DYSARTHRIA
Difficulty in performing the coordinated muscular movements necessary for the articulation of speech; it is caused by anatomical or functional alterations of the effector organs themselves (lips, tongue, palate, pharynx, larynx) or of their central or peripheral innervation apparatus, which results in paralysis, atrophy, spasm or ataxia of the phonatory muscles. Internal language remains normal, so much so that the dysarthric person can express themselves fluently in writing (of course when this is not hindered by paralysis of the right hand). Varying in intensity, dysarthria can reach the point of making speech impossible (anarthria).
The phonatory organs receive their innervation from the facial, glossopharyngeal, vagus-spinal, and hypoglossal nerves, all of which have their origin in the medullary nuclei. These bulbar nuclei receive motor nerve fibers from the operculum of Rolando via the internal capsule, the cerebral peduncle, and the pons. The innervation is bilateral, that is, each nucleus receives fibers from both hemispheres; only the nucleus of the lower facial nerve receives predominantly contralateral fibers. The speech-motor pathway can therefore be distinguished into a first cortico-bulbar neuron and a second bulbo-muscular neuron; any lesion of the first or second can determine dysarthria.
For supranuclear lesions, dysarthria is usually caused by bilateral foci; this is explained by the bilateral nature of the innervation of the phonatory organs. Unilateral lesions can cause transient dysarthria. Dysarthria can also be caused by ataxia of the phonatory muscles.
The distinction between dysarthria (and anarthria) and motor aphasia (v. APHASIA) is made on the basis of the following differential data: in the dysarthric person, internal language is well preserved, hence the ability to express themselves fluently in writing and to understand speech and writing perfectly; in the aphasic person, the cortical moment of language is affected, so that disturbances in the comprehension of spoken and written language frequently occur. The dysarthric person mispronounces all words; singing does not improve their articulation in any way; the aphasic person pronounces some words well and not at all others, of which they have lost the verbal image; often in singing they pronounce words correctly that they are then unable to repeat in speech; the same happens with automatized words (invocations, imprecations). The aphasic person cannot speak, the anarthric person can no longer speak (J. Déjérine).
In dysarthria, the difficulty in articulation concerns especially consonants: labial, palatal, and dental, depending on the affected phonatory organ (facial paralysis, bilateral paralysis of the soft palate, of the tongue); the lesion of the inferior laryngeal nerve, which causes paralysis of the homolateral vocal cord, makes the pronunciation of vowels difficult. In addition to these lesions of individual nerves, certain diseases and syndromes in which dysarthria is a predominant symptom must be considered. 1) Nuclear lesions: labio-glosso-laryngeal paralysis or chronic progressive bulbar paralysis, alone or as part of amyotrophic lateral sclerosis; speech is slow, lacking modulation, with a predominant disturbance in the pronunciation of lingual and labial sounds. 2) Supranuclear lesions: a) progressive paralysis (v. DEMENTIA PARALYTICA PROGRESSIVA) in which speech is slurred, sometimes tending toward scanning with characteristic transposition of syllables; in advanced cases, speech is sometimes unintelligible, reduced almost to vowels alone; b) pseudo-bulbar paralysis, from multiple bilateral cerebral lesions with dysarthria similar to that of bulbar paralysis, but less severe, with a weak, nasalized voice lacking modulation. 3) Lesions of the basal nuclei, even without involvement of the cortico-nuclear pathway (Parkinson’s disease, Wilson’s disease, status marmoratus, chorea, etc.); in the most common form, Parkinson’s disease, the voice is monotonous and words are often pronounced in a progressively accelerated manner. 4) Ataxia of the phonatory muscles (which may be found in multiple sclerosis, hereditary spinocerebellar ataxia, and various cerebellar affections), with scanning, sometimes explosive speech. 5) Myasthenia gravis, in which the first words are well articulated, but as soon as fatigue sets in, speech becomes weak, aphonic, and indistinct. 6) Myopathy (facio-scapulohumeral type): in severe cases, the pronunciation of labial sounds is difficult, as in bilateral facial paralysis.