Clinical manifestations

Presentation and course

	• Patients with dysarthria have difficulty with coordinated movements required for speech although construction and use of proper words is preserved.
	• Dysarthria can be due to several lesions in the nervous system, and the characteristics vary.
	• One of the characteristics of dysarthria in cerebellar lesions is slow, slurred speech, also referred to as speech ataxia.
	• Prognosis of dysarthria depends on the nature of the lesion causing it.

In dysarthria, the utterance of sounds is imperfect. Proper construction and use of words is preserved but enunciation of words is faulty due to difficulty in performing the coordinated movement required for speech. However, in progressive bulbar palsy with multiple cranial nerve involvement both articulation and phonation may be affected. Clinical manifestation depends on the cause of dysarthria and associated diseases. In pseudobulbar palsy due to bilateral cortical lesions, the speech is thick and referred to as "bulbar speech." There is spasticity of muscles of articulation, but it rarely progresses to anarthria. In movement disorders, movements of the muscles of the face and tongue may make the speech jerky. Regardless of the cause, the most important effect of dysarthria is a reduction in speech intelligibility. Application of the World Health Organization's International Classification of Functioning, Disability and Health facilitates a broad understanding of the multidimensional and complex nature of dysarthria as well as the neuroanatomical and physiological basis of reduced speech intelligibility. Manifestations of dysarthria in some diseases are described here briefly.

Cerebellar diseases. Dysarthria in cerebellar lesions is also referred to as speech ataxia and has the following characteristics:

(1) Slow and slurred speech with prolongation of individual speech sounds (2) Sudden uncontrolled alterations in loudness of voice (3) Consonants become imprecise and vowels are distorted (4) Voice tremor

Stroke. Dysarthria is a frequent sign of cerebral ischemia, ranging from 8% to 12.4% in large unselected stroke series. Dysarthria without evidence of aphasia may be the first sign of a stroke in evolution, particularly when it involves the vertebrobasilar system.

Neurodegenerative disorders. Dysarthria is usually 1 of many manifestations of neurodegenerative disorders. Progressive dysarthria can rarely present as the single initial manifestation of a neurodegenerative condition involving posterior inferior frontal lobe structures, mainly in the dominant cerebral hemisphere.

Multiple sclerosis. The speech is scanning with slowness, halting, and ataxia of the cerebellar type. The sing-song speech of multiple sclerosis is characteristic. Midbrain lesions have been associated with paroxysmal dysarthria in multiple sclerosis (03). Quantitative acoustic assessments are biomarkers of multiple sclerosis and can be used as clinical endpoints for diagnosing, monitoring progression, and treatment in disease-modifying clinical trials (30).

Pompe disease. Dysarthria due to lingual weakness is a frequent finding in late-onset Pompe disease with bulbar involvement (20).

Parkinson disease. Approximately 90% of patients with Parkinson disease present with hypokinetic dysarthria manifested by reduced vocal loudness, monotone, reduced fundamental frequency range, consonant/vowel imprecision, and irregular pauses. The speech is slow and slurred because of muscular rigidity. Dysarthria may be due to lingual dyskinesia in Parkinson disease.

Lesions of cranial nerves. Dysarthria is unlikely with unilateral involvement of trigeminal nerve, but bilateral lesions may cause it with a bulbar characteristic of speech. In peripheral facial nerve paralysis, there is difficulty in pronouncing labials. Lingual sounds are difficult to pronounce with lesions of the hypoglossal nerve and speech may be lisping and difficult to understand. Multiple cranial nerves are involved in pseudobulbar palsy, and dysarthria may be accompanied by difficulty in phonation.

Myasthenia gravis. Prolonged speech may cause dysarthria with a decrease in volume that may end up in anarthria.

Drug-induced dysarthria. This may be a part of tardive dystonia. Some chemotherapeutic agents may be associated with dysarthria. There is a report of 4 patients who developed acute dysarthria while receiving intravenous infusion of irinotecan (13). In all cases, dysarthria occurred during the infusion of the first course of irinotecan and then resolved rapidly without any sequelae.

Raynaud phenomenon involving the tongue. Dysarthria can occur during episodes of lingual ischemia and responds to dihydropyridine calcium-channel blockers (07).

Prognosis and complications

Prognosis of dysarthria depends on the nature of the lesion causing it. In degenerative disorders with progression of lesions, prospects for recovery from dysarthria or improvement by speech therapy are poor. In dysarthria due to cerebrovascular ischemia, the prognosis is better in cerebral lesions than in brainstem lesions. In Parkinson disease, excellent prediction of the severity of hypokinetic dysarthria was made using machine learning based on MRI, which showed atrophy of the right precentral cortex and the right fusiform gyrus (08).

Mild to severe dysarthria in children with motor impairments may involve tone, strength, and coordination of any or all the muscles used for speech. The outlook for spontaneous recovery is poor.

Biological basis

Anatomic localization

	• Dysarthria can be due to lesions in the cortical motor-face area, cerebellum, brainstem, or cranial nerves or the innervation of muscles for articulation.
	• Dysarthria can occur in several diseases, both neurologic and nonneurologic.
	• Dysarthria can be drug-induced or a result of exposure to environmental toxins.

Dysarthria can be due to a range of causes that include lesions in the cortical motor-face area, cerebellum, cranial nerves innervating muscles of articulation, and local lesions of the structures involved in articulation of speech. In a man who presented with progressive dysarthria, dysphagia, and ataxic gait, MRI revealed bilateral ventrolateral pontine infarction with a large atherosclerotic plaque in the ventral side of the basilar artery (01). Pure dysarthria can occur due to cortical stroke at the precentral gyrus or middle frontal gyrus. A case of dysarthria without aphasia has been reported to be caused by an acute cortical infarction in the insular cortex (19). Articulation may be affected by lesions of the basal ganglia.

Analysis of voxel-based lesion symptom mapping in patients with basal ganglia hemorrhage has identified several voxel clusters in the pulvinar nucleus of the left thalamus that were significantly related to the presence of dysarthria at admission (22). A prospective study of speech characteristics of dysarthria due to acute unilateral cerebral infarctions showed that left cerebral hemispheric lesions show a more severe overall impairment of speech and articulation. MRI studies indicate that cerebellar dysarthria may be due to damage to lobulus simplex and lobulus quadrangularis of the upper cerebellum (31).

Articulatory movements of the tongue and orofacial muscles are involved in the activation of the rostral paravermal area of the anterior lobe of the cerebellum. Lesions in the upper paravermal area of the right cerebellar hemisphere, the site of coordination of articulatory movements of the tongue and orofacial muscles, may lead to the development of dysarthria. Neural network models and neuroimaging studies in ataxic dysarthria indicate that bilateral superior areas of the cerebellum appear to mediate speech motor control.

Postoperative follow-up for up to 10 years in children who suffered dysarthria following posterior fossa surgery has shown that poorer outcomes were associated with right cerebellar tumors compared to left, which is consistent with the concept that speech is controlled by reciprocal right cerebellar/left frontal interactions, based on studies in adult patients (29).

Several cranial nerves are involved in articulation of speech. These include:

• Trigeminal nerve, which innervates the muscles of mastication • Facial nerve, which supplies the facial muscles • Hypoglossal nerve, which supplies the tongue

Lesions of these nerves, 1 or more, can produce dysarthria.

Pathophysiology

Dysarthria can be a manifestation of several disorders, both neurologic and systemic, as listed in Table 1. There is an overlap of localization as well as pathology.

Table 1. Causes of Dysarthria

Neurologic disorders with dysarthria as a symptom:
	• Stroke: cerebrovascular ischemic disorders
				- Cerebellar infarction - Lacunar infarction - Subcortical ischemic vascular dementia - Vertebrobasilar ischemia
	• Neurodegenerative disorders
				- Amyotrophic lateral sclerosis - Corticobasal degeneration - Frontotemporal dementia - Huntington disease - Progressive supranuclear palsy - Wilson disease
	• Traumatic brain injury
	• Complications of surgery for posterior fossa tumors
				- Dysarthria - Transient cerebellar mutism following surgery on cerebellum
	• Encephalitis
				- Encephalitis lethargica - Herpes simplex encephalitis
	• Cerebral palsy
	• Cerebellar disorders
				- Ataxia telangiectasia - Friedrich ataxia - Olivopontocerebellar atrophy - Paraneoplastic cerebellar degeneration - Spinocerebellar ataxias
	• Brainstem disorders
				- Basilar-type migraine - Brainstem encephalitis - Brainstem tumors - Central pontine myelinolysis - Midbrain hemorrhage
	• Movement disorders
				- Parkinson disease - Oromandibular dystonia - Sydenham chorea
	• Epileptic disorders
				- Lingual epilepsia partialis continua - Congenital bilateral perisylvian syndrome with partial epilepsy - Benign childhood epilepsy with centrotemporal spikes
	• Demyelinating diseases
				- Multiple sclerosis - Pelizaeus-Merzbacher disease
	• Myopathies
				- Myasthenia gravis - Polymyositis - Muscular dystrophy involving facial muscles
	• Cranial nerve lesions
				- Aneurysms of extracranial internal carotid artery
Dysarthria in metabolic diseases with neurologic manifestations:
			• 2-Hydroxyglutaric aciduria • Ornithine transcarbamylase deficiency
Dysarthria as part of syndromes with other manifestations:
		• Fabry disease • Hereditary spastic paraparesis • Di George syndrome or 22q11.2 deletion syndrome (35) • Dysarthria-facial paresis syndrome • Dysarthria-clumsy hand syndrome • Moebius syndrome • Niemann-Pick disease • Pompe disease • Raynaud syndrome involving the tongue • SANDO (sensory ataxic neuropathy, dysarthria, and ophthalmoplegia)
Drug-induced dysarthria:
		• Antiepileptic drugs: phenytoin • Anticancer agents: irinotecan (41) • Atropine overdose during minor procedures • Benzodiazepines • Lithium neurotoxicity • Neuroleptic drugs • Paroxetine, a selective serotonin reuptake inhibitor
Exposure to toxins and metals:
		• Mercury poisoning • Botulism • Nerve agent poisoning

Epidemiology

• True overall incidence of dysarthria is not known, but some approximate figures are available for individual neurologic disorders.

Although dysarthria is present in many neurologic diseases, its true incidence and prevalence are not fully known. Estimates of dysarthria in neurologic disorders include:

	• It is estimated that 8% to 60% of individuals with stroke present with dysarthria.
	• Approximately 10% to 65% of individuals with traumatic brain injury have dysarthria.
	• Dysarthria affects approximately 70% to 100% of individuals with Parkinson disease.
	• Between 25% and 50% of individuals with multiple sclerosis present with dysarthria at some point during the course of their disease.
	• Dysarthria can be observed as an initial sign in up to 30% of patients with amyotrophic lateral sclerosis.

Differential diagnosis

Confusing conditions

Dysarthria can be confused with the following conditions:

	• Dysarthria should be differentiated from other disorders of articulation, for example, difficulty in articulation due to lesions in the oral cavity, such as from primary disorders of the tongue, cleft palate, and surgical procedures in the oral cavity.
	• Dysphasia should be differentiated from dysarthria, as both may occur in 76% to 90% of patients with neuromuscular disease and stroke (39). Diffusion-weighted imaging shows that cortical involvement is more frequent in patients with pure dysarthria than in those with dysarthria and additional neurologic signs, and that pontine involvement is more frequent in patients with additional neurologic signs than in those with pure dysarthria.
	• Apraxia of speech is a motor speech disorder that can occur in the absence of aphasia or dysarthria. It is defined in cognitive terms as an impairment in the translation of phonological representations into specifications for articulation. It should be differentiated from phonological and dysarthric disorders.
	• Neurogenic foreign accent syndrome, in which changes of segmental as well as suprasegmental aspects lead to the perception of a foreign accent in speech. It may occur early in neurodegenerative disorders where aphasia as well as dysarthria may develop later. The autosegmental-metrical method is useful for intonation analysis of motor speech disorders in Parkinson disease, ataxic dysarthria, and foreign accent syndrome (26).
	• Impairment of speech in patients with Parkinson disease resembles that seen in the normal aging process, except prosody and the habitual frequency (25).
	• Dysphonia and aphonia due to vocal cord lesions. Unilateral lesions may not have any appreciable effect on voice. The voice may be hoarse and lost altogether as in bilateral total palsy.

Dysarthria is further differentiated according to the localization of lesion and possible pathology. Because of multiple causes affecting several components of motor speech, differential diagnosis of dysarthria is difficult. Perceptual analysis of dysarthria does not help in the clinical differential diagnosis of corticobasal degeneration and Parkinson disease. Dysarthria is a cardinal feature of Friedreich ataxia and has a distinctive phonetic profile but neurologic findings may vary across individuals depending on the extent of involvement of cerebrocerebellar loops. Various components of speech production and trunk/limb motor functions are differentially susceptible to the pathology of Friedreich ataxia (05).

The syndrome of cerebellar mutism and subsequent dysarthria is known to develop after surgery on the cerebellum. In 1 study, approximately 98.8% of the children displayed motor speech deficits during the recovery phase of speech (14). In a retrospective study of children following posterior fossa surgery, incidence of postsurgical mutism at the time of discharge from hospital was reported to be 33%, and of dysarthria, 30% (28).

A case of dysarthria due to compression of the hypoglossal nerve by an extracranial internal carotid artery aneurysm has been reported, which resolved after internal carotid artery ligation (12).

Drug-induced dysarthria may be a manifestation of extrapyramidal syndrome as suggested in a case report of dysarthria following treatment with paroxetine in a 42-year-old man with pathological laughter resulting from traumatic brain injury (32). Dysarthria disappeared following discontinuation of paroxetine. A mixed hyperkinetic-hypokinetic dysarthria has been reported in ephedrone-induced parkinsonism and is associated with marked dystonia as well as bradykinesia; it is likely due to manganese-induced damage to the basal ganglia (36).

Irinotecan-induced dysarthria in a man being treated for colon cancer resolved with the discontinuation of irinotecan (23). Recurrence could be prevented in this case by modifying the cycles of chemotherapy and prolonging the duration of intravenous administration of the drug. A case of acute dysarthria and delirium with suspected stroke has been reported after a dental procedure with an overdose of sublingual atropine due to miscalculation, but intravenous physostigmine completely reversed the symptoms, confirming atropine toxicity as the cause as well as providing an effective therapy (10).

Although an attempt is usually made to place a patient with dysarthria in 1 of the 6 categories, combinations of these are also common. In motor neuron disease, there may be a combination of upper and lower motor neuron lesions. In multiple sclerosis, articulatory muscles may be both spastic and ataxic. A survey shows that it is unreliable to identify the dysarthria type without the use of additional validated instruments or rating scales (38).

Diagnostic workup

Key tests and procedures

	• General assessment of spontaneous speech during conversation
	• Special testing of speech
	• Assessment of muscles involved in articulation, eg, electromagnetic articulography
	• Quantitative acoustic analysis
	• Diffusion-weighted MRI tractography of speech-related motor pathways

The initial examination of articulation involves assessment of spontaneous speech during conversation. Further special testing of speech involves repetition of test words and phrases and the reading of printed matter aloud.

A tongue pressure transducer system can be used to assess tongue strength, endurance, fine pressure control, and rate of repetitive movement. Tongue-jaw coordination during speech can be studied by electromagnetic articulography.

Quantitative acoustic analysis is used to identify hypokinetic dysarthria in Parkinson disease. Acoustic analysis of the voice dysfunction in dysarthria following stroke has shown significantly larger smoothed pitch perturbation quotient and soft phonation index as compared to healthy young women (40). This finding suggests that a combination of acoustic analysis with perceptual analysis will better illuminate the voice dysfunction in dysarthria following stroke.

Assessment of dysarthria over the Internet is possible using both standardized and informal methods on an Internet-based telerehabilitation system (18).

Diagnostic workup of dysarthria has 2 objectives: anatomical localization of the lesion and identification of the disease process. Diagnostic procedures are selected according to the suspected causes.

Diffusion-weighted MRI tractography of speech-related motor pathways is a useful prognostic tool for dysarthria, as an intact left dorsal corticobulbar tract remains crucial for the normal execution of speech following acquired injury (24).

Management

	• Compensatory speech and language strategies are undertaken in dysarthria as a sequel of nonprogressive or recoverable brain damage, such as in stroke and traumatic brain injury.
	• Deep brain stimulation of the subthalamic nucleus is used for patients with Parkinson disease who have failed medical management.
	• A supplemented speech recognition system for speakers with dysarthria incorporates automatic speech recognition.

Impairment-based therapy and a wide variety of compensatory management strategies are undertaken by speech and language therapists in dysarthria as a sequel of nonprogressive brain damage such as in stroke and traumatic brain injury.

Traumatic brain injury. Speech therapy for dysarthria is usually beneficial in patients recovering from traumatic brain injury. Such therapy could be limited by cognitive deficits. Treatment of dysarthria can provide short-term benefits for speech production during the late stages following traumatic brain injury. A review of interventions for adults with dysarthria following traumatic brain injury showed that they fall into 6 broad categories – behavioral, prosthetic, instrumental, pharmacological, augmentative and alternative communication, and mixed interventions; most of these focused on behavioral interventions (16).

Stroke. Speech supplementation strategies are effective for poststroke dysarthria. Drug therapy has been used in dysarthria due to neurologic disorders. Lamotrigine therapy for paroxysmal dysarthria caused by multiple sclerosis has been reported (37).

Results of a study suggest that persons with dysarthria secondary to stroke respond positively to intensive speech treatments, such as Lee Silverman Voice Treatment, by improvement in voice quality and a more natural speech (27). A systematic metaanalysis of various studies identified methods for measuring the severity of poststroke dysarthria, eg, acoustic parameters affected in dysarthria secondary to stroke and the differences in these parameters after speech therapy (09). The alternating and sequential motion rate and maximum phonation time were significantly improved after speech rehabilitation.

Multiple sclerosis. Levetiracetam has been reported to improve paroxysmal ataxia and dysarthria in multiple sclerosis (17).

Parkinson disease. Use of levodopa seems to improve articulation, voice quality, and pitch variation in dysarthria of Parkinson disease, although some studies show no change in parameters of phonation. Speech therapy can have substantial beneficial effects on dysarthria of Parkinson disease. A combination of behavioral speech therapy, specifically the Lee Silverman Voice Treatment, in medically managed patients with Parkinson disease is currently the most effective type of speech intervention. A fMRI study revealed cortical hypoactivation as a correlate of hypokinetic dysarthria in patients with Parkinson disease and showed increased activation during high- versus normal-intensity speech generation following effective Lee Silverman Voice Treatment (02).

For patients with Parkinson disease who have failed medical management, deep brain stimulation of subthalamic nucleus is currently the most common surgical treatment. Active contact placement within the anterior sensorimotor subthalamic nucleus improves airflow for speech, which in turn, improves how the patients’ voices sound (21).

Management of dysarthria in children. A systematic review found no evidence from randomized trials of the effectiveness of speech and language therapy for improving the speech of children with early acquired dysarthria (34). Lee Silverman Voice Treatment, an intensive voice treatment based on principles of neuroplasticity, has been used in children with dysarthria secondary to cerebral palsy (04). Individual and environmental factors can influence immediate and lasting responses to treatment, which may be weak or strong. Strong responders appear to benefit from the intensity of treatment whereas weak responders may require a longer duration of treatment.

Role of a speech recognition system. Supplemented Speech Recognition system for speakers with dysarthria incorporates automatic speech recognition, which is optimized for dysarthric speech, alphabet supplementation, and word prediction (15). It was reported to achieve significant keystroke saving for typical sentences and atypical phrases.