Definition

Duffy defines mutism traditionally as “the absence of speech” (1995, p. 282). Von Cramon (1981) adds the inability to produce nonverbal utterances. Lebrun (1990) requires normal or relatively preserved comprehension. Gelabert-Gonzalez and Fernandez-Villa (2001) require “unimpaired consciousness” (p. 111). Each of these definitions has strengths, but none dominates the literature. Therefore, the literature can be a bit of a muddle. The literature is also challenging because of the mutism population's heterogeneity. One response to this heterogeneity has been to classify mutism according to relatively homogeneous subtypes.

Traditionally, groupings of mute patients have been organized according to the putative pathophysiology (Turkstra and Bayles, 1992), by syndrome, by etiology, or by a mix of syndrome and medical etiology (Lebrun, 1990; Duffy, 1995). This last approach guides the organization of the following discussion.

Akinetic Mutism

Akinetic mutism (AM) is a syndrome of speechlessness and general akinesia that exists in the context of residual sensory, motor, and at least some cognitive integrity and a normal level of arousal. The designation abulic state may be a synonym (Duffy, 1995), as are apallic state and coma vigil. Persons with AM often are silent, despite pain or threat. Bilateral and occasionally unilateral left or right anterior cerebral artery occlusion with involvement of the anterior cingulate gyrus or supplementary motor area is frequently implicated (Nicolai, van Putten, and Tavy, 2001). Recent data suggest that the critical areas are the portions of the medial frontal lobes immediately anterior to the supplementary motor area and portions of the anterior cingulate gyrus above the most anterior body of the corpus callosum. These regions appear to be involved in gating intention (plans of action) (Picard and Strick, 1996; Cohen et al., 1999). Lesions of the globus pallidus, thalamus, and other subcortical structures can also result in AM. Schiff and Plum (2000) advocate for a companion syndrome of “hyperkinetic mutism” resulting most frequently from bilateral temporal, parietal, and occipital junction involvement in which the patient is speechless but moving. The cause may be any nervous system–altering condition, including degenerative diseases such as Creutzfeldt-Jacob disease (Otto et al., 1998), that alters what Schiff and Plum (2000) posit to be a series of corticostriatopallidal-thalamocortical (CSPTC) loops. CSPTC loops are critical to triggering or initiating vocalization (Mega and Cohenour, 1997) and to the drive or will to speak. AM is to be differentiated from persistent vegetative state, which reflects extensive damage to all cerebral structures, most critically the thalamus, with preservation of brainstem function (Kinney et al., 1994).

Mutism in Aphasia

Mutism can be a feature of severe global aphasia. Patients with severe anomia, most often in relation to thalamic lesions, may initially exhibit no capacity for spontaneous language and little for naming, but they can repeat. Certain types of transcortical motor aphasia (Alexander, Benson, and Stuss, 1989), most particularly the adynamic aphasia of Luria (1970), may be associated with complete absence of spontaneous speech. However, these patients are reasonably fluent during picture description, and this syndrome likely reflects a prelinguistic disorder involving defective spontaneous engagement of concept representations (Gold et al., 1997).

Mutism in Apraxia

Immediately after stroke, a profound apraxia of speech (called aphemia and by a variety of other names in the world's literature) can cause mutism, as can primary progressive apraxia of speech. In the acute stage of stroke, the mutism is thought to signal an apraxia of phonation. The hypothesis is that mutism due to apraxia reflects a profound failure of motor programming.

Mutism in Dysarthria

Mutism can be the final stage of dysarthria (anarthria) in degenerative diseases such as amyotrophic lateral sclerosis, slowly progressive anarthria (Broussolle et al., 1996), olivopontocerebellar atrophy, Parkinson's disease, Shy-Drager syndrome, striatonigral degeneration, and progressive supranuclear palsy (Nath et al., 2001). Speech movements are impossible because of upper and lower motor neuron destruction. Cognitive changes may hasten the mutism in some of these degenerative diseases. Anarthric mutism can be present at onset and chronically in locked-in syndrome (Plum and Posner, 1966) and the syndrome of bilateral infarction of opercular motor cortex. Relatively recently, a syndrome beginning with mutism and evolving to dysarthria, called temporary mutism followed by dysarthria (TMFD) (Orefice et al., 1999) or mutism and subsequent dysarthria syndrome (MSD) (Dunwoody, Alsagoff, and Yuan, 1997), has been described. Pontomesencephalic stroke is one cause.

Mutism in Dementia

Mutism has been reported in Alzheimer's disease, cerebrovascular dementia, and most frequently and perhaps earliest in frontotemporal dementia (Bathgate et al., 2001). It can also occur in other corticosubcortical degenerative diseases, including corticobasal degeneration. Its occurrence in the late stages of these conditions is predictable, based on the hierarchical organization of cognitive, linguistic, and speech processes. When cognitive processes are absent or severely degraded, speech does not occur.

Mutism Post Surgery

Mutism can occur after neurosurgery (Pollack, 1997; Siffert et al., 2000). So-called cerebellar mutism can result from posterior fossa surgery, for example. It is hypothesized that disruption of connections between the cerebellum, thalamus, and supplementary motor area causes impaired triggering of vocalization (Gelabert-Gonzalez and Fernandez-Villa, 2001). Mutism may also occur after callosotomy (Sussman et al., 1983), perhaps because of damage to frontal lobe structures and the cingulate gyrus.

Mutism in Traumatic Brain Injury

Mutism is frequent in traumatic brain injury. Von Cramon (1981) called it the “traumatic midbrain syndrome.” He speculated that the mechanism is “temporary inhibition of neural activity within the brain stem vocalization center” (p. 804) within the pontomesencephalic area. Often mutism is followed by a period of whispered speech in this population.

Summary

Speech depends on myriad general and specific cognitive, motor, and linguistic processes. These processes are widely distributed in the nervous system. However, the frontal lobes and their connections to subcortical and brainstem structures are the most critical. Mutism, therefore, is common, but not inevitable, as an early, late, or chronic sign of damage, regardless of type, to these mechanisms.