Stammering and stuttering are difficult to classify. In some respects they belong to and are customarily included in the developmental language disorders, but they differ in being largely centered in articulation. There is no valid reason to distinguish between these two forms of the inarticulation, as they are intermingled, and the terms stammer and stutter are now used synonymously. Essentially they represent a disorder of rhythm—an involuntary, repetitive prolongation of speech because of an insuppressible spasm of the articulatory muscles. The spasm may be tonic and result in a complete blocking of speech (at one time referred to specifically as stammering) or clonic speech, that is, a rapid series of spasms interrupting the emission of consonants, usually the first letter or syllable of a word (stuttering). Certain sounds, particularly p and b, offer greater difficulty than others; paperboy comes out p-p-paper b-b-boy. The problem is usually not apparent when single words are being spoken and dysfluency tends to be worse at the beginning of a sentence or an idea. The severity of the stutter is increased by excitement and stress, as when speaking before others, and is reduced when the stutterer is relaxed and alone or when singing in a chorus. When severe, the spasms may overflow into other groups of muscles, mainly of the face and neck and even of the arms. The muscles involved in stuttering show no fault in actions other than speaking, and all gnostic and semantic aspects of receptive language are intact.
Males are affected four times as often as females. The time of onset of stuttering is mainly at two periods in life: between 2 and 4 years of age, when speech and language are evolving, and between 6 and 8 years of age, when these functions extend to reciting and reading aloud in the classroom. However, there may be a later onset. Many afflicted children have an associated difficulty in reading and writing. If stuttering is mild, it tends to develop or to be present only during periods of emotional stress, and in 4 of 5 children it disappears entirely or almost so during adolescence or the early adult years (Andrews and Harris). If severe, it persists throughout life regardless of treatment but tends to improve as the patient grows older.
Theories of causation are legion, attesting to a lack of actual explanation. Slowness in developing hand and eye preference, ambidexterity, or an enforced change from leftto right-hand use have been popular explanations, of which Orton and Travis were leading advocates. According to their theory, stuttering results from a lack of the necessary degree of unilateral control in the synchronization of bilaterally innervated speech mechanisms. Fox and colleagues support a theory of failure of left hemisphere dominance. By performing PET studies while a subject was reading, they found that the auditory and motor areas of the right hemisphere are activated instead of those of the left hemisphere. However, these explanations probably apply to only a minority of stutterers (Hécaen and de Ajuriaguerra). It is of interest that stutterers activate the motor cortex prematurely when reading words aloud and, as noted by Sandak and Fiez, affected individuals seem to initiate motor programs before the articulatory code is prepared. More recently, several groups have reported subtle structural anomalies in the gray matter of the perisylvian region, but no common theme has emerged, and others are skeptical of these findings (see editorial by Packman and Onslow). It has been commented in the literature on this subject that speech production is a highly distributed system and that compensatory mechanisms used by stutterers may confound interpretation of functional imaging studies. It is likely that abnormalities in basal ganglia structures and in thalamocortical pathways impair the initiation and termination phases of articulation, and it is interesting that deep brain stimulation for other indications can impact the severity of stuttering, both for better and worse (see Craig-McQuaide et al).