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Based on the book chapter: Hain TC, Micco A. Cranial Nerve 8: Vestibulocochlear Nerve. Textbook of Neurology, Goetz and Pappert Ed, Saunders, 2003, (2nd Edn)
Central hearing loss is generally thought to be extremely rare compared to the sensorineural or conductive types of hearing loss, but recent studies has shown that central components to hearing loss are much more common than previously appreciated (Gates et al, 1996; Gates 2012). There is likely some confusion about this, as audiometry -- the conventional method of detecting hearing loss -- depends on reponses from the person being tested. If one has difficulty understanding, this could impair performance on conventional audiometry. Perhaps because of this confusion, hearing loss is somewhat correlated with the onset of dementia (Lin et al, 2011)
The diagnosis of central hearing loss is usually not made by the pure tone audiogram, which in fact, is often normal. Rather patients with central hearing loss usually have poor scores on speech reception threshold (SRT), or word recognition scores (WRS) portions of the audiogram. Kamerrer et al (2019) reported results in 32 subjects and suggested that "Measures of hearing that involve temporal processing are significantly influenced by cognitive abilities".
Rather than relying on the audiogram, the diagnosis of central hearing loss is best made by combining an audiogram demonstrating good pure-tone recognition, with "speech in noise" testing. Inconsistency between pure-tone recognition and speech is another way to recognized central impairment.
Auditory steady state responses (ASSR) are also sensitive to central hearing loss. Noh and Lee (2019) reported discrepencies in a small number of people (about 2%) between ASSR and PTA hearing even in non-malingerers. 80% of the discrepancies had CNS lesions.
The "central" part of a hearing deficit of course does not respond to hearing aids, but rather should be treated with a program aimed at optimizing their other communication abilities (Gates, 2012).
Patients with central hearing loss typically have inconsistent auditory behavior, that may cause them to be misdiagnosed as having "functional" or psychogenic hearing disturbances. Cortically "deaf" patients may have reactions to environmental sounds, despite absence of reaction to loud noises. As in patients with cortical visual disturbances, patients may consider themselves "deaf" in spite of having reactions to sounds in the room.
We will next consider the sites of central deafness syndromes.
Brainstem lesions are a rare source of central deafness. In general, bilateral brainstem lesions are necessary for central deafness.
|Exterior of brain (From Wikipedia Commons, originally due to Gray), showing the middle temporal gyrus (Heschl's gyrus, Brodmans area 41/42). The Planum temporale is behind Heschl's gyrus.||Transverse section of brain, also from Wikipedia commons, showing again Heschl's gyrus as well as the planum temporale (author: Pancrat).|
Moving up to the cortex, it has been recently appreciated that the cortical representation of hearing is highly complex, contains considerable parallel processing, and involves at least 4 cortical levels including 15 or more areas in the brain (Kaas and Hackett, 1998). The diagram above shows some of the most important areas. The medial geniculate body (not shown) is the major auditory nucleus of the thalamus. Parts of the medial geniculate are hypothesized to function in directing auditory attention.
The medial geniculate sends output to primary auditory cortex, also known as the transverse temporal gyri of Heschl (Brodman's areas 41 and 42, see figure), and association auditory cortex (areas 22 and 52, not shown). The medial geniculate also sends output to auditory motor cortex which controls body responses in response to sound.
The auditory radiations connect the medial geniculate to auditory cortex. Lesions in the area of the putamen (which is a common site of hypertensive bleeds), can cause cortical deafness by interrupting the auditory radiations(Tanaka et al, 1991) . The illustration of the optic radiations above is from Yeh et al (2018)
|Regions of human cerebral cortext that comprise the central cortical auditory area. Area 41/42 is also called Heschl's gyrus.|
Auditory cortex is functionally divided into three areas including a primary area, AI, a secondary area, AII, and a remote projection region, Ep. Authorities vary in assigning area 42 to primary or secondary auditory cortex. The ventral medial geniculate projects almost entirely to AI, while the surrounding auditory areas receive projections from the rest of the geniculate body. Once again, as with the lower auditory systems, tonotopic relationships are maintained.
Pure word deafness is a subtype of central deafness. This disorder is defined as disturbed auditory comprehension without difficulties with visual comprehension. Patients characteristically have fluent verbal output, severe disturbance of spoken language comprehension and repetition, and no problems with reading or writing. Noverbal sounds are correctly identified. The lesion is classically postulated to be a disruption in connections between the dominant Heschl's transverse gyrus and the medial geniculate as well as callosal fibers from the opposite superior temporal region. It commonly presents initially as a Wernicke's aphasia, which on recovering, difficulties in auditory comprehension persist. While usually caused by a stroke, pure word deafness can arise from other causes of focal cortical lesions such as tumors.
Primary progressive aphasia (PPA) is associated with prominent central hearing disturbances. PPA is not a disease in the usual sense but rather a group of symptoms of diverse etiologies connected together by an easy to remember name. It resembles other disorders defined by symptom collections - -such as all psychiatric illnesses. Thus studies of PPA are intrinsicially of a collection of conditions, rather than of a "disease" entity. PPA is associated with some degenerative neurological disorders that are defined by neuropathology, such as FTD. Grube et al (2016) reported in 18 patients with three different variants of PPA and suggested that auditory timing pathways are altered. This inference seems reasonable, though we would think that many other circuits would also be affected in one the the many flavors of PPA.
Sound localization in complex acoustic environments was studied by Zundorf et al (2014). The right planum temporale was important as well as the left inferior and pre and postcentral areas. These areas were said to be "particularly involved in the spectrotemporal analyses crucial for effective segregation of multiple sound streams from various locations, beyond the currently known network for localization of isolated sound sources in otherwise silent surroundings."
Auditory agnosia, another rare subset of central deafness, is typified by relatively normal pure tone hearing on audiometry, but inability to interpret (recognize) nonverbal sounds such as the ringing of a telephone. Inability to interpret nonverbal sounds but preserved ability to interpret speech may be a result of a right hemisphere lesion alone. Amusia is a particular type of auditory agnosia in which only the perception of music is impaired. Again, right sided temporal lesions are thought to be the cause.
|MRI of a patient who had a large meningioma removed on the right side, followed by a stroke including Wernicke's type aphasia about 10 years later. This is a "diffusion" MRI, and the white areas are reduced blood flow. She presently can "hear" noises, but has considerable problems understanding speech. She is fluent.||Another patient with cortical deafness. The black areas on the scan above are strokes in the temporal lobes of both sides.|
Cortical deafness is essentially the combination of word deafness and auditory agnosia. It is characterized by an inability or at least an impairment of ability to interpret either verbal or nonverbal sounds with preserved awareness of the occurrence of sound (as for instance by a startle reaction to a clap. In most instances, the cause is bilateral embolic stroke to the area of Heschl's gyri. It usually results from bilateral lesions and happens when remaining normal auditory cortex is destroyed (as was the case above on the left). It begins as a sudden deafness, which evolves into a picture where patients can hear sounds but are unable to recognize their meaning. Relatively few cases of this disorder have been studied. Note that this syndrome might be difficult to distinguish from a brainstem lesion such as described above. Mendez and Geehan have reviewed this syndrome (1988).
Kaga et al (2015) described a single case of an individual who developed deafness and loss of vestibular sensation after subarachnoid hemorrhage and extensive infarctions. This is a complicated situation due to the very extensive strokes from which very little can be concluded.
|MRI scan of individual with auditory hallucinations, and lesion in auditory cortex.|
Consist of an illusion of a complex sound such as music or speech. Auditory hallucinations are classically found in schizophrenia, however they can also be a result of brain damage. In this context they most commonly occur as a result of an injury to the superior temporal auditory association areas. Penfield discovered that stimulating this area induces an auditory sensation that seems real to patients. Auditory hallucinations can also occur as a result of a temporal lobe seizure. Occasionally auditory hallucinations can occur due to damage to brainstem structures involved in hearing such as the superior olive (Casino and Adams, 1986; Lanska et al, 1987).