Childhood Apraxia of Speech (CAS) is a speech sound disorder (SSD) with a range of severity, resulting in unintelligible speech that often persists into elementary school age and impacts language and literacy skills. Children with CAS demonstrate heterogeneity in their symptoms and diagnostic criteria for the disorder have been highly controversial. The American Speech-Language-Hearing Association (ASHA) has defined CAS as a “…speech disorder in which the precision and consistency of movements underlying speech are impaired in the absence of neuromuscular deficits (e.g., abnormal reflexes, abnormal tone)” [1]. Children with CAS have difficulty selecting and executing motor programs for speech, resulting in inconsistent speech production, difficulty sequencing syllables, and abnormal prosody [1], often accompanied by language and literacy difficulties [1–6]. CAS is a disorder of the central nervous system [7]. The prevalence estimates in the United States range between 0.01–0.02% [8], putting it in the categories of genetic rare disorders, and is reportedly found in 3.4%-4.3% of children referred for speech sound disorders [9].
There is controversy in the field whether or not CAS should be classified as a “syndrome”, i.e., a symptom complex, or solely a motor-speech disorder. Velleman [10] states “It is important to note, however, that CAS is a syndrome (i.e., a symptom complex), not a unitary disorder. That is, not all children will demonstrate the same symptoms and symptoms will change over time in a given individual. Thus, a checklist approach to diagnosis is not possible; rather, a pattern of symptoms is key to identification” (p. 59). Gillon and Moriarty [3] refer to CAS as a “symptom cluster of speech, motor, and/or language characteristics” and note that there is no single characteristic that can be used in isolation to diagnose it. The concept of CAS as a syndrome is further discussed by Nijland [11]. While CAS may co-occur with known neurodevelopmental disorders and/or dysarthria [12], it may also be caused by genetic mutations, many of which are still uncharacterized [13, 14], or it may be idiopathic. A genetic study of two multigenerational families with histories of SSD associated with CAS revealed findings suggesting a spectrum of CAS phenotypes [13]. Another genetic study of 10 unrelated participants with CAS observed significant genetic heterogeneity and a high degree of phenotypic diversity[14].
As stated by Stackhouse and Wells [15], the motoric deficit seen in CAS may have “flow-on” effects for a child’s language and literacy development. Inconsistent and inaccurate speech in CAS may provide inadequate input to the child’s developing linguistic system and may thus also affect auditory processing and vocabulary knowledge. This cascading effect of motor-speech difficulties on a child’s developing language and literacy development may be due to poor linguistic abilities and the ensuing reading and spelling deficits for school-age children with CAS [1, 6, 16, 17]. One effect of motor-speech impairment on language and literacy is that poor speech output interferes with the accurate encoding of complex words, which affects the child’s developing lexicon and linguistic system [15]. Previous studies have demonstrated language and reading disabilities of varying prevalence and severity among children with CAS [3–6]. This illustrates how articulation, language, and reading are inter-dependent.
In addition to characteristic difficulties in speech production and associated reading and language difficulties, CAS may also present with other neurological signs, including early difficulties with feeding, sensory processing issues, a paucity of vocal play, babbling, and imitation in infancy, gross or fine motor in-coordination, body dyspraxia, dysarthria, and other “soft” neurological signs in addition to slow progress in therapy and limited repertoire of sounds[10, 11, 18–21]. These clinical characteristics present differently at various stages of child development and are highly variable across children with CAS. While many papers have characterized the speech deficits associated with CAS, few studies have explicitly examined these other clinical characteristics. As illustrated above, there is much to be learned about the phenotypic diversity of CAS, which could potentially reveal clues about its biologic underpinnings.
Because of the potential heterogeneity within CAS manifested in difficulties in written language [3, 11, 22–24], it is of interest to see if there are subgroups within CAS; this could have implications for treatment of other communication domains. Our objective was to use speech, language, and reading assessments to identify comorbid subgroups within CAS of varying severity and associated clinical characteristics, based on the aforementioned literature suggesting that these are variable within children with CAS. Specifically, we exploited the full range of phenotypic variation to identify more homogeneous subgroups of children, an approach that was advocated for autism, another neurodevelopmental disorder, several years ago for genetic mapping [25]. Subgroup classification of CAS and other communication disorders has been a long-standing interest for communication disorders professionals as it may potentially lead to differential diagnosis and treatment [3, 26]. In addition, recognition of clinical characteristics associated with varying severity levels within CAS could help identify children in need of early or more intense remediation, thus improving later academic outcomes. First, using endophenotypes of communication disorder severity, specifically articulation, vocabulary, phonological memory and reading, we used an unsupervised multivariate clustering method to mine the data for potential comorbid subgroups. Then, we conducted a chart review of other clinical symptoms associated with CAS to examine if these symptoms were associated with comorbid subtypes of CAS as defined by the degree of impairment in language and reading ability. This analysis presents an unusually large cohort of children with CAS. Together, these data suggest that there may be comorbid subgroups within CAS that can be defined by language and reading ability as well as the presence of specific clinical symptoms.