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Stuttering: A View from Psychiatry
By Nathan E. Lavid

What causes stuttering?
The etiology of stuttering is unknown, though throughout history, medicine has proposed many theories on the cause of stuttering. Roman physicians attributed stuttering to an imbalance of the four “humors,” yellow bile, blood, black bile, and phlegm. Treatment was directed as correcting an imbalance. Humoral manipulation continued to be the mainstay of treatment up until the eighteenth century. By the 1700s, anatomy and its pathology became the template for disease, and these concepts were applied to stuttering. Giovanni Morgani (1682-1771), the Italian pathologist, attributed stuttering to deviations in the hyoid bone. He came to this conclusion via autopsy – making a causal relationship between variations in the hyoid bone and stuttering. Anatomy continued to be the focus of study for the next century. Surgical intervention, via resection of a triangular wedge from the posterior tongue to prevent spasms of the tongue, was tried. Also, tubes were placed behind the tongue to ensure proper airflow – as abnormal airflow was thought to contribute to stuttering. By the early twentieth century, psychological and neurological conflict became implicated in stuttering. For example, Joseph Sheehan, the American speech pathologist, proposed that a component of stuttering was a conflict between a desire to speak and a fear from speaking. Also, Lee Edward Travis, the American speech pathologist, presented a theory of cerebral dominance that was based on conflict between the two brain hemispheres. This conflict created mistiming of nerve impulses to the bilateral speech musculature.

There have been other theories of stuttering – ranging from psychodynamic formulations to disturbances of the voice apparatus. None have been ideal, and treatments based on these concepts have not ameliorated stuttering. However, recent progress in medical research has shed some more light on the nature of the stuttering and offers the chance for new, efficacious treatments.

Because of the observation that individuals who stutter can speak fluently under certain conditions; for example, while singing, investigation has switched from anatomical abnormalities in speech mechanism to evaluation of the system of speech production. Since the brain orchestrates the motor production of speech, it has become the forefront of stuttering research. Some of the strongest tools used in psychiatric research have been applied to stuttering, revealing some interesting observations. Below is a summary of some findings of recent inquiry.

Genetics
As some people who stutter already know, stuttering tends to run in families. This observation has been also been documented in scientific study. Familial studies reveal that about two-thirds of those who stutter have a family member who also stutters. Also studies of identical twins, who have the same genetic constitution, and fraternal twins, who share about fifty percent of their genes, reveals differences in the incidence of stuttering among the different pairs. The incidence of identical twins who both stutter is higher than the incidence of fraternal twins who both stutter. In about three quarters of the population of identical twins, where one twin stutters, the other twin also stutters. The percentage of fraternal twins who both stutter is lower, ranging from nine to nineteen percent. Because the incidence of dual stuttering in identical twins is not one hundred percent, nongenetic factors must contribute to the expression of stuttering and thus stuttering cannot be completely ascribed to genetics. Though, these familial and twin studies support a genetic connection to stuttering.

Brain imaging
Structural brain imaging, where the gross anatomy of the brain can be visualized and analyzed has not shown an anatomical difference between the brains of those who stutter and those who do not. However, functional neuroimaging, where processes in the brain can be observed, has shown some differences in the state of stuttering. There is no consensus on which areas of the brain are active or inactive during the state of stuttering, but several positron emission tomography (PET) studies of this state have found distinct changes brain activity. Differences in brain activity have been observed in areas that are associated with speech motor function, e.g., the area of the primary motor cortex that controls mouth movements and areas associated with perceiving and decoding sounds. Also, the areas involved with the formulation and expression of language, which in the vast majority of individuals are located in the dominant left cerebral hemisphere, appear to be expressed in the right hemisphere or bilaterally in those who stutter.

PET has also been used to measure the activity of neurotransmitters (the molecules in the brain that allow brain cells to communicate with each other) in stuttering. Increased dopamine, a neurotransmitter, activity has been found in the medial prefrontal cortex – an area that modulates the motor production of speech, areas of the limbic system, and in part of the auditory cortex. Recently reported, is a PET study of four individuals who stutter that reports the same pattern of brain activity in stutterers when they stutter and when they imagine they are stuttering.

Psychopharmacology
Psychopharmacology refers to the effects of medication on brain function. The induction of stuttering in individuals by medicines known to affect brain function, and the subsequent return of fluency when these medications are discontinued is evidence that the brain is involved in stuttering. A wide variety of medications have been reported to induce stuttering.

There are also observations of medications inducing fluency. These medications target specific chemicals and circuits in the brain and are discussed in the below treatment section.

Observations of Acquired stuttering
Further implication of brain involvement in stuttering are reports in the medical literature of brain damage with the onset of stuttering. Brain cell injury affects brain function and head trauma and stroke have been associated with acquired stuttering. Observations of where the damage in the brain has occurred have not revealed a specific brain area where damage induces stuttering. For example, there are multiple reports of patients with strokes who develop stuttering, who have lesions affecting different areas of the brain. However, there are reports of patients who have undergone neurosurgical procedures, and as a complication, there are those who have developed stuttering and who have also had their stuttering alleviated. In these patients, the area where the neurosurgical intervention occurred was the thalamus. Simply, the thalamus is an area of the brain that processes sensory information. Surgical destruction or stimulation of the thalamus does not always induce or ameliorate stuttering, but these cases implicate that the thalamus is involved in speech production and may have a role in stuttering.

So, in light of the above observations, the question arises again: What causes stuttering? The answer is still unknown, but there is strong evidence that genetics and the brain contribute to the phenomenon.

For a very comprehensive list of resources visit The Stuttering Homepage by Judith Kuster at MNSU.
 

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