Central Deafness: A Synopsis

Frank Musiek, PhD
University of Arizona

 

Introduction

Recently, one of our articles was published in the International Journal of Audiology (IJA) pertaining to central deafness [Musiek, F., Chermak, G., & Cone, B. (2019). Central Deafness: A Review of Past and Current Perspectives. International Journal of Audiology, 58, 605 – 617]. Much to our delight, it has received considerable attention. Therefore, it seemed appropriate to generate a synopsis of this article for the Pathways readership who have not accessed the original article.

Central deafness is a rare disorder that is seen in circumstances that are most often a result of bilateral damage to the auditory cortices of the temporal lobes. This is usually quite extensive and often includes more than just Heschl’s gyrus and planum temporale. This disorder can also result from bilateral damage along the central auditory pathways, including the brainstem auditory pathways. There have also been reports of extensive damage to one temporal lobe that has resulted in central deafness or similar types of disorders, but these are relatively rare. The pathophysiology of central deafness most commonly is related to strokes affecting bilateral auditory regions. However, other disorders such as head injury, infections, and tumors, have all been reported as a basis for this disorder (see Musiek & Lee, 1998; Mendez & Geehan, 1988).

A comprehensive review of previous literature demonstrates that there are a variety of auditory processing disorders related to lesions affecting the central auditory nervous system (CANS). Central deafness (CD) can cause dysfunction of processing auditory information, including hearing sensitivity, despite the periphery remaining totally intact. Though thresholds can be obtained, and pure tones and perhaps some environmental sounds can be heard, in most cases speech cannot be understood. Complete central deafness (CCD) is when the damage to the central auditory system is so severe that the individual is rendered functionally deaf. Improvements in task performance are noted over time, in part due to neuroplasticity and central reorganization. 

There are multiple reasons that limit our knowledge and understanding of CD.   These include: 1) the anatomical structures involved are not always well defined; 2) lack of well- defined lesions across patients; 3) inconsistencies in the use of terminology; 4) variability of auditory deficits across individuals; 5) inconsistent and incomplete testing protocols; 6) overall patient health and test reliability; and 7) the multitude of auditory functions affected by the considerable area of cortex and subcortex. Discussing all of these facets is beyond the scope of this short communication but several key points are addressed.

 

Terminology and Clinical Symptoms

Because these patients are seen by a variety of medical specialists with differing backgrounds, semantic differences and confusions abound. A few common confusing terms regarding central deafness follow.

First of all, the term “central deafness” is preferred over central hearing loss because “hearing loss” suggests an involvement of the periphery. Characterization of auditory deficits are often worse in the ear opposite the affected hemisphere. Pure-tone thresholds may be completely absent for CCD and show normal to severe deficits for CD (see discussion below). 

Cortical deafness is sometimes used interchangeably with CD and is often represented as quite similar to auditory agnosia.  Some investigators differentiate this term noting that cortical deafness is limited to areas of the auditory cortex, although this type of anatomical isolation is extremely rare. Cortical deafness can transition from severe to more specific auditory disorders such as word deafness. Auditory agnosia represents a group of disorders where the patient lacks the capacity to identify presented sounds when there are no difficulties identifying spoken language. Even though there is a severe deficit in processing auditory information, hearing sensitivity typically falls within normal range.  Pure word deafness or verbal auditory agnosia is the inability to understand spoken words despite normal or near normal hearing sensitivity. Though the term pure is used, often other auditory functions are affected, such as auditory discrimination of frequency duration and time (see Wirkowski et al., 2006)

The symptoms surrounding central deafness may not always be obvious. Much depends on the extent and type of questions asked of the patient. The site and size of the lesion are important, as well as any previous deficits in communication. In CCD, the patient is essentially unresponsive to sound. In those with CD, patients may be able to hear some things but often not understand or recognize what they hear. This is usually a result of severe distortion created by an improperly functioning CANS. These patients may also be hypersensitive to whatever sound they can hear and often have various forms of tinnitus and may also experience auditory hallucinations. Voice alterations have been reported as well as speech and language changes. The latter is often related to whether or not the patient has a true aphasia. Duration of the disorder and how much hearing and recovery in general is gained will often determine speech and language characteristics (see Mendez & Geehan, 1988).

The audiogram in patients with CD and CCD is particularly intriguing. Those with CCD usually present an audiogram with severe to profound hearing loss bilaterally, even though the basis for this is totally central. Those with CD often present hearing loss, but better hearing sensitivity than those with CCD. In some cases, essentially normal hearing sensitivity bilaterally is seen in those with CD. In both of these situations, tests such as otoacoustic emissions show essentially normal findings bilaterally unless there is some pre-existing peripheral hearing loss. Speech recognition ability in CD and CCD is severely reduced or even untestable bilaterally. 

It is worth noting that peripheral hearing loss and central deafness can coexist. Compromise of the peripheral auditory system, of course, makes the situation of CD worse. It is also worth noting that any audiological test, including the pure tone audiogram, requires cognitive and attention functions that may have been affected by the CANS disorder causing CD or CCD. This must be kept in mind and when testing these individuals, extra effort must be made to ensure results are accurate and not affected by compromise of attention and/or cognitive abilities that may also be involved.

Using the traditional central auditory test battery in individuals with types of central deafness is often futile, at least at the early stages of recovery, due to the complexity of these tests and abilities required to perform the tests, which exceed the capabilities of the patient with central deafness. (After some recovery, these tests may offer some useful data). Because the behavioral central auditory test battery exceeds the patient with central deafness’ capacity, the diagnosis of central deafness is highly dependent on electrophysiological auditory procedures. Acoustic reflex testing can be valuable, especially if the patient does not have any peripheral or low brainstem involvement. Reflex testing will usually demonstrate normal findings bilaterally. Again, if the periphery is normal and the brainstem is not involved, the auditory brainstem response (ABR) will essentially be normal bilaterally as well. In cases of cortical lesions, thalamus or midbrain, acoustic reflexes and ABR results are usually normal. The middle latency evoked response (MLR,) which is generated within the thalamic cortical pathway, will generally be abnormal or even absent bilaterally. The long latency evoked potentials, often termed cortical potentials (P1, N1, P2 and P3) are usually compromised significantly or even absent bilaterally in cases of CD and CCD (see Musiek et al., 2007).

 

Summary

In summary, the terminology used to define what we call central deafness can be a semantic jungle. Understanding these related terms can be confusing, but necessary to understand the patient’s symptoms and underlying CANS damage. Central deafness is a rare disorder but one from which we can learn a great deal about how the brain works in response to auditory signals. Understanding the underlying anatomy, physiology and pathophysiology is critical to understanding that severe damage to the central mechanism can actually reduce hearing sensitivity. This knowledge assists the audiologist in evaluating patients and rendering the correct diagnosis. Indeed, the diagnosis of central deafness leans heavily on electrophysiological procedures and interpreting these results within the context of the patient’s history.

 

References and Suggested Readings

  1. Kaga, K., Shindo, M., Tanaka, Y., & Haebara, H. (2000). Neuropathology of auditory agnosia following bilateral temporal lobe lesions: A case study. Acta Oto-Laryngologica, 120(2), 259-262.
  2. Kussmaul, A. (1877). Disturbance of speech. Cyclopedia of the Pract. Med, 581-875.
  3. Medez, M., Geehan, G. (1988). Cortical auditory disorders: Clinical and psychoacoustic features Journal of Neurology, Neurosurgery and Psychiatry, 51, 1-9
  4. Musiek, F. E., Baran, J. A., & Pinheiro, M. L. (1994). Neuroaudiology: Case studies. San Diego: Singular Publishing Group.
  5. Musiek, F. E., Baran, J. A., Shinn, J. B., Guenette, L., Zaidan, E., & Weihing, J. (2007). Central deafness: An audiological case study: Sordera Central. Un caso audiológico de estudio. International Journal of Audiology, 46(8), 433-441.
  6. Musiek, F., Chermak, G., & Cone, B. (2019). Central Deafness: A Review of Past and Current Perspectives. International Journal of Audiology, 58, 605-617.
  7. Musiek, F. E., & Lee, W. W. (1998). Neuroanatomical correlates to central deafness. Scandinavian Audiology, 27(4), 18-25.
  8. Pinard, M., Chertkow, H., Black, S., & Peretz, I. (2002). A case study of pure word deafness: Modularity in auditory processing? Neurocase, 8(1), 40-55.
  9. Shivashankar, N., Shashikala, H. R., Nagaraja, D., Jayakumar, P. N., & Ratnavalli, E. (2001). Pure word deafness in two patients with subcortical lesions. Clinical Neurology and Neurosurgery, 103(4), 201-205.
  10. Wirkowski, E., Echausse, N., Overby, C., Ortiz, O., & Radler, L. (2006). I can hear you yet cannot comprehend: a case of pure word deafness. The Journal of Emergency Medicine, 30(1), 53-55.

About Pathways

Pathways is both a column that covers topics related to CAPD and Neuroaudiology and a society for people interested in central auditory disorders that regularly meets to discuss these issues.

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