DIFFERENTIAL DIAGNOSIS

Outer and Middle Ear Pathologies:

Pathologies of the outer ear or the ear canal generally result in a conductive type hearing loss which is usually flat or semi-flat across all tested frequencies. There is often a low frequency, or stiffness tilt, to the audiogram as well. Problems with the outer ear may result from a blockage of the opening, a reduction in the size of the opening, foreign bodies within the ear canal, impacted cerumen, or a congenital problem such as congenital atresia. The degree of hearing loss depends on the extent of the closure of the canal. Usually, however, only a small opening is necessary for sensitivity to be adequate. Some typical audiograms and losses for various pathologies follow:

1. Impacted cerumen against the tympanic membrane will result in an approximate 40 dB hearing loss at all frequencies.

2. A collapsed ear canal will result in a loss between 15 - 25 dB. It should be noted that a canal will collapse only when pressure is applied to it such as the ear cushion around the head phone. On visual inspection without the phone in place, it may appear normal. To check for this possibility, remove the cushion from the phone and press against the auricle and view the canal through the center hole.

3. A perforated tympanic membrane may result in a 30 - 40 dB loss. The loss will be greatest in the lower frequencies because the perforation creates a high pass filter effect. Note, however, that a perforation in the pars tensa will have a more pronounced affect on hearing sensitivity than a perforation in the pars flacida. Perforations, however, can be quite complex. Anthony and Harrison in 1972 studied perforations and placed them into one of four groups. (1) Small central perforations (less than 2 mm) which are in contact with the manubrium; (2) Small peripheral perforations (less than 2 mm) not in contact with the manubrium; (3) Large central perforations (>2 mm) which are in contact with the manubrium; and (4) Large peripheral perforations (>2 mm ) not in contact with the manubrium. They found the following:

(1) The average loss for all frequencies was about 20 dB, with the greatest loss in the lower frequencies, with the greatest loss in the lower frequencies except for the small central group where the loss at 250 Hz was less than at any other frequency.

(2) Those with central peripheral perforations showed more loss in the middle frequencies while those with a more peripheral perforation showed the greatest loss at 250 Hz.

(3) Those with a perforation in the posterior inferior quadrant showed an overall greater loss than those with a perforation in the anterior inferior quadrant.

Concerning pathologies of the middle ear, the resultant conductive hearing loss may not be flat across all frequencies, but may display a more marked depression the in the lower frequencies. A sampling of middle ear pathologies are presented below.

1. Discontinuity of the Ossicular Chain usually results in a high frequency hearing loss because of the loss of stiffness which interferes with high frequency transmission. There is often a drop per octave of about 8.6 dB from 500 Hz on up.

2. Clinical Otosclerosis is an heredity problem that occurs when a spongy growth (osteospongiosis) occurs and invades the area around the stapedial footplate. If it begins to anchor the footplate, it will hinder the transmission of acoustic signals into the inner ear. Normally, the growth develops just anterior and inferior to the oval window, and may also be present around the outer border of the round window. There are both otologic and audiologic symptoms: (1) Progressive conductive hearing loss which usually begins unilaterally but may become bilateral shortly after clinical onset. Age of onset is around 20 years; (2) Most common in white females; (3) Tinnitus is present in 80% of the cases and vertigo in 25% of the cases; (3) Schwartze's sign is present (a reddish or pinkish hue observed behind the drum as a result of the increased vascularization in Otosclerosis); and (4) There is often a soft speaking voice resulting from the conductive loss and the concurrent enhancement of the bone conduction in the lower frequencies.

Audiologically, there are usually five distinct stages:

1) There is a very slight conductive drop at 250 Hz resulting from an slight increase in middle ear stiffness. There also is no apparent change in immittance.

2) There is a conductive drop at all frequencies below 1000 Hz with the greatest depression at 250 Hz. The tympanogram may begin to show a Type As tracing. The magnitude of the acoustic reflex may be less.

3) There is an increasing depression in the lower frequencies resulting from the additional stiffness of the ossicular chain. The loss is conductive, but there may be a slight depression in the bone score at 2000 Hz. This is an artifact and is referred to as the Carhart Notch. In terms of immittance, the tympanogram will continue to show a Type As tracing - perhaps a bit more noticeable than at stage 2. The acoustic reflex will be elevated with reduced amplitude or not observed at all. In some cases, it may be reversed.

4) The audiogram begins to level off at about 40 - 60 dB. This occurs because of the continued increase in the stiffness of the system as well as the increase in the mass of the system which now begins to bring down the higher frequencies. The Carhart Notch is now very noticeable at 15 - 20 dB. Immittance will show a Type As tympanogram and acoustic reflexes probably will not be observed.

5) There is a relatively flat 60 dB conductive hearing loss. Even so, there are noticeable depressions in the bone conduction: 5 dB at 500 Hz, 10 dB at 1000 Hz, 15-20 dB at 2000 Hz, and 5 dB at 4000 Hz. NOTE: The depression in the bone scores does not mean that there is a cochlear problem (although, in some forms of Otosclerosis there is an invasion of the cochlea), but that the expression reflects an artifact in the transmission of the bone conduction signal.

3. Otitis Media refers to a broad category of middle ear infections and the labeling is often confusing, but the following is a fairly consistent representation of terminology.

a) Otitis Media with effusion (a fluid-like discharge)

b) Otitis Media without effusion

c) Otitis Media with an accompanying perforation.

Each of the above categories is also classified by duration.

a) Acute: Zero to 21 days

b) Subacute: 22 days to 8 weeks

c) Chronic: Over 8 weeks

If effusion is present, it is also categorized:

a) Serous: A sterile thin straw colored liquid

b) Purulent: Thin, but clouded with leukocytes (infected)

c) Mucoid: Thick mucus-like fluid

The greatest hearing loss occurs with otitis media with effusion and, within this category, the most severe losses occur with the mucoid effusion. In many cases, the loss may range up to 60 dB across the frequency range. An effusive ear may remain that way for many weeks or months after the original infection has been treated.

4. Cholesteatoma is a lesion of the middle ear composed of a squamous epithelial sac filled with concentric layers of desquamated keratin (epithelial debris) in which cholesterol crystals are formed within the middle ear or other pneumatized areas of the temporal bone. Clinically, cholesteatomas can range from static benign lesions to very rapidly growing entities with extensive invasion of the intracranial area. While an exact understanding of the pathogenesis of cholesteatomas is elusive, they have been observed to arise from congenital epidermoid cysts, epithelial migration, papillary ingrowth, or traumatic implantation. The first observation of a cholesteatoma behind an intact tympanic membrane was noted by Howard House in 1953. A cholesteatoma may also be formed by some inflammatory disease medial to the intact drum which disrupts the membrane and lamina proper permitting proliferation of basal epithelial cell columns into the middle ear. In certain instances, cholesteatomas may be secondary to otitis media or other bacterial invasions.

Most commonly, cholesteatoma invades the middle ear through the development of retraction pockets or directly through a perforation. Some retraction pockets are very deep and appear to be perforations and if they do contain debris, they may be labeled pre-cholesteatoma (Schucknecht). For perforations, marginal perforations are more likely to lead to cholesteatomas than central perforations. Once established, a cholesteatoma can invade surrounding structures including the Eustachian Tube. Recently Eisenman and Parisier (AM J OTOL, 19: 20-25, 1998) have noted that cholesteatomas may also result in sensory hearing loss through invasion of the cochlea, and Magliulo et al (AM J OTOL, 18:697-701, 1997) have described labyrinthine fistulas secondary to cholesteatomas.

The audiological manifestations depend on the location within the middle ear space. If it is anterior to the malleus, there is very little hearing loss and most of that is in the higher frequencies because of the additional mass within the middle ear. If the cholesteatoma is posterior to the malleus, there may be a severe conductive hearing loss. Surgical removal is required.

5. Glomus Tumors were first studied at Johns Hopkins. Glomus tumors (paragangliomas) are related to chemoreceptor tissue throughout the head and neck regions. These ganglia are normal cell collections and found in discrete areas of the cochlear promontory near the nerves of Arnold and Jacobson in the middle ear, in the hypotympanum near the jugular bulb, and near the carotid body.

Glomus Jugulare tumors arise in the hypotympanum near the Jugulare bulb while tumors arising on the nerves of Arnold and Jacobson along the promontory of the middle ear are called Glomus Tympanicum. In these tumors, granular epithelioid cells are clustered around the endothelium of capillaries. The usual size of the cluster is 6 - 8 cells.

Audiologically, there is unilateral conductive hearing loss ranging from 30 - 40 dB and patients may complain of pulsating or swishing tinnitus. On otoscopic examination, a bluish or purplish mass is noted reflected through the posterior inferior quadrant of the drum. This tumor must be surgically removed.

Sensory Hearing Losses:

Hearing losses resulting from damage to the cochlear hair cells are referred to as sensory losses. The term "sensorineural" is also used, but current practice is to use "sensory" to refer to cochlear insults and "neural" to refer to extra axial brainstem lesions.

In contrast to middle ear pathologies, the audiological manifestations of cochlear impairment are: (a) immittance audiometry will reveal normal tympanograms and acoustic reflexes will usually be present for all but severe losses, (b) bone conduction is superimposed on the air conduction responses or there is a complete lack of the bone response in severe losses, © suprathreshold word recognition is frequently reduced relative to a comparable conductive hearing loss, (d) the audiometric configuration will usually show, with certain exceptions, a greater hearing loss in the higher frequency regions, (e) loudness recruitment is usually present to some degree as measured on the SISI, ABLB, or MLB, and (f) Bekesy tracing are usually Type II, but there is much variability here.

Speech or word recognition ability is probably one the of primary differences when comparing sensory with conductive losses. As a conductive hearing loss increases in severity, word recognition ability does not change dramatically - as long as amplification compensates for the degree of loss. However, such is not always the case with a sensory loss. Generally, as a sensory loss increases, word recognition ability decreases even with compensatory amplification. The degree of impairment in terms of word recognition ability may be classified as follows:

Normal: 90% - 100%

Slight Difficulty (such as over the phone): 75% - 90%

Moderate Difficulty: 65% - 75%

Severe Difficulty: 50% - 65%

Very Severe and unable to follow normal speech: <50%

Of course, these classifications are approximate and each individual case must be judged on its merits.

Loudness recruitment is also a very distinguishing feature of sensory losses compared to either conductive or neural losses. Recruitment is defined as an abnormal sensitivity to loudness. For example, assume a 50 dB sensory loss in the right ear at 2 kHz and a 0 dB threshold level in the left ear at 2 kHz. If a pulsed tone is presented to the right ear at 40 dB SL (90 dB Htl) it may very well sound just as loud to the patient as a 90 dB SL (90 dB Htl) presented to the left ear at the same frequency. Thus, the two signals sound equally as loud to the patient even though one is presented 40 dB over threshold and the other 90 dB over threshold. What this is showing is that the impaired ear performs similar to the normal ear in terms of loudness growth at suprathreshold levels after the loss has been surmounted. This was first noted clinically in the literature by Neil Goetzinger at the University of Kansas. Subsequent research has confirmed this phenomena and has suggested that it is related to the different functions of the inner and outer hair cells. The most common behavioral tests used to detect and measure recruitment are the Alternate Loudness Balance Test (ABLB), the Monaural Loudness Balance Test (MLB), and the Short Increment Sensitivity Index (SISI). Probably, the most common test is the SISI and is incorporated into most clinical audiometers. Other tests which may be used to detect recruitment are Bekesy audiometry, measurement of most comfortable listening levels, and the PI/PB function.

The most common sensory problems are presented below:

1. Meniere's Disease may be defined as a disorder of the endolymphatic labyrinth characterized by attacks of rotary vertigo, hearing loss, tinnitus, and fullness in the impaired ear. Vertigo is quite common, especially in the early phases of the disease. Initially, the attacks are isolated with no vertigo between attacks. As the disease progresses, the attacks become more frequent and more severe. Most attacks begin quite suddenly and may persist for several hours at a time. The incidence of attacks varies widely with some patients experiencing a single attack while others may experience an episode every few months. In most cases, however, it is progressive as is hearing loss with the greatest loss occurring within the first 5 - 10 years of the disease. Initially, the hearing loss is primarily low frequency, but may eventually affect all frequencies. There are two subtypes of Meniere's Disease: (1) Cochlear Meniere's Disease and (2) Vestibular Meniere's Disease. If the problem is confined solely to the vestibular system with no auditory symptoms, it is the latter. If the problems is confined solely to the cochlear system with no vestibular symptoms, it is the former. Many patients who initially demonstrate one of the subtypes eventually develop full-fledged Meniere's.

As noted above, the hearing loss is usually characterized as a unilateral fluctuating hearing loss initially confined to the lower frequency range with hearing often returning to normal following an attack. As the disease progresses and the fluctuations decrease, the hearing loss may spread to the higher frequencies as well. In both early and later stages, recruitment is present and positive results are obtained on the SISI, ABLB, and Bekesy. Additional objective tests include electrocochleography and electronystagmography.

2. Ototoxicity results from the action of certain drugs or chemicals that damage the cochlear/vestibular system resulting in hearing loss, vertigo, and tinnitus. The damage may be permanent or temporary depending on the drugs or chemicals involved. Ototoxicity may be acquired as an adult or child or it may be congenital from the mother.

Probably the largest family of ototoxic drugs are the aminoglycosides.

Among the more common drugs in this family are: streptomycin,

dihydrostreptomycin, neomycin, kanamycin, and gentamycin. While the exact damage depends on the specific drug ingested, there are some commonalities:

a) These drugs are concentrated in the perilymph and reach peak level within 2-5 hours with a half life of 3-15 hours.

b) Hair cell damage begins in the basal turn and moves progressively toward the apex.

c) The inner row of the outer hair cells are affected first followed by the outer two rows.

d) The inner hair cells are damaged only in the most severe cases.

e) In addition to the hair cells, damage frequently occurs to the stria vascularis, the outer sulcus, and Reissner's membrane.

f) The resultant hearing loss may be unilateral, but is usually bilaterally symmetrical. The degree of loss can range from mild to profound.

g) Recruitment is present and is reflected in positive SISI results, Type II Bekesy pattern, and negative tone decay.

h) Tinnitus is present and often precedes a hearing loss.

I) In the vestibular system, the crista ampularis within the semi-circular canals are damaged initially. The utricle is also sensitive to the amino glycosides while the saccule is less susceptible to damage.

j) Vestibular reactions vary from some unsteadiness to severe vertigo.

Some specific information for each of the drugs is presented below:

Streptomycin: This was originally developed in 1945 to fight TB and is considered to be highly ototoxic - especially to the vestibular system. As little as 2 - 3 grams/day will destroy vestibular function in 2 - 4 weeks. Cochlear function will be destroyed in 28 days.

Dihydrostreptomycin: This drug is more ototoxic than streptomycin. Damage may occur to either the cochlear or vestibular system up to 5 months after the drug is stopped and patient has taken as little as 4 grams. Dihydrostreptomycin also causes much greater damage to outer hair cells than streptomycin.

Neomycin: This drug has the widest spectrum and is highly ototoxic - so much so that it should only be used in life threatening situations. Most damage occurs to the auditory system and reactions may be delayed up to 7 -10 months after cessation of the drug.

Kanamycin: This drug is not quite as severe as neomycin, but does attack primarily the auditory system. It's action on the vestibular system, while not as pronounced as on the cochlear system, is also damaging.

Gentamycin: This is less toxic than kanamycin, but does have an adverse affect on both the auditory and vestibular system.

In addition to the above, certain chemotherapeutic drugs are highly ototoxic. One of the most widely used chemotherapeutic drugs is Cisplatin which is used for treating adult and pediatric malignancies. Research has shown that Cisplatin may result in a progressive bilateral sensory hearing loss. The loss begins in the higher frequencies but may quickly spread to the lower frequencies as well. The incidence and degree of hearing loss is quite variable and is dependent on factors such as the cumulative dosage, method of administration, age of the patient, and interaction with other ototoxins or cranial irradiation.

3. Noise Induced Hearing Loss This reflects long term damage to the auditory system from prolonged exposure to high intensity noise. While there is a definite relationship between dB level and duration of exposure - the higher the intensity the shorter the duration that will cause permanent hearing loss - any noise at an intensity level of 80 dB or higher should be considered dangerous.

Typically, the initial hearing loss from noise exposure is temporary and the term used is a Temporary Threshold Shift (TTS). After the cessation of noise exposure, hearing levels gradually return to normal. The recovery time is dependent upon the intensity and frequency of the noise exposure as well as the degree of damage already existing within the auditory system. With repeated exposures, the magnitude of the TTS will increase, the recovery time lengthen, and, eventually, a portion of the TTS will become permanent (PTS).

The hearing loss is usually bilateral and begins in the 4 kHz to 6 kHz range.

With repeated exposure, the loss at 4k and 6k will increase to a level and then the loss will plateau. As the loss is increasing at 4k and 6k, adjacent frequencies are also affected. Recruitment is present and positive results are observed on the behavioral test battery already mentioned. Initially, word recognition ability is minimally affected; however, as the loss spreads to adjacent frequencies, word recognition declines significantly.

4. Presbycusis is the most common hearing loss in the elderly. This term means literally old age hearing loss. Presbycusis ranks second only to arthritis among chronic conditions affecting the physical well being of the elderly and is the most common auditory disorder found in the entire population.

Unfortunately, the term "Presbycusis" can infer a multitude of audiological problems, thus making it difficult to accurately categorize degree of individual hearing impairment. In general, the following characteristics are commonly found in most cases diagnosed as presbycusis:

a) A pure tone hearing loss that has increased gradually with age until about age 64, at which time there is a more rapid decline in hearing sensitivity,

b) A hearing loss that is symmetrical between the two ears,

c) A hearing loss with the greatest loss in the higher frequencies,

d) The hearing loss is greater in males than in females.

In addition, there may be a variety of other auditory difficulties present in varying degrees. The reason for the variability is unknown, but it is thought to be related, in part, to hereditary predisposition coupled with selected environmental factors. Depending on the extent of auditory dysfunction, the elderly may also experience a significant decrement in speech discrimination ability - especially in the presence of background or environmental noise. These problems are also much more severe than might be predicted from the pure tone audiogram alone.

In general, presbycusis has been subdivided on the basis of involvement determined on post-mortem histologic investigations. While subdivisions vary among researchers, they generally fall into four distinct categories.

1) Sensory Presbycusis is characterized by degeneration of the hair cells beginning at the basal (high frequency) end of the cochlea. As the aging process continues, the apical hair cells also become involved in the degenerative process. The degeneration is enhanced by the gradual atrophy of the organ of Corti characterized by a gradual flattening of the tectorial membrane over the supporting structure along with weakening of the supporting cells of Claudius, Deiter, and Hensen. Once again, this process begins basally and moves apically. Audiometrically, there is typically a sloping bilateral symmetrical hearing loss with no recovery in the high frequencies such as might be found in noise induced hearing losses. Speech discrimination is markedly affected, especially when presented in background noise (Mangham and Yarrington, 1984). The speech discrimination problems are felt to result from the damage to the hair cells on the basal turn of the cochlea. Without these sensory receptor cells functioning properly, those 2nd and 3rd order neurons coded for high frequencies cannot be activated (Mangham and Yarrington, 1984). Other problems common in Sensory Presbycusis include recruitment (more common in males than in females), loss of tonal discrimination, and temporal gap detection (Corso, 1985). The cause of the degeneration is not known, but Schucknecht (1974) felt that it may be related to the accumulation of metabolic waste that in turn adversely influences the activity of various enzymes which normally provide nourishment to the cochlear receptive structures.

2) Neural (Central) Presbycusis results from the gradual atrophy of first order neurons in the cochlea as well as the loss through atrophy of second, third, and fourth order neurons throughout the central auditory system. The loss of neurons has minimal effect on hearing sensitivity until the loss exceeds about 50%. When hearing loss is evident, it is more prominent in the higher frequencies. Word discrimination is very poor and "processing" disorders may be evident. Central auditory test results are positive.

3) Strial (Metabolic) Presbycusis results from atrophy of the stria vascularis. The stria consists of a network of capillaries that secrete endolymph and generate the DC potential that sensitizes both the inner and outer hair cells. The stria is most sensitive to the oxygen deprivation resulting from arteriosclerosis common to the elderly. The atrophy begins in the basal end of the cochlea, but is also quite common in the mid and apical aspects. The resultant hearing loss is rather equal across all frequencies with good speech discrimination abilities. This is in marked contrast to sensory presbycusis with the steeply sloping hearing loss and much poorer than expected speech discrimination ability. Other audiological problems include mild recruitment and some problems with tonal discrimination. In addition, there may be some difficulties with sound localization.

4) Cochlear Conductive (Mechanical) Presbycusis results from mechanical changes occurring within the inner ear system. These changes include alterations of the spiral ligament and changes in the mass and stiffness of the Basilar membrane. While there may be some neural or sensory losses as well, the resultant pure tone hearing loss cannot be supported by the degree of sensory and neural loss. As with strial presbycusis, speech discrimination ability is very good and the resultant loss of sensitivity is equal across all frequencies.

In addition to the above, there are concomitant lesions throughout the central auditory system which are equally represented in all four types of presbycusis and may exacerbate difficulties resulting from the peripheral involvement.

Neural Hearing Losses:

Tumors of the VIIIth nerve are usually unilateral and always benign. In their development, they pass through three stages. In stage one, the tumor is confined to the internal auditory meatus, in stage two the tumor has advanced to the cerebellopontine angle and this is referred to as the early cerebelopontine angle stage, and in the third stage the tumor has increased in size within the cerebellopontine angle - referred to as the middle to late cerebellopontine angle stage.

In stage one, the primary symptoms are auditory with some brief periods of vertigo. Often the first sign is a slight depression in the higher frequencies in the affected ear. At other times, however, the first sign might be a decrease in word recognition ability even before a loss of sensitivity is noticed. Other symptoms include the elevation or absence of the acoustic reflex, acoustic reflex decay, and positive results on the tone decay test along with negative results on the SISI and ABLB and a Type III or IV Bekesy. In addition, a PI/PB function will show significant roll over >.25. At this stage, the most sensitive test is the ABR which will show a Wave V latency increase >.2 msec.

In stage two, the tumor may put pressure on the trigeminal nerve resulting in facial numbness or weakness. The auditory symptoms increase with a continued drop in sensitivity and significant decrease in word recognition scores. Tone decay scores are significant and ABR may show an absence of all waves except Wave V. In some cases, Wave V may also be unrecognizable.

In stage three, the tumor has grown to a size sufficient to compress the brainstem and obstruct the flow of cerebrospinal fluid to the brain. This may result in nausea, headaches, hydrocephalus, vomiting, and visual deficits. At this stage, ABR tracings are likely to be flat in the affected ear. Sensitivity may range from the moderate to profound range and word recognition ability will approach 0%.

Summary:

The standard auditory test battery consists of the pure tone air and bone tests, immittance audiometry, and speech audiometry. Generally, these procedures will be all the testing that is necessary for a routine audiological evaluation. However, on those occasions where the referral source, the case history, or the routine results suggest a specific problem, additional testing will be necessary. For example, if there is a sensory hearing loss present and the patient has complained that loud noises hurt his ears, it would be appropriate to evaluate the degree of recruitment present using SISI, ABLB/MLB, or generating a PI/PB function. This is especially important if hearing aid use is a consideration. To further confirm the diagnosis of a cochlear loss with complete or partial recruitment, Bekesy tracings could be generated and a tone decay test completed.

If, after the standard test battery, there is normal or near normal hearing sensitivity bilaterally, but poorer than expected word recognition scores in one ear, or if the patient has complained about tinnitus, headaches, or vertigo, behavioral retrocochlear testing is indicated. Remember, retrocochlear testing includes tests for cochlear function as well as tests designed specifically for the VIIIth nerve. If the results of the behavioral test battery support a retrocochlear lesion, electrophysiological testing should be done.

Finally, remember that these tests do not always give the expected results even when a pathology is present. For example, even if a stage one tumor is present, the tone decay may be normal, word recognition may be normal, and the immittance battery may be normal. It is even conceivable that the ABR will be negative. On the other hand, an individual with a confirmed cochlear lesion may not show positive results on the SISI, ABLB, or any of the other behavioral tests for cochlear function. For this reason, a test battery must be used. To give an example, in some cases (not too many, but some) there will be a 100% SISI score which suggests positive recruitment or even hyper-recruitment. However, the 100% score may not be the result of recruitment, but be indicative of severe tone decay. This may happen because the carrier tone used in the SISI may fade or decay out, but the 1 dB increments may still be audible and this is what the patient is responding to. So to be conservative, when a 100% SISI score is present, a tone decay test should also be completed -- it only takes a few minutes. If the tone decay is positive, does this mean there is no recruitment? No it does not, but it does mean that there is a good chance that there is something that should be medically evaluated. It is possible to have both a cochlear and retrocochlear problem present at the same time in the same ear.

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