Hearing loss

Remit

This tutorial examines the different types of hearing loss and their causes.

After the introduction it is divided into three sections: Sensorineural hearing loss, conductive hearing loss (including mixed losses) and presbycusis, which is the commonest sensorineural loss and deserves a separate discussion.

Introduction

We will start with a few words about deafness and its impact. It is estimated that around 1:20 of the world's population have a disabling hearing loss. Disabling is defined as a loss greater than 35dBHL in the better hearing ear. The worse ear hears even less.

Untreated deafness causes different problems depending on circumstances. For example, it affects language acquisition in children, social integration at all ages, economic potential for working age people, communication for everyone. It is linked with mental decline and is a significant issue in dementia.

Worldwide, much of it is preventable through public health strategy and medical treatment. Identifying it, once it is present, is important so that rehabilitation can happen early, mitigating many of its effects on the individual.

Broadly speaking deafness may be sensorineural, conductive or mixed. The type of hearing loss depends on the site of disease. Diseases that affect the outer or middle ear cause conductive loss and those that affect the cochlea or cochlear nerve cause sensorineural losses. There are a few diseases that can cause both types of deafness (e.g. otosclerosis, cholesteatoma and acute suppurative otitis media).

Hearing loss is discussed at length on this page. The image below summarizes this.

Sensorineural Hearing Loss.

Causes

The following diseases cause sensorineural hearing loss. It would be prudent to know a little about all of them but only the ones highlighted are covered here. Note that they are not in order of frequency. Presbyacusis is by far the commonest cause of sensorineural hearing loss.

Presbyacusis
Drug related
Traumatic
Congenital
Acoustic Neuroma
Ménière's Disease (and other vestibular syndromes)
Noise Induced hearing loss
Sudden deafness

Drug related deafness

The drugs that can cause hearing loss are many and only the common ones are listed here together with examples in the class and whether the deafness is usually reversible or not. The hearing loss may be unilateral or bilateral and typically affects the high frequencies first. Drugs that cause hearing loss are termed ototoxic drugs.

Aminoglycosides (gentamicin, neomycin) - irreversible
Loop diuretics (furosemide, bumetamide) - largely reversible
Chemotherapeutic agents (cisplatin, carboplatin) - irreversible
Aspirin - reversible

The first aminoglycoside was streptomicin and it was used for the treatment of TB. It became apparent that it caused hearing loss and it is now known that other drugs in the same family cause the same problem.

These days it is gentamicin that is most commonly quoted as an ototoxic drug but other aminoglycosides such as neomycin and kanamycin are also important. In general, ototoxicity occurs when the serum level of the drug is high and this may occur in patients on systemic therapy who also have impaired renal function (as this is the principle route of excretion). Hence patients on gentamicin have drug levels monitored.

Ototoxic drugs can be subdivided into those that predominantly cause deafness (cochleotoxic) and those that cause vestibular damage (vestibulotoxic). Remember that predominantly cochleotoxic drugs also cause some vestibulotoxicity and vice versa.

Interestingly, the ototoxic nature of gentamicin can be used for therapeutic benefit in patients with Ménière's disease. Gentamicin is predominantly vestibulotoxic (with a small ototoxic potential) and can be instilled into the middle ear in order to poison the vestibular apparatus on that side. See the Ménière's tutorial for more.

Acoustic neuroma

Acoustic Neuroma

This is a badly named tumour as it is, in reality, a schwannoma and seldom occurs on the cochlea nerve (usually it arises on a vestibular nerve). So, really it should be called a vestibular schwannoma. It is benign.

There are no specific risk factors except rare cases when they may occur after ionising radiation. They generally present in a patient's middle age. They are also associated with Neurofibromatosis Type 2, when they are bilateral but unilateral, sporadic tumours are by far the most common.

The figure shows the anatomy of the right Internal Acoustic Meatus as seen from the brainstem. SV is the superior vestibular nerve and IV the inferior. C is the cochlear nerve. VII is the facial nerve.

Although the tumour is benign, it is situated in the bony internal auditory meatus and, as it expands, it grows towards the brain stem. Hence, it can be dangerous by virtue of its position and could cause death by brain stem compression.

Such an outcome is very rare, however, and the majority of acoustic neuromas cause only hearing loss, tinnitus and some vague imbalance.

Characteristically, acoustic neuromas cause unilateral ear symptoms. For example, the patient may suffer with unilateral sensorineural hearing loss or unilateral tinnitus. Interestingly, although the tumour usually arises form the vestibular nerve, vertigo is uncommon. Patients may notice a mild imbalance only. This is due to the slow growing nature of the tumour and the ability of the brain to compensate for balance disturbance (ask your tutor for more details on this).

Although the facial nerve is also in the IAM, an acoustic neuroma causes facial weakness relatively rarely. This is because motor nerve fibres are resistant to pressure. This is in contrast to sensory fibres (hearing) as these are thin and easily damaged by pressure.

Investigation of acoustic neuromas is relatively simple - MRI. No other modality has the sensitivity, specificity and convenience. Audiograms are done to confirm sensory asymmetry but more arcane audiological techniques are seldom used now.

This is the audiogram of a patient with an acoustic neuroma in their left IAM. Note how the right side hearing is normal and that there is a drop-off in hearing thresholds on the left.

Almost all patients with asymmetric sensorineural hearing loss have an MRI. Asymmetry is variously defined but the currently accepted definition in the UK is that asymmetry is a difference in at least 15dBHL in two adjacent frequencies.

In this audiogram there is greater than 20dBHL asymmetry at 2,4 and 8kHz and an MRI would normally be ordered and may look like the scan below.

MRI scan through the posterior cranial fossa.

The right internal acoustic meatus (IAM) has a normal calibre. The left IAM is widened by the presence of a large acoustic neuroma (AN). It has grown out of the IAM and enlarged towards the brainstem creating a filling defect in the CSF. Most acoustic neuromas are a lot small than this when they are diagnosed. LSCC - lateral semi-circular canal. PSCC - posterior semicircular canal.

Acoustic neuromas are treated in a number of ways depending on their size and the symptoms that they are causing:

Conservative. The tumour is slow growing and in patients with other disease there may be no need to remove it. This is the usual way of managing a small acoustic neuroma in the UK.
Surgery. This requires a major neurosurgical procedure with it's attendant risks. It is considered when other options have failed. It can also be removed by drilling through the inner ear but the patient will use all hearing too.
Radiosurgery. This stops the tumour growing any further and may shrink it a little.

The conservative approach is by far the commonest. About 70% of acoustics do not change after diagnosis so observation is all that is needed. This is achieved by MRI scanning at intervals.

The following are aspects of management that Primary Care can offer:

1. Understand the nature of the problem and offer the necessary psychological support

2. Refer for a hearing aid

3. Offer advice on the management of tinnitus or make a referral for this

4. Support the patient if they are referred for stereotactic radio surgery or surgical removal of tumour.

Do a quick quiz to assess your recall of acoustic neuromas here.

Noise Induced Hearing Loss

Daily exposure to high intensity noise over a prolonged period will lead to NIHL. This is seen in people who have worked in the mining industry, steel works, panel beating etc. It can also happen to musicians and commonly occurred in members of the armed forces from gunfire.

These days companies must provide hearing protection to their workforce if they work in a loud environment for long periods and severe NIHL is becoming progressively less common.

The hearing loss is characteristically sensorineural and at its maximum at 4kHz. It is almost always present in both ears but it does not have to be symmetrical. Someone who shoots a rifle without protection may develop worse deafness in one ear than the other. Since the damage is irreversible only hearing aids offer any help to the severely affected.

The audiogram on the left shows normal hearing thresholds on the right (red circles) and on the left (blue crosses). The one on the right is typical of a patient with noise induced hearing loss. Note that at 4kHz there is a marked increase in the threshold of hearing on both sides. The 8kHz threshold has suffered a little as well.

Congenital Hearing Loss

This is deafness present at birth and is distinct from hereditary deafness (where a deafness may or may not be present at birth).

In the UK 1:1000 babies are born with hearing loss. This may be in isolation or as part of a variety of other abnormalities. The deafness may be sensorineural or conductive.

It is essential that these babies are identified as early as possible. The sooner they get appropriate hearing help the better. Normal hearing helps us with language acquisition but also with social, emotional and intellectual development.

Here is a brief list of the causes of congenital hearing loss. Some of these will be familiar to you if you have done your Obs & Gynae blocks.

Toxoplasmosis
Rubella
CMV
Herpes
Syphilis
Neonatal hypoxia
Rhesus incompatibility

Babies that have been admitted to Special Care Baby Unit (or equivalent) have 10 times the risk of deafness than their peer group.

Sudden Deafness

Sudden deafness is loss of hearing that develops over a three day period or less. It is usually unilateral and may be caused by outer, middle or inner ear disease.

If the loss is inner ear in nature, an immediate referral is needed to the ENT Department or an Accident and Emergency Department. Sudden inner ear loss can be treated with oral or intratympanic steroids but treatment must be quick. After a few weeks, treatment will not help and the patient may be left with permanent hearing loss.

Sudden loss due to wax or middle ear infection can be treated by the Primary Care teams very successfully. Middle ear effusion can be referred to ENT without urgency (unless there are signs or symptoms of rare nasopharyngeal malignancy).

The difficulty for the Primary Care or Emergency teams is deciding whether the patient has a conductive or sensorineural hearing loss without access to a hearing test. The best that can be done in this situation is to examine the ear for obvious signs of outer or middle ear disease and perform tuning fork tests. These aren't perfect but will give a useful pointer to the nature of the loss when used in conjunction with otoscopy.

What causes sudden sensorineural deafness? This is seldom known for an individual patient but the most likely culprits will be viruses or ischaemia.

All patients will require an MRI scan of their internal acoustic meatus because a vestibular schwannoma is another possible cause.

Presbyacusis

Presbyacusis is the decline in hearing thresholds that is associated with age. Note that age does not cause it.

It is estimated that 15% of those 55-64 years of age, 30% of those 65-74 years of age, and 40% of those over 75 years of age have a hearing loss that affects communication.

Pathophysiology

Presbyacusis is a process that affects both the cochlea and the nerves that run from it into the brainstem.

Below is a cross section of the cochlea zoomed in on the scala media. It is in the scala media that the organ of Corti is found. This organ is responsible for the transduction of vibration into neural activity.

Key to diagram of cochlea

IHC - Inner hair cells

OHC - Outer hair cells

In the cochlea, both the outer and inner hair cells of the organ of Corti are affected. This is especially so at the basal turn of the cochlea where high frequency sound is detected. This means that the patients get a high frequency hearing loss. As time passes the hair cells further up the cochlea are affected and progressively lower frequencies are lost.

The audiogram on the left is typical of presbyacusis. Note that the thresholds for hearing in the high frequencies are elevated. At 2kHz this patient can only hear when the sound is louder than 25dBHL. At 4 and 8kHz the sound must be 50 and 70dBHL before the patient can hear it.

Note also that presbyacusis is a bilateral process.

With loss of hair cells is associated loss of neural cells in the spiral ganglion and hence in the cochlear nerve.

In the diagram below the cochlea is 'unravelled' and the tonotopic organisation shown. Musicians should note that the scale shown below represents only part of the range a human can hear. In fact we can hear frequencies between 64 and 23,000Hz. Look at the diagram carefully and note that high frequency sound is detected in the basal turn near the stapes and low frequencies at the thin apex of the cochlea (this is the opposite way round to the strings in a piano where the high notes come from short wires and the low notes from long ones).

Loss of high frequency hearing is a particular problem when listening to speech.

Speech consists of consonants and vowels. Consonant sounds tend to be higher frequency sounds. Try saying the following letters aloud: 'F', 'S', 'Ch', 'Sh' and you will see.

Vowel sounds are low frequency. Try saying: 'Oo', 'ee'.

The patient with presbyacusis can hear (low frequency) vowel sounds quite well but not (high frequency) consonants. The problem is that consonants provide a lot more information about the content of a sentence than vowels do. Read the following two sentences. In the first the consonants have been removed and just the vowels remain, in the second the vowels have been removed. The sentences have nine words.

'e ui o o u oe e a o'

'Th qck brwn fx jmps vr th lzy dg'

Understanding the first is very difficult but the second is easily recognisable. In fact, both sentences are the same:

'The quick brown fox jumps over the lazy dog'

This illustrates the importance of high frequency hearing. Our environment has continual noise. It tends to be low frequency background noise and this poses another problem for the patient with high frequency hearing loss - the frequencies in speech that they can hear are drowned in background noise.

Management

Sensorineural hearing loss of this type is irreversible so the patient requires assessment as to the degree of disability and provision of a hearing aid and hearing rehabilitation.

1. Hearing aids
All hearing aids have the same basic design: a receiver, a processor (including and amplifier) and a speaker. There are different models: some are hidden in the ear canal while others fill the conchal bowl of the pinna. Hearing aid design and technology is not within the scope of this article.

Ear mold

Amplifier & power

This aid is a 'behind the ear' (BTE) aid. The amplifier, power source, switches and volume control sit behind the pinna. The sound output passes down the clear plastic tubing, through the plastic insert and down the ear canal to the ear drum. Gold plated ones are rare and are used when the skin of the ear reacts to the moulding material.

Other types of hearing aid are: Bone Anchored Hearing Aid (BAHA) and Cochlear Implant.

2. Rehabilitation

Lip reading, enhancing the environment to favour the patient and counseling about hearing aid use are sometimes needed for the patient.

Conductive hearing loss

This subtype of hearing loss is encountered when there is disruption of the sound waves passing through the external auditory canal, tympanic membrane or middle ear. There are multiple causes such as wax, tympanic membrane perforation, otitis media or even cholesteatoma.

Brackets on an audiogram identify bone conduction. This is measured by placing a probe behind the ear on the mastoid bone and measuring the threshold that the patient hears the noise.

In a patient with normal or sensory deafness the bone conduction line will be the same as the air conduction line. The right ear in this hearing test is normal so the bone conduction line has been left off the audiogram.

In this patient the left ear is deaf and so bone conduction testing has been done. The bone conduction testing shows that the inner ear is working well (all brackets are in the normal range except the one at 2kHz).

Another way of saying this would be there is a discrepancy between what is heard through the bone and what is heard through the air (an air-bone gap). An air-bone gap means a conductive hearing loss.

Mixed hearing loss

A mixed hearing loss is a combination of the hearing losses described. There are both sensorineural and conductive components.

Opposite is an audiogram showing an example of a mixed hearing loss. The right ear is normal.

In this example, the left ear demonstrates a mixed loss. An air-bone gap exists, however, the brackets are also below the normal hearing thresholds.

Examples of conditions that may cause this sort of hearing loss are a cholesteatoma and trauma.

Audiology - How to perform a hearing test.

Have a look at this excellent video that describes how to read an audiogram. It is written and presented by Century Hearing Aids, a private hearing aid dispenser in the USA, and it describes audiometry clearly.

Test yourself at our audiograms quiz section, based on audiograms commonly examined in a medical school curriculum.

Hearing Loss

MRI scan through the posterior cranial fossa.

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Ear mold

Amplifier & power