Normal speech production involves the integration and coordination of five primary physiological subsystems: respiration, phonation, articulation, resonance, and prosody. Impairment of any of these elements may lead to dysarthria or slurring of speech. Dysarthria may occur secondary to lesions along the neuroaxis that produce motor dysfunction of any of these five speech systems. Lesions can be unilateral or bilateral and localize to the cerebral cortex, subcortical structures, brainstem, cerebellum, basal ganglia, cranial nerves, upper cervical nerves, or even the neuromuscular junction or musculature.
I. DEFINITION
Dysarthria is defined as “difficult, poorly articled speech resulting from interference in the control and execution over the muscles of speech usually caused by damage to a central or peripheral motor nerve.” Dysarthria is a group of speech disorders characterized by muscle control disturbance leading to impaired articulation of sounds. The words are slurred, but the language content is normal. Dysarthria should be differentiated from mutism, dysphonia, aphasia, and apraxia of speech.
II. CLINICAL PICTURE
A normal speech pattern is achieved through the smooth coordination of respiration, phonation, articulation, resonance, and prosody. Adequate breath support and forced exhalation gives way to changes in vocal fold length, position, and vibratory pattern. As exhalation occurs, changes in the size and shape of the oral cavity in conjunction with the articulators produce phonemes for speech production. At the same time, changes in prosody attach meanings to phonemes with alterations in pitch, intonation, stress, and rate. Together, these speech mechanisms allow us to effectively participate in daily conversation. The semiology of dysarthria may include: slurred speech, slow or rapid speech, whispering speech, abnormal intonation, and changes in quality such as nasal or hoarse sounding speech, breathy sounding speech. Associated clinical findings include limited or abnormal movements of the tongue, jaw or lips, drooling, and difficulty chewing or swallowing.
III. TYPES OF DYSARTHRIA
Differentiating among the dysarthrias is not simple as there is much overlap in the semiology of the various types of dysarthia, although certain speech characteristics are often associated with specific types of dysarthria. The flaccid, spastic, mixed, ataxic, hypokinetic, and hyperkinetic dysarthria are best characterized and described below (see also Table 19.1):
A. Flaccid dysarthria, as seen in bulbar palsy, is characterized by excessive hypernasality. Articulation of consonants and/or vowels is imprecise with slow and labored speech rate. It is typically caused by damage to cranial nerves V (motor division), VII, X, and XII that supply muscles of articulation and mastication. Muscle weakness, hypotonia, and/or atrophy may be observed. One common cause of flaccid dysarthria is idiopathic seventh nerve (Bell’s) palsy. Often these patients complain of changes in speech, drooling, and oral dysphagia. The face may appear asymmetrical during range of motion tasks even if normal at rest.
TABLE 19.1 Mayo Clinic Classification of Dysarthria
B. Spastic dysarthria, as seen in pseudobulbar palsy, results from damage to the upper motor neuron (UMN) pathways in the frontal cortex, subcortical regions, or brainstem. Speech is shallow and labored with imprecise articulation. Pitch is low with a strained vocal quality, and hypernasality can be considerable. Dysphagia may be documented as well. In addition to spastic dysarthria, the patient with pseudobulbar palsy may often exhibit emotional lability that may exhibit spontaneous outbursts of laughter or crying known as “pseudobulbar affect.” Spastic dysarthria may also result from ischemic insults. Isolated or “pure” dysarthria results mainly from lacunar infarcts involving the internal capsule or corona radiata. Isolated dysarthria and facial paresis are considered a variant of the dysarthria-clumsy hand lacunar syndrome. Occasionally, an isolated lacune will interrupt the corticolingual fibers from the motor cortex, causing dysarthria but without hemiparesis. Other common causes of spastic dysarthria include traumatic brain injury, spastic cerebral palsy, multiple sclerosis (MS), and amyotrophic lateral sclerosis (ALS).
C. Mixed dysarthria is caused by simultaneous damage to two or more primary motor components of the nervous system, such as combined UMN and lower motor neuron (LMN) lesions. This form of dysarthria is common in patients with MS, ALS, or severe traumatic brain injury. The patients may speak with very slowly and with great effort. Articulation is markedly impaired with considerable hypernasality. Pitch continues to be low with a strained or strangled vocal quality. Prosody is completely disrupted with intonation errors and inappropriately shortened phrases/sentences. Bulbar involvement in ALS often presents in this fashion with dysarthria, hypophonia, drooling of saliva, and progressive swallowing difficulties (Fig. 19.1).
D. Ataxic dysarthria is usually associated with cerebellar disorders. Patients present with decreased motor coordination for accurate articulation with abnormal speech rhythm and syllable repetition. Likewise, patients are unable to accurately complete dysdiadochokinetic tasks with variations in pitch and loudness. Prosody impairment is characterized by prolonged intervals between syllables or words with excess stress on certain syllables and words. Ataxic dysarthria is caused by damage to the cerebellum or cerebellar connections to other parts of the brain. Isolated cerebellar dysarthria has also been reported with small infarcts in the left paravermian zone of the ventral cerebellum (lobulus simplex and semilunaris superior).
E. Hypokinetic dysarthria, most typically seen in parkinsonism, is associated with hypophonia or reduced vocal loudness, in addition to monotonous speech with a slow and flat rhythm. Initiation of speech is difficult, resulting in inappropriate silences intermixed, however, with short rushes of speech. The rate is variable with wide fluctuations in pitch.
F. Hyperkinetic dysarthria also occurs secondary to damage to the basal ganglionic pathways and is typified Huntington’s disease. Damage to this system causes involuntary movements such as tremors, dyskinesias, athetosis, and dystonia. Vocal quality may be described as harsh, strained, or strangled and is often associated with spasmodic dysphonia.
FIGURE 19.1 Diffuse tongue atrophy and fasciculations in a patient with bulbar MND.
IV. DIFFERENTIAL DIAGNOSIS
The major clinical distinctions are between dysarthric, dysphonic, apraxic, and aphasic disorders. Both dysarthria and apraxia are considered to be motor speech disorders, and it may be sometimes difficult to differentiate among them. Apraxia of speech is a motor programming or planning disorder involving speech production tasks. Automatic and involuntary tasks are usually spared. Errors in articulation are inconsistent with primarily vowel and consonant distortions. Initiation is very difficult with obvious effortful grouping in an attempt to achieve accurate movement of the articulators. Patients are often aware of the errors and make attempts at correcting them. However, they are often unsuccessful in achieving initial articulatory configurations or transitioning from one sound to the next.
Aphasia is a loss or impairment of language processing caused by brain damage (see Chapter 3), whereas dysarthria is a problem in speech articulation. It is not uncommon for aphasia and dysarthria to coexist. A person with aphasia may be able to communicate with adequate breath support, voicing, and articulation, but may be unable to comprehend others or name, repeat, or express themselves adequately. Patients may also have isolated anomia (word finding difficulty) with inability to state certain words or name specific persons or objects.
Dysphonia is a term used to describe hoarseness or other phonation disorders. It is a characteristic of certain types of dysarthria; however, it may stand alone when describing other voice disorders. Spasmodic dysphonia is a neurological voice disorder that involves involuntary tightening or constriction of the vocal cords, causing interruptions of speech and affecting the voice quality, which can be strained or strangled. Voice overuse, illness, trauma, and stress can also result in chronic spasm, paresis, or vocal cord paralysis. Laryngitis and gastroesophageal reflux are common non-neurologic causes of chronic hoarseness.
V. DIAGNOSTIC EVALUATION
A detailed history and thorough neurological examination are necessary to determine the possible underlying etiology of the different types of dysarthria. The presenting symptoms, duration, pattern of speech disturbance, and progression of symptoms may help elucidate the mechanism of dysarthria. Other neurologic symptoms, medical comorbidities, and knowledge of contributory medications or exposures may also help determine the etiology of the dysarthria. For example, patients with extrapyramidal disorders have slow, quiet, and monotonous speech, which is gradually progressive and is associated with slowness of movement, falls, and tremors. Scanning speech with dysprosody is suggestive of a cerebellar disorder, especially when lack of coordination and gait unsteadiness are present. Patients with an LMN lesion may have pronounced tongue atrophy and fasciculations, with gradual and progressive muscle weakness, whereas an UMN disease is characterized by spastic and explosive speech. Palatal palsy and decreased gag reflex with tongue weakness may indicate bulbar involvement, whereas a brisk jaw jerk, hyperactive gag reflex, and emotional lability are suggestive of pseudobulbar palsy. Mechanical factors contributing to dysarthria including pharyngeal, vocal cord, tracheal, and other airway lesions including trauma and masses in these areas must also be considered.
Neuroimaging studies of the head or neck are helpful in diagnosing central and peripheral causes (see Chapter 32) with MRI with contrast enhancement as the preferred modality. Electromyography (EMG) and nerve conduction studies are an important tool in the diagnosis of MND, peripheral nerve injury or focal dystonia, polyneuropathy, myopathy, or neuromuscular junction disorders such as myasthenia gravis or the Lambert–Eaton myasthenic syndrome (see Chapter 33). Repetitive nerve stimulation or single fiber EMG to help diagnose neuromuscular junction syndromes should be done (when indicated). Lumbar puncture and CSF analysis are discussed in Chapter 33. Other tests include pulmonary function studies and certain blood tests that may be specific to the diagnostic possibilities producing dysarthria including blood tests to assess for inflammatory or infectious causes, genetic diseases, or acquired and autoimmune disorders.
VI. EVALUATION BY A SPEECH–LANGUAGE PATHOLOGIST
In the evaluation of speech disorders, a speech–language pathologist (SLP) is often consulted to differentiate among the various types of dysarthria and help determine the best treatment strategies. Several core components are included in the evaluation. The SLP, after reviewing the neurological and medical evaluation, conducts an interview with either the patient and/or the closest relative. This interview helps to further define the time of onset, pattern of symptoms, previous assessments completed or treatment received, and the course of symptom improvement of the dysarthric disorder over time. An examination of the physical structures of the speech mechanism, as well as an assessment of articulation, respiration, phonation, resonance, and prosody is then performed. This includes a thorough oral mechanism examination assess strength, rate of movement, range of motion, and coordination of the speech mechanism including the jaw, lips, tongue, and velopharyngeal function. Deviations from the norm give way to articulation errors. Articulation can further be assessed in dysdiadochokinetic tasks and by listening to a brief speech sample. Abnormalities are noted with the production of imprecise consonants, producing voiced for voiceless syllables, repeated or prolonged phonemes, or vowel distortions. Speech intelligibility can be rated as well. Examples of improperly voiced syllables are “BAT for PAT; DIME for TIME; GOAT for COAT.”
With decrease in laryngeal control, a patient may be unable to produce voiceless syllables. Abnormalities in respiration are often observed in sustained phonation tasks. A patient may be unable to sustain a vowel, such as “ah,” with normal loudness for a period of time (approximately 5 seconds). Verbal output may also be limited to single words or short phrases due to a lack of expiratory effort. Vocal quality may be breathy, and a patient may be unable to maintain voicing throughout the length of a phrase or sentence. Voicing may start strong, but gradually fade with increased phrase or sentence length.
Appropriate phonation is very dependent on adequate respiration. Adequate breath support is required to achieve functional vocal fold closure in order to produce a sound. Abnormalities in voicing may be attributed to unilateral or bilateral vocal fold paralysis, a vocal cord mass, or vocal cord edema. Vocal fold adduction may be compromised resulting in a breathy vocal quality (hypofunction). Excessive adduction of the vocal folds gives way to possible strained or strangled output in addition to increased pitch (hyperfunction). If there is a suspicion of an abnormal approximation of the vocal folds, a consultation by an otolaryngologist (ENT) may be recommended. Assessment of phonation by an SLP includes sustained phonation tasks. Having the patient sustain a vowel (“ah” or “ee”) for as long as they can allows the SLP to discriminate between variations in pitch, breath support, loudness, and voice quality. Of note, measuring maximum length of phonation provides limited direction in differentiating among dysarthrias because sustained phonation is not characteristic of functional conversational speech.
Hypernasality and hyponasality are characterized by abnormalities in resonance. Hypernasality may be evidenced by an excess escape of air into the nasal cavity resulting from reduced velopharyngeal closure or soft palate weakness. It is also important to watch the soft palate at rest, and during sustained phonation, for functional movement or possible fatigue. Hyponasality results from inadequate velopharyngeal opening, which may be caused from a complete or partial blockage of nasal airway. If a blockage is suspected, further ENT evaluation may be required. Prosody can be analyzed by assessing the coordination of respiration, phonation, and articulation. Errors in prosody may present as abnormally slowed or rapid rate of speech, decreased stress or emphasis patterns, intonation errors, or inappropriately shortened phrases/sentences that can be mixed with intervals of silence. Prosody can be assessed within the speech sample or by having the patient imitate various phrases. One sample of stress or intonation variations is: “Is THAT your car?” “Is that YOUR car?” “Is that your CAR?” Clearly, varying the stress/intonation within this short sentence may result in significant changes in the meaning of a sentence.
Impairments to any of the above listed speech mechanisms resulting in dysarthria often coexist with dysphagia. Current standards require that a swallow screening be documented on all stroke patients; however, this should not exclude screening for dysphagia in any of the other disorders when examining dysarthria. If there are noted dysphagic symptoms with a brief swallow screening, a formal swallow evaluation is needed including a thorough history for possible dysphagic symptoms and assessment of oral mechanisms for strength, movement, and coordination of the muscles for swallowing. If then deemed safe, the patient is given various consistencies of liquids and/or solids, and tolerance is observed. The SLP, through observation and hands-on assessment, notes oral, pharyngeal, and sometimes esophageal difficulties. Depending upon the clinical results of the bedside exam, the SLP may recommend oral feeding with the least restrictive liquids and/or solids, a video fluoroscopic swallow evaluation, or both. When indicated, a fiberoptic laryngoscopic examination may also be helpful for the assessment of swallowing function as well as actual movement of the vocal cords and tracheopharygneal muscles involved in the mechanics of swallowing and speech production.
VII. MANAGEMENT
The underlying etiology of the dysarthria and prognosis for improvement must be taken into consideration when devising a treatment plan. Treatment includes patient and family education and training in compensatory strategies. Treatment of respiratory/phonatory deficits may include improving breath support to increase vocal volume. Slowing the rate of speech may be necessary to improve articulation and intelligibility. Non-speech oral motor training may be recommended for strengthening muscles and increasing mouth, tongue, and lip range of motion and movement. Changes in loudness and prosody, through intonation and stress patterning tasks, may be targets of intervention as well. In severe cases, augmentative and alternative communication strategies, such as computerized voice production systems, may be needed.
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