Dysarthria

Etiology

Various neurological disorders can cause dysarthria. Dysarthria can arise from disorders at various locations of the neuroaxis, including the cerebral cortex, basal ganglia, cerebellum, cranial nerve nuclei, or peripheral nerves, and from a primary motor disorder of the tongue, larynx, and pharynx.

Mayo Clinic Classification (commonly used - groups dysarthria based on the location)[4]

Etiopathological Causes

1. Infections: Creutzfeldt–Jakob disease, acquired immune deficiency disease.
2. Vascular disorder: Ischemic and hemorrhagic strokes, arterio-venous malformations.
3. Neoplasm: Primary and metastatic brain tumors.
4. Demyelinating: Multiple sclerosis, Guillain–Barre disease.
5. Degenerative: PD, progressive supra nuclear palsy, corticobasal degeneration, multiple system atrophy, Huntington disease (HD), ataxia telangiectasia.
6. Trauma: TBI, chronic traumatic encephalopathy, cerebral palsy.
7. Toxic: Heavy metal poisoning (Minamata disease due to methylmercury poisoning can cause dysarthria), alcohol, drugs
8. Genetic: Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) due to a mutation in the gene encoding the mitochondrial DNA polymerase gamma enzyme (POLG1)[5][6]

In addition to the neurological causes, nonneurological causes, such as cleft lip or palate and laryngeal tumors, also cause difficulty with articulation. However, the condition is not termed dysarthria if it stems from nonneurological causes.

Epidemiology

The exact incidence of dysarthria is not known, and incidence varies based on the underlying cause. About 90% of patients with PD develop dysarthria during the illness. In ALS patients, dysarthria may predate limb weakness by about 3 to 5 years; dysarthria affects about 70% of patients with limb weakness.[7][8] 

In one study evaluating stroke patients, 28% had both aphasia and dysarthria, and 24% had dysarthria only. In a study of children with neuromuscular diseases, the prevalence of dysarthria was 31.5%.[9] It is estimated that 10 to 60% of patients with TBI have dysarthria.[10]

Pathophysiology

The motor control of speech occurs at multiple levels. The cranial nerve nuclei receive cortical supply through the corticobulbar tract. All other cranial nerves are innervated bilaterally except for the lower face, which receives contralateral innervation. The facial nerve terminates in 5 branches; the branches that contribute to the muscles of speech are the buccal, mandibular, and, to an extent, cervical. The glossopharyngeal nerve (IX), through the stylopharyngeal nerve, innervates the stylopharyngeus muscle and, through the pharyngeal branches, innervates the muscles of the pharynx.[11]

Through the pharyngeal branches, the vagus nerve innervates the pharyngeal muscles, which elevate the palate and cause pharyngeal constriction. The glossopharyngeal nerve provides a sensory supply to the stylopharyngeus, which elevates the pharynx during speech. The cricothyroid muscle supplied by the vagus’ superior laryngeal branch is the vocal cords’ chief tensor. The recurrent laryngeal nerve separates the vocal cords and opens the glottis through the posterior cricoarytenoids, closes the glottis through lateral cricoarytenoids, and relaxes the vocal cords through the vocalis.

Hypoglossal nerve nuclei originating in the medulla provide motor branches to the tongue, supplying the intrinsic and extrinsic muscles (except the palatoglossus).

Suprahyoid muscles: These influence tongue movements by altering the position of the hyoid bone.

1. C1 fibers supply the geniohyoid muscle.
2. The trigeminal nerve supplies the mylohyoid muscle and the anterior belly of the digastric muscle.
3. The facial nerve supplies the stylohyoid muscle and posterior belly of the digastric muscle.

In the neuromuscular junction, acetylcholine produced in the presynaptic nerve terminal binds to receptors, ultimately creating an endplate potential strong enough to propagate action potential over the surface of the skeletal muscle membrane, resulting in muscle contraction. In myasthenia gravis, various autoantibodies interrupt these processes resulting in dysarthria and other symptoms.[12]

History and Physical

Multiple neurological conditions cause dysarthria, so the natural course and clinical features can differ. The presentation can be acute in patients with acute ischemic stroke, whereas it can be delayed in neurodegenerative diseases like ALS.

Based on the Mayo classification, there are salient features of dysarthria[13] described below:

Flaccid: Speech is slow, with hypernasality and breathy vocal quality. One of the most common examples in clinical practice is idiopathic peripheral facial paralysis, in which the patient presents with facial paralysis and drooling. Another common cause is Guillain–Barre syndrome.

Spastic: Speech is harsh, with low pitch and constant errors. Speech evaluation shows hypernasality, reduced intelligibility, palatal elevation, and slow speech rate. Patients have signs of pseudobulbar palsy with dysphagia, hyperactive jaw jerk, and pseudobulbar affect. Patients with dysarthria-clumsy hand syndrome are noted to have facial weakness, dysarthria, and extremity dysmetria.[14]

Hypokinetic dysarthria: This is seen in PD due to the loss of dopaminergic neurons. Speech is monotone and poorly articulated and tends to be quiet. Delays in speech initiation mixed with rushing of words can be seen. Other characteristic signs, such as masked facial features, resting tremors, cogwheeling, and festinating gait, can be observed at the examination.

Hyperkinetic dysarthria: This is seen with basal ganglia lesions and associated hyperkinetic movement disorders, such as HD. Speech is harsh, with variation in loudness and rate of speech. There are occasional stoppages while speaking.

Ataxic dysarthria: This is commonly seen with disorders of the cerebellum or its connections. Speech has a “scanning” quality or irregular rhythm with the explosion of syllables. Prosody is impaired, with each syllable being pronounced slowly, and there is a pause after every syllable. There is decreased motor coordination manifested by axial and appendicular ataxia depending on the part of the cerebellum affected.

Mixed dysarthria: Two or more central nervous system components are affected in this type. This can be seen with ALS and multiple sclerosis. Speech is slow, prosody is disrupted, voice is strained, and there is marked hypernasality.

Evaluation

A thorough history and a detailed physical examination are crucial in evaluating patients presenting with dysarthria. A comprehensive initial speech evaluation is comprised of (1) history, (2) oral motor/speech mechanism exam, (3) screening of subsystems (respiration, phonation, articulation, resonance, and prosody), (4) perceptual assessment, and (5) intelligibility evaluation.

The water glass manometer test provides a gross assessment of pressure-generating capabilities for speech production. The patient must blow into a water-filled drinking glass with the straw secured at a certain depth inside the glass. If the patient can maintain a stream of bubbles for 5 seconds, breath support is adequate for most speech purposes. To be valid, the patient must maintain velopharyngeal closure and a tight lip seal around the straw.[15]

Perpetual assessment of speech helps observe the function of all speech subsets. This assessment provides insight into deficits and serves as a comparison tool. Passages such as "my grandfather" and "caterpillar passage" help assess perceptual speech.[16][17] These passages help evaluate the speech repertoire, examine the subsystems of speech, contemporary vocabulary, and simple syntax, and assess polysyllabic word form.

The Caterpillar Passage

"Do you like amusement parks? Well, I sure do. To amuse myself, I went twice last spring. My most MEMORABLE moment was riding on the Caterpillar, which is a gigantic roller coaster high above the ground. When I saw how high the Caterpillar rose into the bright blue sky, I knew it was for me. After waiting in line for thirty minutes, I made it to the front, where the man measured my height to see if I was tall enough. I gave the man my coins, asked for change, and jumped on the cart. Tick, tick, tick, the Caterpillar climbed slowly up the tracks. It went SO high I could see the parking lot. Boy, was I SCARED! I thought to myself, "There's no turning back now." People were so scared they screamed as we zoomed fast and faster along the tracks. As quickly as it started, the Caterpillar came to a stop. Unfortunately, it was time to pack the car and drive home. That night I dreamt of the wild ride on the Caterpillar. Taking a trip to the amusement park and riding on the Caterpillar was my MOST memorable moment ever!" 

Speech Intelligibility Evaluation

Assessment of intelligibility in dysarthric speakers (AIDS), sentence intelligibility test (SIT), and word intelligibility test are used to assess speech intelligibility. AIDS is the most commonly used and comprises word and sentence tasks. The patient reads or imitates 50 unsystematically chosen words from 12 phonetically similar words for each word. In the sentence task, the patient reads or imitates two sentences each for 220 words. The sentences are chosen from a collection of 100 sentences of each length. The judge derives an intelligibility score based on the percentage of words transcribed accurately.

SIT is an improved Windows version of the sentence portion of AIDS. It examines the intelligibility of words and sentences and estimates efficiency by examining the rate of intelligible words per minute in sentences.

The onset and progression of dysarthria and associated neurological complaints, such as tremors, dysphagia, and gait instability, can provide clues for the diagnosis. A medication list review should include potential overdoses and exposure to toxins, such as alcohol and cocaine. Having the patient count from 1 to 100 can bring out respiratory muscle fatigue in myasthenia gravis. Counting numbers without interruption (1–30) is another bedside tool to assess respiratory status.[18] Sustaining an "ah" sound is a functional bedside test to assess laryngeal function.

Imaging techniques, such as computed tomography (CT) of the head and magnetic resonance imaging (MRI) of the brain, are helpful initial tools in evaluation. In patients suspected of neuromuscular junction disorders, electromyography (EMG) and nerve conduction studies (NCS) are indicated. CBC (complete blood count), basic metabolic profile (BMP), and urine drug screening are indicated based on history and pretest probability. If GBS is suspected, evaluation of pulmonary function (vital capacity and negative inspiratory force) is indicated.[18]

Speech Assessment Tools

Frenchay Dysarthria Assessment is one of the most commonly used commercially available tools for dysarthria assessment. It was initially devised in 1980 and revised in 2008. It incorporates a series of tasks used to identify the dysarthria subtype. Speech pathologists rate the patient's speech employing a 5-point scoring system on the following (a) reflexes, (b) respiration, (c) lips, (d)palate, (e) laryngeal, (f) tongue, and (g) intelligibility, (h) influencing factors.[19]

For subjective assessment, self-report questionnaires such as Living with Neurologically Based Speech Difficulties (Living with Dysarthria - LwD) may be used. The severity of dysarthria may not always correspond to the extent of perceived communicative difficulties.[20] 

Treatment / Management

The overall goals of speech and language treatment are to facilitate the recovery of communication, to assist patients in developing strategies to compensate for communication disorders, and to counsel and educate people in the patient's environment on assistive communication supports to facilitate communication, decrease isolation, and meet the patient's wants and needs.

When developing the plan, it is essential to consider dysarthria's cause, severity, and underlying co-morbidities. Speech-language pathologists and physicians work together to formulate an individualized approach for the patient. Recent studies indicate that speech rehabilitation significantly improves speech in adults with stroke-related dysarthria and that interventions such as Lee Silverman Voice Treatment are effective for hypokinetic dysarthria in individuals with PD.[21][22](A1)

Broadly, types of therapy could be grouped as follows:

• Therapy targeting the speech-production subsystems
• Communication strategies
• Environmental adaptations
• Augmentative and alternative communication (AAC)
• Medical/surgical interventions.

Targeting the Speech-Production Subsystems

Speech pathologists can target the five individual subcomponents that produce speech.

Lee Silverman Voice Treatment and Pitch Limiting Voice Treatment target phonation; the former is a program that improves loudness and intelligibility and has been widely studied in PD, while the latter increases vocal loudness without increasing pitch.[23][24] Articulation can be improved by increasing loudness, pausing, exaggerating articulation, and altering pitch variation. Respiratory muscle strength training supports breathing by altering posture to target respiration.

Communication Strategies

Providing feedback, clarifying, and encouraging are practical approaches for the partner. For the patient, setting up the conversation by gaining the partner's attention, slowing and repeating the phrases, and using nonverbal gestures, such as eye contact and facial expressions, are helpful strategies.

Speech supplementation, such as alphabet, syntactic, and topic, are helpful strategies. In alphabet supplementation, the speaker uses an alphabet board to identify the first letter of the spoken word. In topic supplementation, a phrase or cue word is uttered before speaking. Information about the grammar or word class is provided with each spoken word in the syntactic supplementation.

Behavioral communication intervention techniques such as biofeedback are shown to improve intelligibility. In stroke patients, biofeedback techniques increased the volume, decreased the speed, and improved the intelligibility.[25][26]

Environmental Adaptations

Setting up optimal environmental conditions to increase understandability includes ensuring a quiet conversation background, intimate seating, and face-to-face interaction.

Augmentative and Alternative Communication (AAC)

AAC may include low-tech aids, such as picture boards or pen and paper, or high-tech aids, such as smartphones, voice synthesizers, digital records, and speech-generating devices.[27] 

Computer-based interventions offer an exciting step toward dysarthria management. Some examples are:

1. A mobile application for PD patients. It includes an assessment of the speech in addition to various other aspects of disease management[28]
2. Feedback and individual practice using computers were as effective as traditional therapy in patients with stable dysarthria[29]
3. Improvement in articulation and intelligibility was assessed in Virtual articulation therapy[30]

These small studies hold good promise for further expansion of computer-based interventions.

Medical/Surgical Interventions

Medical therapy should target the underlying neurological cause. In dysarthria arising from PD, therapy should focus on repleting dopaminergic therapy. The effects of PD drugs on dysarthria are variable. Subthalamic nucleus stimulation has some efficacy among surgical treatments, but speech intelligibility worsens, as with most surgical interventions in PD.[31] Improving spasticity in ALS with baclofen, tizanidine, and botulinum toxin type A has been tried.[8]

Laryngoplasty is an option if hoarseness is associated with recurrent laryngeal nerve palsy and does not improve with conservative management. Palatal lift improves resonance by surgically lifting the weak palate. Since the 2019 COVID pandemic, telehealth has become more relevant than ever. Telerehab (TR) is less expensive and equally effective in improving functional stroke outcomes, including speech, compared to traditional rehab.[32] 

In another review of speech therapy administered via teletherapy in patients with PD, the patient reported overall increased satisfaction due to increased convenience and accessibility. Further research is needed, however, as the studies lacked double blinding, and there was heterogeneity in the protocol.[33]

Differential Diagnosis

The differential diagnoses for dysarthria include aphasia, apraxia of speech, and aphemia.

Apraxia, in general, is a dysfunction of a learned motor task. Patients need help initiating speech and transitioning between sounds; they speak through trial and error, and their errors are inconsistent. When patients are asked to repeat a phrase exactly, especially a polysyllabic word (eg, television), different errors are noted in each attempt. Isolated apraxia of speech is rare; it is usually seen with aphasia. Imaging might show lesions in the dominant insula and Broca’s area.[34]

Aphasia is a language impairment. Depending on the location, language comprehension or production may be impaired. Patients also have difficulty writing and reading.[35]

Aphemia is a motor speech disorder resulting in near muteness. Patients have normal comprehension, reading, and writing.

Prognosis

Dysarthria is described as chronic if persistent for greater than 5 years. Dysarthia is considered stable in patients with nonprogressive etiologies.

Recovery also appears to be dependent on the etiology. One study evaluating dysarthria following stroke showed recovery in about half of the patients.[36] No estimates of the long-term prognosis of various diseases are available. However, from various anecdotal reports, it is evident that dysarthria is progressive in most neurodegenerative diseases.

Functional Communication Measures (FCMs) refer to rating scales used to define an individual’s functional abilities. They are seven-point rating scales, ranging from least functional (Level 1) to most functional (Level 7). They help measure a patient’s functional communication and swallowing abilities throughout speech-language pathology intervention.

Complications

Speech difficulty can have a significant impact on the patient’s psychosocial life. Patients report stigmatization, changes in self-identity, and social and emotional disturbances due to post-stroke dysarthria. In children, behavioral problems and lack of access to education can lead to decreased future employment opportunities. Tools such as the Dysarthria Impact Profile (DIP) are available to assess the psychosocial impact of dysarthria.[37] Given the possibility of far-reaching effects, timely intervention should be the goal.

Dysarthria Impact Profile is used to assess the psychosocial impact of dysarthria.[38] Patients report how their condition hinders them in multiple circumstances, such as talking to people they do not know and ordering a meal in a restaurant. The tool is used for outcome measurement and for planning interventions. Communicative Participation Item Bank (CPIB) is a self-reported tool designed for adults with various communication disorders; this tool has clinical and research applications.[39] Patients report how their condition hinders them in multiple circumstances, such as talking to people they do not know and ordering a meal in a restaurant.