Sensorineural hearing loss (SNHL) accounts for about 90% of all hearing loss. SNHL is sometimes also called "nerve deafness", although the term is not entirely accurate, as it leaves out disorders of the hair-cells of the cochlea. It is found in 23% of the population older than 65 years of age. The term "sensorineural" is used to indicate that there is either a cochlear or an eighth nerve lesion. The diagnosis of a sensorineural pattern hearing loss is made through audiometry, which shows a significant hearing loss without the "air-bone gap" that is characteristic of conductive hearing disturbances. In other words, air conduction is equal to bone conduction. Persons with cochlear deficits fail OAE testing, while persons with 8th nerve deficits fail ABR testing.
The most important structures involved in hearing are shown on the figure below. Sensorineural hearing loss is always due to damage to either the cochlea or the auditory nerve (6 and 9 below)
|Figure: The outer ear consists of the auricle (unlabelled), the external auditory canal, and the lateral surface of the tympanic membrane (TM). The middle ear includes the medial surface of the eardrum, the ossicular chain, the eustachian tube, and the tympanic segment of the facial nerve. The inner ear includes the auditory-vestibular nerve, the cochlea and the vestibular system (semicircular canals). The auditory nerve, also called the cochlear nerve, transmits sound to the brain.|
Common causes include old age, where the hearing pattern is often called presbycusis (see following section), Menieres disease, ototoxic medications (such as high-dose aspirin or certain strong diuretics), immune disorders, and noise exposure (see following section). Trauma (inner ear concussion) can cause both temporary and permanent hearing loss.
13-30% of those with meningitis develop hearing loss (Dichgans et al, 1999; Wellman et al, 2003; Wang et al, 2005). Postmeningitic hearing loss can be due to lesions of the cochlea, brainstem and higher auditory pathways, but usually is related to suppurative labyrinthitis (cochlear). Syphilis can cause hearing loss.
Surprisingly perhaps, hypertension, obesity and diabetes is not associated with an increased risk of hearing loss(Shargorodsky et al 2010).
There are many rare individual causes of sensorineural hearing loss.
Noise is a common cause of hearing loss in the US. It is a subtype of sensorineural hearing loss, and because it is due to cochlear damage, it is more specifically a cochlear problem. Twenty-five percent of the US work force is regularly exposed to potentially damaging noise (Suter and von Gierke, 1987). Because of occupational risk of noise induced hearing loss, there are government standards regulating allowable noise exposure. People working before the mid 1960's may have been exposed to higher levels of noise where there were no laws in the USA mandating use of devices to protect hearing. An example of an audiogram showing noise induced hearing loss is shown below. There is a clear "notch" at 3000 hz, with better hearing at both lower and higher frequencies. Otoacoustic emission testing is very sensitive to noise induced hearing loss.
|Example audiogram showing noise induced notch.|
Noise can also cause a reversible hearing loss, called a temporary threshold shift. This typically occurs in individuals who are exposed to gunfire or firecrackers, and hear ringing in their ears after the event (tinnitus). Non-occupational noise are also regularly encountered during recreational activities and are a source of premature hearing reduction. Portable CD players have been evaluated and it has been determined that for most players, 1 hour of listening at 70% of the maximum volume level for a typical player is equivalent the peak recommened noise exposure by the government for occupational exposure (Fligor and Cox 2004) . Peak noise levels, in DB, are provided in the following table taken from Smith et al, 1999).
Dance clubs are often very loud and may be the main source of loud noise exposure outside of occupational exposure. We have encountered several patients in our practice with transient hearing loss after going to a loud club.
Hearing loss and tinnitus (usually temporary), can also be associated with high doses of aspirin or other ototoxic drugs such as the nonsteroidal anti-inflammatory drugs.
Musical instruments can generate considerable sound and thus can also cause hearing loss. The most damaging type of sounds is in the high-frequencies. The piccolo generates sound levels up to 112db…roughly equivalent to a jackhammer at 30 feet.
Violins and violas can be sufficiently loud to cause permanent hearing loss. This is typically worse in the left ear which is nearer the instrument. Unlike other instruments, the ability to hear the high-frequency harmonics is crucial to these musicians. Mutes can be used while practicing to reduce long term exposure. (Karlsson, Lundquist et al. 1983; Ostri, Eller et al. 1989; Royster, Royster et al. 1991; Sataloff 1991; Palin 1994; Teie 1998; Obeling and Poulsen 1999; Hoppmann 2001; Kahari, Axelsson et al. 2001). In a study of rock/Jazz musicions, almost 3/4 had a hearing disorder, with hearing loss, hyperacusis and tinnitus being the most common maladies. (Kaharit, Zachau et al. 2003)
|Musician's earplugs are available in ER-9, ER-15, and ER-25, according to ones need for noise reduction.|
There are a number of strategies that can be used to reduce the chance of noise injury from other instrumentalists. Musicians ear plugs are generally "flat" so that bass and treble notes are not relatively favored, thus distorting perception. Nevertheless, a"vented" ear plug can be used to tune the ear cavity to low frequencies, which are less damaging. Drummers should use musicians ear plugs, such as the ER-25. Guitarists and vocalists can use the less attenuating ER-15 (see figure above). These ear plugs require a visit to a clinic to get an impression made, so that they are specific for the ear being fitted. A typical cost for a musician's ear plug is about $125. We offer these in our practice in Chicago. Too much ear protection can result in overplaying and not enough protection can result in hearing loss.
Plexiglass baffles can be used to reduce the noise from other instruments.These are particularly relevent for drummer's high-hat cymbals. Drums and brass can be particularly a problem. Ear monitors are small in-the-ear devices that look like hearing aids, that can be used to electronically protect hearing, while allowing the musicians to hear themselves. Acoustic monitors are stethescope like devices that block sound from other in the group, but allow the instrumentalist to hear their own instrument.
Loudspeakers produce both high and low frequency sounds. High frequencies tend to emanate in almost a straight line, while low frequencies are present in nearly all directions. Thus, standing besides a high-frequency source may provide some protection. Humming just prior to, and through a loud noise such as a cymbal crash or rim shot may provide some protection. Small protective muscles in the ear contract naturally when we sing or hum, and thus humming may protect from other noises.
While very unusual, acoustic neuromas or metastatic cancer (particularly breast) can be a source of hearing loss. Typically there is a combination of a hearing problem, as well as mild dizziness and imbalance. Audiometry, as below, generally shows an asymetrical sensorineural hearing loss.
A recent study examined 674 patients (naval recruits) with ASNHL (Khan et al, 2018). Of these, 661 had an MRI, and 0.3% had some sort of pathology on the MRI. Not a single one had an acoustic neurinoma. (Khan et al, 2018). This would suggested that the "yield" of doing MRI scans for ASNHL is very low.
Sudden hearing loss (SHL), covered separately here, is defined as greater than 30 DB hearing reduction, over at least three contiguous frequencies, occuring over 72 hours or less. It occurs most frequently in the 30-60 year age group. Differential diagnosis includes viral disease, vascular disease (1%), autoimmune phenomena, perilymph fistulae and Meniere's disease, and acoustic neuroma. Because hearing tends to recover spontaneously at such a high rate, treatment is not always felt necessary, especially when impairment is minor.
Although diabetes per se is not associated with an overall greater risk of hearing loss (Shargorodsky et al 2010), hearing loss starts at an early age than the normal population. but eventually the general population catches up, so that by the age of 60, they are difficult to distinguish (Vaughan et al, 2006). Pathological studies suggest that it is due to microangiopathic involvement of inner ear blood vessels and subsequent stria vascularis atrophy and hair cell loss (Fukushima et al, 2004). Good control of diabetes seems to be associated with slower progressing of hearing loss.
Diabetics are also more prone to get external and middle ear infections, as well as more prone to develop cranial nerve palsies and stroke. These problems can greatly complicate the hearing care of diabetics.
As diabetes is commonly an autoimmune disorder, autoimmune inner ear disease may also contribute.
Presbycusis is defined as hearing loss associated with degenerative changes of aging. Presbyacusis is the most common type of hearing loss in the United States. This type of hearing loss is typically gradual, bilateral, and characterized by difficulty hearing high frequencies.
There are four types:
An example of presbycusis is shown below. Presbycusis can be inherited -- a recent study suggested that heritability of medium and low frequencies was .38 and .31 (DeStefano et al, 2003). More information about presbycusis is found here. Statistical data from the US vital statistics can be found here.