Developmental Apraxia of Speech

Developmental apraxia of speech (DAS) is a devel-opmental speech disorder frequently deﬁned as di‰culty in programming of sequential speech movements based on presumed underlying neurological di¤erences. Theo-retical constructs motivating understanding of DAS have been quite diverse. Motor-based or pre-motor planning speech output deﬁcits (e.g., Hall, Jordon, and Developmental Apraxia of Speech 121

Robin, 1993), phonologically based deficits in represen-tation (e.g., Velleman and Strand, 1993), or deficits in neural tissue with organizational consequences (e.g., Crary, 1984; Sussman, 1988) have been posited. Reflect-ing these varied views of causality, a variety of terms have been employed: developmental apraxia of speech, developmental verbal dyspraxia, and developmental articulatory dyspraxia. Clinically, DAS has most often been defined by exclusion from functional speech disor-der or delay using a complex of behavioral symptoms (e.g., Stackhouse, 1992; Shriberg, Aram, and Kwiat-kowski, 1997).

The characterization of DAS was originally derived from apraxia of speech in adults, a disorder category based on acquired brain damage resulting in di‰culty in programming speech movements (Broca, 1861). Morley, Court, and Miller (1954) ﬁrst applied the term dyspraxia to children based on a proposed similarity in behavioral correlates with adult apraxic symptoms. A neurological etiology was implied by the analogy but has not been conclusively delineated, even with increasingly sophisti-cated instrumental techniques for understanding brain-behavior relations (see Bennett and Netsell, 1999;

LeNormand et al., 2000). Little coherence and consensus is available in this literature at present. In addition, de-spite nearly 40 years of research, di¤erential diagnostic correlates and range of severity levels characterizing DAS remain imprecisely defined. Guyette and Deidrich (1981) have suggested that DAS may not be a theoreti-cally or clinitheoreti-cally definable entity, as current empirical evidence does not produce any behavioral symptom not overlapping with other categories of developmental speech disorder or delay. In addition, no currently available theoretical constructs specifically disprove other possible theories for the origins of DAS (see Davis, Jakielski, and Marquardt, 1998). In contrast to devel-opmental disorder categories such as hearing impair-ment or cleft palate, lack of a link of underlying cause or theoretical base with behavioral correlates results in an

‘‘etiological’’ disorder label with no clearly established basis. Evidence for a neurological etiology for DAS is based on behavioral correlates that are ascribed to a neurological basis, thus achieving a circular argument structure for neural origins (Marquardt, Sussman, and Davis, 2000).

Despite the lack of consensus on theoretical motiva-tion, etiology, or empirical evidence precisely deﬁning behavioral correlates, there is some consensus among practicing clinicians as well as researchers (e.g., Shriberg, Aram, and Kwiatkowski, 1997) that DAS exists. It thus represents an incompletely understood disorder that poses important challenges both to practicing clinicians and to the establishment of a consistent research base for overall understanding. An ethical di¤erential diag-nosis for clinical intervention and research investiga-tions should, accordingly, be based on awareness of the current state of empirically established data regarding theories and behavioral correlates deﬁning this disorder.

Cautious application of the diagnostic label should be the norm, founded on a clear understanding of positive beneﬁts to the client in discerning long-term prognosis,

appropriate decisions regarding clinical intervention, and valid theory building to understand the underlying nature of the disorder. Use of DAS as an ‘‘umbrella term for children with persisting and serious speech di‰culties in the absence of obvious causation, regardless of the precise nature of their unintelligibility’’ (Stackhouse, 1992, p. 30) is to be avoided. Such practice continues to cloud the issue of precise deﬁnition of the pres-ence and prevalpres-ence of the disorder in child clinical populations.

Accordingly, a review of the range of behavioral cor-relates presently in use is of crucial importance to careful deﬁnition and understanding of DAS. The relationship of behavioral correlates to di¤erential diagnosis from

‘‘functional’’ speech disorder or delay is of primary im-portance to discriminating DAS as a subcategory of functional speech disorder. If no single deﬁning charac-teristic or complex of characcharac-teristics emerges to deﬁne DAS, the utility of the label is seriously questionable for either clinical or research purposes. In every instance, observed behaviors need to be evaluated against devel-opmental behaviors appropriate to the client’s chrono-logical age. In the case of very young clients, the di¤erential diagnosis of DAS is complicated (Davis and Velleman, 2000). Some listed characteristics may be normal aspects of earliest periods of speech and language development (e.g., predominant use of simple syllable shapes or variability in production patterns at the onset of meaningful speech; see Vihman, 1997, for a review of normal phonetic and phonological development).

Before the clinical symptoms presently employed to define DAS are outlined, specific issues with available research will be reviewed briefly. It should be empha-sized that behavioral inclusion criteria are not con-sistently reported and di¤ering criteria are included across studies. Criteria for inclusion in studies then be-come recognized symptoms of involvement, achieving a circularity that is not helpful for producing valid char-acterization of the disorder (Stackhouse, 1992). Subject ages vary widely, from preschoolers (Bradford and Dodd, 1996) to adults (Ferry, Hall, and Hicks, 1975).

Some studies include control populations of functional speech disorders for di¤erential diagnosis (Stackhouse, 1992; Dodd, 1995); others do not (Horowitz, 1984).

Associated language and praxis behaviors are included as di¤erential diagnostic correlates in some studies (Crary and Towne, 1984), while others explicitly exclude these deﬁcits (e.g., Hall, Jordon, and Robin, 1993). Se-verity is not reported consistently. When it is reported, the basis for assigning severity judgments is inconsistent across studies. A consequence of this inconsistency is lack of consensus on severity level appropriate to the DAS label. In some reports, the deﬁning characteristic is severe and persistent disorder (e.g., Shriberg, Aram, and Kwiatkowski, 1997). In other reports (e.g., Thoonen et al., 1997), a continuum of severity is explored. In the latter conceptualization, DAS can manifest as mild, moderate, or severe speech disorder.

Despite the foregoing critique, the large available literature on DAS suggests some consensus on

behav-ioral correlates that should be evaluated in establishing a di¤erential diagnosis. The range of expression of these characteristics, although frequently cited, has not been speciﬁed quantitatively. Accordingly, these behaviors should not be considered deﬁnitive but suggestive of directions for future research as well as guidelines for the practicing clinician based on emerging research.

Exclusionary criteria for a di¤erential diagnosis have been suggested in the areas of peripheral motor and sensory function, cognition, and receptive language. Ex-clusionary criteria frequently noted include (1) no peri-pheral organic disorder (e.g., cleft palate), (2) no sensory deﬁcit (i.e., in vision or hearing), (3) no peripheral muscle weakness or dysfunction (e.g., dysarthria, cerebral palsy), (4) normal IQ, and (5) normal receptive language.

Phonological and phonetic correlates have also been listed. Descriptive terminology varies from phonetic (e.g., Murdoch et al., 1995) to phonological (Forrest and Morrisette, 1999; Velleman and Shriberg, 1999) accord-ing to the theoretical perspective of the researcher, com-plicating understanding of the nature of the disorder and comparison across studies. In addition, behavioral cor-relates have been established across studies with highly varied subject pools and di¤ering exclusionary criteria.

The range of expression of symptoms is not established (i.e., what types and severity of suprasegmental errors are necessary or su‰cient for the diagnosis?). Some characteristics are in common with functional disorders and thus do not constitute a di¤erential diagnostic char-acteristic (i.e., how limited does the consonant or vowel repertoire have to be to express DAS?). In addition, not all symptoms are consistently reported as being necessary to a diagnosis of DAS (e.g., not all clients show ‘‘groping postures of the articulators’’). Long-term persistence of clinical symptoms in spite of intensive therapy has also frequently been associated with DAS.

Phonological/phonetic correlates reported include (1) limited consonant and vowel phonetic inventory, (2) predominant use of simple syllable shapes, (3) frequent omission of errors, (4) a high incidence of vowel errors, (5) altered suprasegmental characteristics (including rate, pitch, loudness, and nasality), (6) variability and lack of consistent patterning in speech output, (7) increased errors on longer sequences, (8) groping postures, and (9) lack of willingness or ability to imitate a model.

Co-occurring characteristics of DAS in several related areas have also been mentioned frequently. However, co-occurrence may be optional for a di¤erential diagno-sis, because these characteristics have not been con-sistently tracked across available studies. Co-occurring characteristics frequently cited include (1) delays in gross and ﬁne motor skills, (2) poor volitional oral nonverbal skills, (3) inconsistent diadokokinetic rates, (4) delay in syntactic development, and (5) reading and spelling delays.

Clearly, DAS is a problematic diagnostic category for both research and clinical practice. Although it has long been a focus of research and a subject of intense interest to clinicians, little consensus exists on deﬁnition, etiology, and characterization of behavioral or neural correlates. Circularity in the way in which etiology and

behavioral correlates have been described and studied does not lend to precision in understanding DAS. Re-search utilizing consistent subject selection criteria is needed to begin to link understanding of DAS to ethical clinical practices in assessment and intervention and to elucidate the underlying causes of this disorder.

See also motor speech involvement in children.

—Barbara L. Davis

References

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Bradford, A., and Dodd, B. (1996). Do all speech-disordered children have motor deﬁcits? Clinical Linguistics and Pho-netics, 10, 77–101.

Broca, P. (1861). Remarques sur le siege do la faculte du lan-guage articule, suives d’une observation d’aphemie (peste de la parole). Bulletin de la Societe d’Anatomique, 2nd se´rie, 6, 330–337.

Crary, M. A. (1984). Phonological characteristics of devel-opmental verbal dyspraxia. Seminars in Speech and Lan-guage, 6(2), 71–83.

Crary, M. A., and Towne, R. (1984). The asynergistic nature of developmental verbal dyspraxia. Australian Journal of Hu-man Communication Disorders, 12, 27–37.

Davis, B. L., Jakielski, K. J., and Marquardt, T. M. (1998).

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Dodd, B. (1995). Procedures for classiﬁcation of sub-groups of speech disorder. In B. Dodd (Ed.), Di¤erential diagnosis and treatment of children with speech disorders. London: Whurr.

Ferry, P. C., Hall, S. M., and Hicks, J. L. (1975). Dilapidated speech: Developmental verbal dyspraxia. Developmental Medicine and Child Neurology, 17, 432–455.

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Guyette, T. W., and Deidrich, W. M. (1981). A critical review of developmental apraxia of speech. In N. J. Lass (Ed.), Speech and language advances in basic practice (No. 11).

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Horowitz, J. (1984). Neurological ﬁndings in developmental verbal apraxia. Seminars in Speech and Language, 6(2), 11–18.

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Stackhouse, J. (1992). Developmental verbal dyspraxia: I. A review and critique. European Journal of Disorders of Com-munication, 27, 19–34.

Sussman, H. M. (1988). The neurogenesis of phonology. In H. Whitaker (Ed.), Phonological processes and brain mech-anisms. New York: Springer-Verlag.

Thoonen, G., Maasen, B., Gabreels, F., Schreuder, R., and de Swart, B. (1997). Towards a standardized assessment pro-cedure for DAS of speech. European Journal of Disorders of Communication, 32, 37–60.

Velleman, S. L., and Shriberg, L. D. (1999). Metrical analysis of the speech of children with suspected developmental apraxia of speech. Journal of Speech, Language, and Hear-ing Research, 42, 1444–1460.

Velleman, S. L., and Strand, K. (1993). Developmental verbal dyspraxia. In J. E. Bernthal and N. W. Bankson (Eds.), Child phonology: Characteristics, assessment, and interven-tion with special populainterven-tions. New York: Thieme.

Vihman, M. M. (1997). Phonological development. Cambridge, UK: Blackwell.

References

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