research at Nottingham University (MRCIHR)。 He is conducting a large-scale evaluation of the effectiveness of the different treatments for glue ear, which will be completed in the autumn.

    [13] First, the long-term developmental effects of glue ear are not very severe. Language development is slightly delayed in children under four, and in children between four and seven there are "modest but definite adverse effects on anxiety, social confidence and general coordination of behaviour, Haggard says.

    [14] The second problem is more difficult. Although glue ear affects 83 per cent of children at some time in their lives, it fluctuates enormously in its severity and most children recover quickly and spontaneously. Nobody can identify in advance the children who will have persistent glue ear.

    [15] "Deciding when to intervene is difficult because it is only when it's too late that you know you should have intervened. Another problem is that non-treatment also works, so it's a finely balanced issue," Haggard says.

    [16] In contrast, even the best treatments do not cure the deafness caused by prolonged exposure to noise. According to Adrian Davis, also of the MRCIHR, deafness is the commonest disability in the UK and affects 8.5m people.

    [17] Noise exposure in the work-place is the most common preventable hazard to hearing. The hazard depends both on the intensity of the noise and the duration of the exposure.

    [18] The UK Health and Safety Executive recommends that precautions should be taken by those who work 40 hours a week in sound levels of 85 decibels, and requires an annual check for those who work in noise levels between 85db and 90db. In working environments where the noise is above 90db, ear protection is mandatory.

    [19] "For every 3db increase in intensity you halve the exposure limit," says Davis, "30 minutes' exposure to l00db can produce deafness that lasts several days." These levels make leisure almost as risky as work: many personal stereo players can produce l00db, and disco dance-floor levels are usually about 103db.

    [20] Noise damages the mechanisms in the inner ear that amplify quiet sounds and damp down loud sounds. It particularly affects sensitivity to the high-frequency components of the sound spectrum carrying the information that distinguishes different consonant sounds such as K, S and T. So the intelligibility of speech is severely affected. Just amplifying the sound with a traditional hearing aid doesn't help because it makes the unaffected frequencies too loud.

    [21] Sophisticated hearing aids that selectively amplify affected frequencies are better, but even they are limited by the fact that the amplifiers in the healthy inner-ear turn loud sounds down as well as turning quiet sounds up. Without them, many sounds become uncomfortably loud almost as soon as they can be detected.

    [22] This narrowing of the gap between detectability and discomfort is the reason most hearing aid users spend large amounts of time adjusting the volume. In principle, it is possible to produce a hearing aid that adjusts itself according to the user and to his situation, but the cost of such a device would be more than 10 times the cost of a National Health Service hearing aid.

    [23] Davis is part of a consortium to evaluate the cost-effectiveness of leading-edge technology in the NHS. In doing so, they hope to exploit economies of scale: there are 500,000 hearing-aid wearers in the UK. However, even the best hearing aid will be no match for an undamaged ear, so keep the music turned down!

    人的耳朵构造精巧,反应敏捷,是个不可思议的器官。多次计算表明,我们所能听到的最微弱的声响,给耳鼓带来的振幅还抵不过一个氢原子的直径。

    [2]在大脑对那些振动进行分析之前,它们要通过好几道机械程序和神经程序——包括选取我们想要听的声音,把它们调到舒适清晰的程度,还要把令人分神的声音压低。

    [3]但在英国,有850万人在某道听觉程序中存在着障碍。年龄不同,在听觉处理上各有不同的薄弱环节。引起听觉障碍的原因有些是可以避免的,很多则可以医治。对早期出现于听觉处理环节的问题,医治起来更为直接有效。

    [4]最早出现于听觉环节的问题处理起来最容易。耳垢堵塞耳道会引起轻度的听力下降。若非完全堵塞耳道(这是极为罕见的),这类失听不足为虑,只需清除耳垢就能轻易地恢复听力。

    [5]在鼓膜背后、充满空气的中耳室内所发生的问题就复杂得多了。

    [6]中耳的问题较普遍,可以治疗,并且是有关谁该治、怎样治的激烈争论的主题。任何损害中耳传导的东西——哪怕只是飞机机舱气压的变化,或感冒引起的耳咽管堵塞——都会引起听力下降。

    [7]关于听力治疗的争议,其焦点就是医学上所说的OME (渗出性中耳炎)这种儿童常见耳疾。

    [8]这种耳疾通常由中耳感染所致,往往发生于感冒后期,此时中耳室内积留着一种渗出液。积液可能有些粘稠,使这种疾病俗称“胶耳”。由于影响声音传导到内耳,这种积液就会导致不同程度的听力下降。

    [9]胶耳可以运用外科方法治疗,就是把一根细管子(一种索眼)插进鼓膜。此法可让胶液排出,还可使中耳通气,从而减少粘稠的积液。有证据表明,通过插管既能消除腺肿,同时还有助于减少日后感染的可能性。

    [10]胶耳的治疗存在争议。此疾波及数百万年龄在1~4岁的幼儿,而此时他们正处于学习说话的阶段。

    [11]20世纪80年代,关于此疾有损于语言及其他认知能力的开发的证据,引起人们热衷于选取插管这种最普遍的小儿外科手术。自1992年起,人们对该疗法的诸多好处是否能证明手术危险的合理性表示怀疑,从而使形势发生了改变。

    [12]据诺丁翰大学听力研究所的医学研究理事会主任马克。哈格德说,致使评估变得错综复杂的有两大问题。他正在主持对胶耳不同疗法的疗效进行的 一项大型评估,可望在今年秋季完成。

    [13]首先,胶耳的长远影响不太严重。哈格德说,它对4岁以下幼儿的语言开发略有耽误,而对于4-7岁的儿童则具有“不大不小且又明显存在的”负面效应,表现为情绪不安、处世缺乏信心、举止整体失调等。

    [14]第二个问题更难。虽说83%的儿童在某个年龄段会受到胶耳的影响,但其严重程度差别极大,而且绝大多数孩子会很快自行康复。谁也没法事先就把可能患上胶耳顽症的孩子识别出来。

    [15]哈格德说:“很难决定何时介入,因为一旦你认识到有必要介入,往往已是为时过晚。还有一个问题是,胶耳也可以不治而愈,因而它是一个机体精细平衡的问题。”

    [16]相比之下,对于长期接触噪音所导致的耳聋来说,即便是给予最佳的治疗也无济于事。据阿德里安。戴维斯(此人也是该听力研究所的)说,耳聋是英国最普遍的残疾,其患者多达850万。

    [17]工作场所发出的噪音是最常见的可防性听力公害。其危害程度取决于噪音强度大小和接触时间的长短。

    [18]英国健康与安全监管署建议,凡每星期在85分贝音量下工作40小时的人需要采取防范措施,凡在85-90分贝噪音下工作的人需要每年查一次听力。凡是噪音大于90分贝的工作环境,均应强制保护耳朵。

    [19]“音量每增加3分贝,人承受噪音的极限便缩减一半。”戴维斯说,“接触100分贝的噪音达30分钟,则会产生持续多日的耳聋。”在这么高的分贝下娱乐休闲,其危险程度与在噪音下工作是差不多的,因为不少的立体声单放机可以产生100分贝的音量,而迪斯科舞厅的音量通常约有103分贝。

    [20]内耳具有放大静音、压低高音的功能,噪音有损这些功能。它尤其影响我们对带有区别K、S和T这类不同辅音信息的高频音谱的分辨。理解语言的能力就这样受到严重的影响。只凭传统助听器来放大声音是无济于事的,因为它把不相干的音频放得过大。

    [21]技术先进的助听器要好一些,它们可以有选择地放大所需音频,不过,即使采用这种助听器依然有其局限性,因为放大器在健康内耳里把静音放大的同时还把高音压低了。在没有高音的情况下,诸多可检声讯随即变大,令人难受。

    [22]为了缩小可检声讯与刺耳声讯之间的差距,大多数助听器用户把大量时间耗在了音量调节上。从原则上讲,要造出一种可按用户要求及其处境自动调节的助听器是可能的,但这类装置的成本会比国家保健服务所的助听器贵十倍多。

    [23]戴维斯是对国家保健服务所前缘技术成本效益进行评估的一个国际财团的成员。他们希望通过评估来拓展规模经济:在英国有着50万助听器用户。然而,即使最好的助听器也比不上一只完好无损的人耳,所以还是把音乐的音量开小一点吧!

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