By Frank E. Musiek, Ph.D.
In this month’s issue of Pathways I have decided to write on a topic that has become of interest to me and that certainly plays a key role in neuroaudiology as well as the diagnostic medical community such as neurology, neuroradiology and neurosurgery. The topic is what I will term the concept of “lesion undercut” – a term probably foreign to most in audiology; hence the reason to present it in the Pathways column.
Of course, selecting a topic like this poses the risk that many will not be interested because of its lack of relevance to those in routine practices of audiology. However, I am betting on appealing to the general curiosity of many and supplying useful knowledge to a few.
My heightened interest in the lesion undercut phenomenon is that I believe it can explain some of the diagnostic enigmas of the central auditory system. It also may help us in making sense out of peculiar audiological–anatomical findings. Finally, it may boost our understanding of functional anatomy and pathoanatomy.
Most of what is written here is “my take” on the lesion undercut phenomenon. In some circles it is a term “taken for granted” that it is understood, but in other academic and clinical areas it seems not to be known at all. Certainly the literature is scant – hence the liberty taken in this article to offer my views of lesion undercut.
One of the common uses of the term “undercut” stems from the field of radiology. By definition: It is the effect of scatter on a radiographic image resulting from an unattenuated x-ray beam striking the film screen detector. This indeed is somewhat different from what will be discussed here.
The term undercut in the medical arena was popularized in the early years of psychosurgery. Years ago it was thought to be helpful to perform frontal lobectomies on patients with severe mental illness. By “undercutting” part of the frontal lobe, the procedure would create a disconnection to other parts of the frontal lobe and brain. In essence, this created a lesion in part of the brain to disallow proper function or, in the case of psychosurgery, disallow improper function.
Though this background on psychosurgery provides an insight into the “lesion undercut” concept this article will focus on the neurological-neurosurgical use of the term. In his book Molecular Neurology, Waxman (2010) uses undercut to mean a (lesion) “producing an isolated, deafferented cortical island” (pp 361). In other words, regions of tissue can be isolated from necessary inputs, which then gives the appearance that the tissue area is no longer functioning properly. This situation can be confusing and misleading.
One of the key factors in lesion undercut is the proximity of the lesion to the area that is undercut. Neural inputs to the undercut area must be disrupted by the lesion (Figure 1). This generally means that the lesion is near the undercut area, though this author does not know of any defined distance that qualifies the use of this term.
It appears that the use of “undercut” is rather subjective but understood — at least in some circles of professionals.
One of the factors regarding lesion undercut that impacts diagnostic efficiency is that it can mislead. By this I mean that from radiologic views the area of the brain that is undercut may seem perfectly normal. In fact, the seemingly normal area may be in a state of dysfunction due to a lesion that is separating it from appropriate neural input or output. One way to mitigate this problem of misinterpretation is a thorough understanding of relevant anatomy. Knowledge as to a particular brain structure’s neural input as well as its projections can help in the correct understanding of function and dysfunction secondary to a lesion undercut. Let me provide some examples of lesion undercut and the worth of knowing surrounding anatomy.
One of the common undercut lesion types is that of white matter lesions undercutting the cortex or grey matter. The white matter is, in essence, composed of the axons of nerve cells that conduct impulses. If white matter is affected it can deprive the cortex of input if the pathways supplying the cortex are disrupted.
In a study we recently reported a case where a lesion inferior and medial to as well as involving Heschl’s gyrus resulted in severe auditory deficits (Musiek, Guenette & Fitzgerald, 2013). In this case, part of the deficit may have been an undercut lesion compromising the input from the medial geniculate body to the auditory cortex via the internal capsule. Lesions of the internal capsule –especially the posterior segment are at risk for creating an undercut lesion to the auditory cortex due to profuse neural connections carrying acoustic impulses to this region. The convergence of many auditory inputs to the auditory cortex within the internal capsule forms a “bottleneck” phenomenon, which greatly enhances the possibility of undercut. This condition is one that deserves strong consideration in interpreting undercut results.
Another example of undercut involves the anatomical area of the corpus callosum. The corpus callosum is a large white matter structure connecting the two hemispheres (see Musiek and Baran, 2007). The callosal fibers course across the brain from one gray matter area (cortex) to the other. A lesion near and medial to the cortex in the callosal pathway results in an undercut lesion to the cortex. In audiology this condition in the left hemisphere results in the classic paradoxical left ear deficit for dichotic listening. This condition deprives the left hemisphere from input from the right hemisphere in a dichotic listening paradigm.
Perhaps the most interesting example of potential lesion undercut is the ectopic areas of the brain in dyslexia. These nests of non-functioning cells are often found in the left hemisphere and in auditory areas (Galaburda et al, 1985). These ectopic areas, though small on an individual basis, often are high in number. Therefore, unlike strokes or mass lesions, many ectopic nests are needed to result in significant dysfunction. These ectopic structures can prevent proper connectivity with the cortex. Therefore, though not classic in appearance to most undercut lesions, this occurrence can be considered this type of disorder.
These examples, though only a few of a long list, highlight the nature of the undercut concept. That is, that lesion undercut isolates and deprives a healthy area of the brain and results in dysfunction – with this dysfunction often attributable to this seemingly healthy part of the brain.
The clinician and the scientist need to be aware of the lesion undercut concept presented here. Advances in diagnostic audiology are, have been, and will be dependent on establishing relationships between the anatomy of site of lesion and test findings.
To do this properly, knowledge of lesion undercut is important. Undercut lesions as reviewed here can create a physiological isolated area in the brain resulting in dysfunction of an area that may appear unaffected radiologically. Neural inputs and outputs from auditory cortex are key to the understanding of the undercut phenomenon. As in most facets of complex audiology, the audiologist who is well oriented to fundamental auditory anatomy will be best prepared to properly interpret effects of the lesion undercut.
- Waxman, S., (2010) Molecular Neurology, Academic Press, New York, pp. 361
- Musiek F1, Guenette L, Fitzgerald K, (2013) Lateralized auditory symptoms in central neuroaudiology disorder. J Am Acad Audiol. , 24(7):556-63
- Musiek, F. and Baran, J. (2007) The Auditory System: Anatomy, Physiology and Clinical Correlates. Plural Publishing, San Diego.
Dr. Frank Musiek is a renowned hearing researcher, scholar, teacher and clinical audiologist. His research on electrophysiology and central auditory processing has led to the discovery and implementation of numerous tools that are widely used for assessment of the auditory brainstem and central auditory pathways. His research career has contributed in a substantial way to our fundamental understanding of the anatomy, physiology and neurophysiology of the human auditory system. In addition to his immense contributions to clinical science and practice, Dr. Musiek has demonstrated an untiring dedication to educating students, from undergraduates to post-doctoral research associates and medical students.
Dr. Musiek has published over 160 refereed articles and presented more than 275 invited lectures and seminars and nearly 350 papers at national and international conferences, research symposia and other venues around the world. He has developed four clinical audiologic tests, three of which are mainstays of the clinical central auditory test battery. He has published nine books and authored 43 book chapters. His keen research and insight into the areas of central auditory processing and dysfunction, anatomy and physiology of the auditory system and hearing assessment and diagnosis have earned him an international reputation as an authority on the human auditory system and hearing.