Speech disorders may be classified according to their causes or symptoms. The major causes are organic, imitative environmental, and psychogenic. Organic disorders may result from disease, impairment, or absence of the organs of speech. Imitative disorders occur when the child imitates defective speech. A speech disorder has a psychogenic origin when there is a psychological basis for its presence. A classification includes disorders of articulation, rhythm, voice, and symbolization.

Disorders of articulation may be so severe that the resultant speech is unintelligible. Specific sounds or groups of sounds may be omitted, added, substituted, or distorted. The following are some examples of this type of disorder: Lalling, involving misarticulated r, l, t, and d sounds, may be caused by poor control of the tongue tip. In lisping, misarticulated sibilant sounds, particularly s and z, are often substituted by the th sound. Delayed speech, involving the absence of many consonants and poor intelligibility, is often caused by slow physical or psychological maturation. Dysarthria, generalized sound substitutions and distortions, is caused by lesions in the peripheral or central nervous system.

Disorders of rhythm are characterized by disruptions of the normal rate of speech. Two common disorders of rhythm are stuttering or stammering and cluttering.

Voice disorders are usually described as defects of pitch, loudness, and voice quality. Improper use of the voice may cause an injury to the vocal folds and intensify an existing voice disorder. Disorders of pitch are characterized as too high, too low, monotonous, and repeated pitch patterns. A high-pitched voice is most often caused by psychological tension. The muscles of the larynx are contracted so that the pitch is raised beyond its normal range. Voice quality disorders are usually described as hoarseness, nasality, denasality, and similar conditions. Hoarseness may be caused by pharyngeal or laryngeal pathologies. Cleft palate speech is a striking example of nasality. Nasal speech may also follow a paralysis of the palatal muscles. The nasal speech results because the palate is unable to aid in achieving nasopharyngeal closure.

Symbolization disorders involve an impairment of language formulation and expression. They may occur without a concomitant impairment of speech production. Common types of these disorders are aphasia and delayed speech. See also Speech.

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