Dysarthria is a speech diagnostic term that can be used to classify various types of neuromuscular speech disturbances. Dysarthria results from notable degrees of one or more abnormalities involving speech musculature, including weakness, paralysis, incoordination, sensory deprivation, exaggerated reflex patterns, uncontrollable movement activities, and excess or reduced tone. Generally speaking, the dysarthrias are considered motor speech disorders because speaking difficulties are largely due to breakdowns in movement control of one or more muscle groups that compose the speech mechanism. The name of each dysarthria subtype is partially derived from the basic characteristics of the overlying movement disturbances. Notably, normal speech production involves the integration and coordination of five primary physiological subsystems: respiration (breath support); phonation (voice production); articulation (pronunciation of words); resonation (nasal versus oral voice quality); and prosody (rate, rhythm, and inflection patterns of speech).
The pioneering works of Darley, Aronson, and Brown in 1975 led to the general model of dysarthria classification that continues to be used to date. These clinical researchers from the Mayo Clinic studied individuals with different neurological disorders for the primary purpose of identifying and describing in detail the various speech problems that they exhibited. These analyses helped to formulate predictable subtypes of speech abnormalities in individuals with specific kinds of neuropathologies. Besides the six primary forms of dysarthria identified, a seventh type has been added to the differential diagnostic scheme in the past decade. The seven dysarthria subtypes are spastic, unilateral upper motor neuron, ataxic, hypokinetic, hyperkinetic, flaccid, and mixed.
There are no known figures regarding the overall incidence of the various dysarthrias in the general population. Moreover, because numerous possible neuropathological conditions can result in dysarthria, it is unproductive to speculate about either the specific or overall demo-graphics of this multi-varied disorder.
Causes and symptoms
Spastic dysarthria is caused by damage to the primary voluntary motor pathways, which originate in the frontal lobes of the brain and descend to the brainstem and spinal cord. These central tracts constitute the pyramidal or upper motor neuron (UMN) system. Virtually all individuals with spastic dysarthria present with a broad spectrum of speech disturbances, including:
- abnormally excessive nasal speech quality
- imprecise articulation behaviors such as slurred sound productions and periods of speech unintelligibility
- slow-labored rate of speech
- strained or strangled voice quality
- limited vocal pitch and loudness range and control
- incoordinated, shallow, forced, uncontrolled, and overall disruptive speech breathing patterns
Individuals with spastic dysarthria often suffer from co-occurring weakness and paralysis of all four limbs. This occurs because the nerve tracts that supply movement control to these structures run in close parallel to those that regulate muscles of the speech mechanism, thereby making them equally susceptible to damage. The specific combination and severity of these features tend to vary from person to person based on the extent of associated UMN damage. In general, people with spastic dysarthria struggle with these speech difficulties because of widespread involvement of the tongue, lip, jaw, soft palate, voice box, and respiratory musculature. Problems with emotional breakdowns, such as unprovoked crying and laughing, also occur in many cases, due to uncontrolled releases of primitive reflexes and behaviors normally regulated, in part, by a mature and healthy UMN system. Finally, swallowing difficulties, known as dysphagia, are not uncommon in this population, because of underlying weakness and paralysis of the tongue and throat wall muscles.
The most common causes of spastic dysarthria include spastic cerebral palsy , multiple sclerosis , amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease), multiple strokes, and closed head injuries (particularly those that cause damage to the brainstem where the UMN tracts converge on the way to nerves that directly connect with the various muscles of the head, neck, limbs, and girdle).
Unilateral upper motor neuron (UMN) dysarthria
Unilateral UMN dysarthria is caused by damage to either the left or right UMN tract, anywhere along its course to the brainstem and spinal cord. The individual with this diagnosis generally presents with mild to moderate weakness and paralysis of the lower face, tongue, arm, and leg on the side of the body opposite the damaged UMN tract. The hemiplegia may necessitate use of a cane or wheelchair, and the facial and tongue musculature disturbances usually only result in mild speech production and swallowing difficulties because the unimpaired opposite half of the lips and tongue often compensate well for this unilateral problem.
Speech breathing and inflection patterns, voice characteristics, and nasal resonance features are not typically abnormal in the individual with unilateral UMN dysarthria. However, it is not uncommon for this person to suffer from a significant language processing disorder (i.e., aphasia ) and/or apraxia in which the brain damage also involves areas of the cortex that normally regulate motor programming and language formulation abilities.
The most common causes of this dysarthria subtype are cerebral vascular accidents (i.e., strokes) and mild-to-moderate head injuries.
Ataxic dysarthria is caused by damage to the cerebellum or its connections to the cerebral cortex or brain-stem. This component of the central nervous system is chiefly responsible for regulating the force, timing, rhythm, speed, and overall coordination of all bodily movements. When the cerebellum is damaged the affected person may exhibit drunk-like motor patterns, characterized by a wide-based and reeling gait and slurred articulation patterns with intermittently explosive voice pitch and loudness outbursts. During purposeful movement efforts, this individual often suffers from intention tremors , which cause under- or overshooting of the intended target.
However, this shaking phenomenon tends to disappear at rest. Swallowing is not usually disturbed.
Hypokinetic dysarthria is caused by damage to the upper brainstem in a region that is richly composed of darkly pigmented (nigra) nerve cells. These neurons contain the neurochemical agent dopamine, which helps regulate muscle tone and smooth and complete bodily movements. When various speech muscles are involved, numerous communication deficits occur, including imprecise articulation of sounds, harsh-hoarse voice quality, and abnormal bursts of speech that sound like the individual is tripping over his or her tongue. These common dysarthric features are the result of widespread rigidity (i.e., stiffness and limited range of motion [hypokinesia]), tremors, and incoordination of the tongue, lip, jaw, and voice box musculature.
Because the most common cause of hypokinetic dysarthria is Parkinson's disease , patients with these types of speech problems also exhibit numerous trunk and limb disturbances such as rest tremors of the hands, stooped posture, shuffling gait, and mask-like facial expressions due to involvement of associated body musculature. Swallowing difficulties may co-occur.
Hyperkinetic dysarthria is generally caused by damage to nerve pathways and centers within the depths of the brain (subcortex) known as the basal ganglia. These integrated central nervous system components form complex feedback loops between one another and the cerebral cortex. The basal ganglia are largely responsible for helping to maintain posture, muscle tone, bodily adjustments, and overall stability during gross voluntary movement patterns. Damage to these structures and their circuitry generally produces two different types of symptoms, depending upon the site(s) of injury: increased muscle tone and very slow movement, known as rigidity, as seen in patients with Parkinson's disease, or involuntary, excessive, and uncontrollable quick-jerky, slow-twisting, or trembling limb and speech musculature behaviors.
Patients with Huntington's disease and tic disorders frequently exhibit the quick and jerky forms of movement abnormalities. The slow, writhing, and twisting movement disorders are usually observed in patients with histories of dystonia , athetosis, torticolis, and dyskinesia . In fact, spasmodic dysphonia, characterized by strainedstrangled or abnormally breathy vocal quality and episodes of periodic arrests of voice, is a form of hyperkinetic dysarthria in that dystonia involves the vocal cords. Tremors are common in patients with essential (organic) tremor disorders. In general, when tongue, lip, and jaw muscles are afflicted by such breakdowns, the articulation of speech sounds is inconsistent and imprecise, voice is hoarse-harsh in quality, the rhythm of speech is flat and irregular, and breathing patterns are sudden, forced, and shallow. All of these disturbances contribute in total to variable, but often-marked degrees of speech unintelligibility in these clinical populations.
Whereas in most cases the underlying cause of muscle hyperactivity is associated with one of the above listed disease-specific entities, occasionally severe head injuries and deep brain tumors can result in any of these types of movement control disorders. Swallowing difficulties can be a significant problem for these types of patients.
Flaccid dysarthria is caused by damage to nerves that emerge from the brainstem (cranial) or spinal cord and travel directly to muscles that are involved in speech production. These nerves are generically referred to as lower motor neurons. Cranial nerves V, VII, X, and XII are of great importance because they supply the chief muscles of speech production, namely, the jaw, lips, voice box and palate, and tongue, respectively. The cervical spinal nerves innervate the diaphragm, and the thoracic spinal nerves stimulate the chest and abdominal wall muscles, all of which are involved in speech breathing activities. The types of neuromuscular problems that arise as a result of injuries to these nerves depend upon which and how many nerves are disturbed. In general, the types of abnormal muscle signs occurring in patients with damage to lower motor neurons include paralysis, weakness, reduced speed of movement, depressed tactile feedback, limited reflex behaviors, and atrophy or shrinkage of muscle tissue.
Analyses of the electrical activity of involved muscles using needle electrodes frequently reveal disturbed firing patterns or twitch-like behaviors known as fasciculations. In a structure like the tongue, which is not covered with thick overlying skin, fasciculations can sometimes be evident by shining a flashlight on the surface at rest. This pathologic feature is an important differential diagnostic sign of damage to the cranial nerve XII. Patients with limited lower motor neuron damage usually exhibit less severe flaccid dysarthria than those with more widespread damage. Additionally, the actual nerves that are damaged dictate the specific types of speech difficulties that may occur. For example, if a focal lesion involves only the cranial nerve VII, as in Bell's palsy , only the lip musculature will be weakened. The result in this case usually produces minimal dysarthria. However, damage to multiple cranial nerves, as often occurs in certain degenerative conditions like Lou Gehrig's disease, will likely cause severe speech difficulties. The most common speech signs observed in patients with flaccid dysarthria, regardless of the cause or severity, include articulation imprecision, hypernasal voice, hoarse and breathy vocal quality, and slow-labored speech rate.
Brain stem strokes, tumors on the brain stem or along the course of the cranial or spinal nerves, muscular dystrophy , and general injuries to these nerves as a result of head trauma or surgical complications are among the most frequent causes of flaccid dysarthria. If spinal nerves that supply the limbs are also damaged, as may be the case in some of these clinical populations, co-occurring paralysis of these structures is likely to complicate the rehabilitation program. Swallowing problems may occur in some cases, depending upon which and how many cranial nerves are involved.
Mixed dysarthria is caused by simultaneous damage to two or more primary motor components of the nervous system, such as the combined upper and lower motor neuron lesions that typically occur in Lou Gehrig's disease, or the co-occurring degeneration of the upper motor neuron and cerebellum pathways seen in patients with multiple sclerosis. In the first example, the patient usually suffers from mixed spastic-flaccid dysarthria. In the second case, the MS patient often presents with mixed spastic-ataxic dysarthria. The exact mixture of neurological damage governs the characteristic speech (and overall body) musculature difficulties.
It is not uncommon for severe head injuries to cause multi-focal nervous system lesions and nonspecific mixed dysarthrias. Many such patients also struggle with limb and trunk motor problems, as well as coexisting swallowing, cognitive, language, perceptual, and psychosocial deficits that worsen their underlying motor speech problems and complicate the rehabilitation course. The mixture may be of two or more of the previously described single-entity dysarthrias.
In addition to clinical examinations, many dysarthric patients will need to submit to various laboratory studies for a thorough appraisal of the possible underlying causes, areas of brain damage, and overall prospects for improvement with appropriate treatment. Such testing might include:
- computed tomography (CT ) or magnetic resonance imaging (MRI) scans of the head, neck, and/or chest
- skull x rays
- arteriography (imaging of arterial flow dynamics)
- spinal tap for cerebral spinal fluid analysis
- electroencephalography (EEG)
- electromyography (EMG)
- videoendosocopy of the vocal cords and soft palate
- pulmonary function studies
- videofluoroscopic examinations of swallowing proficiency
- speech aerodynamic and acoustic analyses
These diagnostic tests require the cooperation of many different clinical practitioners from various fields of study.
Familiarity with the variable speech subsystem abnormalities exhibited by dysarthric patients is indispensable to differential diagnosis. Additionally, because dysarthria is only a speech diagnostic term, and the underlying cause is some form of neurological problem, a medical examination, usually performed by a clinical neurologist , is critical both to the overall diagnosis in any given case and for effective treatment recommendations. Family members and friends can, however, facilitate this process by cursory investigations of the speech difficulties prior to visiting with diagnosticians for formal testing. This preparatory process may involve having the patient perform several physiologic tasks, as well as noting any generalized walking, balance, and limb coordination difficulties exhibited by the affected individual. If the possible cause is understood from the outset, it may help pinpoint the speech diagnosis. The individual can be engaged in general conversation to judge overall speech intelligibility. The listener can listen for signs of poor pronunciation of sounds, excessively nasal voice, hoarseness or strained vocal quality, breath support difficulties, and limited pitch and loudness inflection patterns. Any one or more of these problems may be evident in the speech profiles of individuals with different forms of dysarthria.
The rehabilitation team for an individual with dysarthria often varies, depending on the severity and cause of the dysarthria and the extent of associated limb and trunk musculature disabilities and co-occurring language, cognitive, and psychosocial deficits. In general, those individuals with multi-system breakdowns require a more complex array of team constituents than those who have more focal or mild problems. Most teams consist of the clinical neurologist, speech-language pathologist, physical therapist, occupational therapist, neuropsychologist , nurse practitioner, and social worker. In school-age patients, teachers and guidance counselors will also play very important roles in the treatment program. Naturally, the role of the speech pathologist is usually most critical in the communication treatment plan for dysarthric patients.
Physical and occupational therapists focus on improving limb and trunk coordination, balance, and range of motion, particularly in relation to daily living functions such as walking, self-dressing, and feeding. Neuropsychologists often facilitate memory strategies, perceptual processes, and overall organizational skills required in various work-related settings and daily social circumstances. The administration of certain medications, daily health care and personal hygiene needs, and general tracheostomy care and feeding-tube monitoring may be indicated.
The speech pathologist must design specific speech musculature exercises to improve the strength, tone, range of motion, coordination, and speed of integrated tongue, lip, jaw, and vocal musculature contractions. These general objectives are often achieved following a hierarchy of exercises that may require two or more sessions of therapy per week. In some cases, when oral speech skills fail to improve with both speech and non-speech exercises, use of an alternative or augmentative communication system is required, such as computerized speech synthesizers and/or form or picture boards. These tools are most useful for those patients who possess at least some control of an upper limb to activate a keyboard or point to a picture. In very severely affected patients, a head pointer may be devised so that head movements meet these objectives.
The prognosis for speech improvement in any individual with dysarthria usually depends on the severity of the problem and the underlying cause. If the speech difficulties are mild to moderate, and the cause has been treated successfully through proper medical avenues and is non-progressive, the prognosis for notable improvements with good speech therapy is often very good. However, in the case of severe dysarthria, with a medically uncontrollable or progressively deteriorating etiology, the prognosis for significant gains, even with the best therapeutic programs possible, is almost always very guarded.
Depending on the cause and the severity of the dysarthria, and any coexisting motor, language, cognitive, intellectual, and psychosocial deficits, the affected individual may require many different methods of care. Formal nursing or group home settings are sometimes necessary for those individuals who are not self-sufficient or who lack home care assistance and supervision. Special education classes may be required in those cases with associated learning disabilities. Structural modifications of a wheelchair to facilitate upright head posturing and abdominal support during speech breathing efforts may be helpful for some patients, and construction of ramps in the home may also be necessary to accommodate wheelchair mobility requirements. Arrangements for use of a bell or light switch activator may be indispensable to certain patients who cannot verbally, or otherwise, get the attention of caregivers.
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Duffy, J. R. Motor Speech Disorders: Substrates, Differential Diagnosis, and Management. St. Louis: Mosby, 1995.
Dworkin, J. P. Motor Speech Disorders: A Treatment Guide. St. Louis: Mosby, 1991.
Dworkin, J. P., and R. A. Cullata. Dworkin-Culatta Oral Mechanism Examination and Treatment System. Farmington Hills, MI: Edgewood Press, 1996.
Robin, D. A., K. M. Yorkston, and D. R. Beukelman. Disorders of Motor Speech. Baltimore, MD: Paul H. Brookes Publishing, 1996.
Vogel, D., and M. P. Cannito. Treating Disordered Speech Motor Control (2nd Ed). Austin, TX: Pro-Ed, 2001.
Yorkston, K., D. R. Beukelman, E. Strand, and K. Bell. Management of Motor Speech Disorders in Children. Austin, TX: Pro-Ed, 1999.
James Paul Dworkin, PhD
"Dysarthria." Gale Encyclopedia of Neurological Disorders. . Encyclopedia.com. (July 9, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/dysarthria
"Dysarthria." Gale Encyclopedia of Neurological Disorders. . Retrieved July 09, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/dysarthria
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"dysarthria." A Dictionary of Nursing. . Retrieved July 09, 2018 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/dysarthria