Language Delay in 3-Year-Old Children With ADHD Symptoms

Abstract

Objective: Little is known about cognition in preschoolers with ADHD and language delay (LD). The objective was to investigate cognitive functions in preschoolers with ADHD symptoms and LD compared with children with ADHD symptoms only and to estimate the frequency of children with ADHD symptoms, co-occurring language delay, and delays on cognitive measures. Method: Participants were recruited from the Norwegian Mother and Child Cohort Study. The teacher report of expressive language and the cognitive tests from 119 3-year-old children with parent reported ADHD symptoms and LD were compared with those of 258 children with ADHD symptoms only. Results: The ADHD + LD group performed significantly worse than the ADHD group on most language-related measures. There were no differences between the groups on most nonverbal measures. Single measures had a limited potential of differentiating between the groups. Conclusion: ADHD symptoms and co-occurring LD in preschoolers were characterized by cognitive deficits associated with both disorders, not with global neurodevelopmental delay.

Keywords

preschool ADHD coexisting disorders language impairment cognitive ability

Particularly slow language development in young preschool children is often labeled language delay (LD). LD is a risk factor for specific language impairment and a variety of other disorders (Whitehurst & Fischel, 1994). It often co-occurs with ADHD (Posner et al., 2007; Tirosh & Cohen, 1998). Both have negative consequences for cognition, education, and social functioning (Cohen, 2001; Larson, Russ, Kahn, & Halfon, 2011), and children with ADHD and co-occurring LD have been shown to experience worse functioning across domains compared with children with ADHD only (Cohen et al., 2000; Larson et al., 2011).

To ensure the best possible prognosis for children with ADHD, early interventions are important (Brandau & Pretis, 2004; Halperin, Bédard, & Curchack-Lichtin, 2012) because of sensitive periods for effects on brain structure and function (Andersen & Navalta, 2011). Similarly, an early remediation of language deficits and language delays may improve communication and social skills (Thiemann & Warren, 2004). Interventions as early as in toddlerhood have shown promising results for children with LD (Van Agt, Van Der Stege, De Ridder-Sluiter, Verhoeven, & De Koning, 2007). For children with ADHD and comorbid disorders, early interventions for ADHD are even more highly recommended than for children with ADHD only (Hinshaw, 1992) and best initiated as soon as ADHD symptoms emerge (Halperin et al., 2012). However, early interventions require that children with debilitating levels of ADHD symptoms and co-occurring LD can be reliably identified at an early age. LD is assumed to be underdiagnosed in children with attention difficulties (Bishop & McDonald, 2009; Cohen, 2001) and therefore children with ADHD and LD are at risk for not receiving the speech-language interventions they need. In addition to not receiving interventions for LD, children with ADHD and LD may lack the underlying skills to benefit from language-based treatments for ADHD (e.g., cognitive behavioral therapy).

Early identification is challenging because delayed language skills are easily misinterpreted as ADHD symptoms, especially when focusing on behavioral categories during assessment, which is common (Redmond, 2002). For example, delayed receptive language skills (e.g., not understanding instructions) are easily mistaken for inattention (Redmond, 2002). Neuropsychological tests and preschool teacher reports of language may help to identify LD in children with significant ADHD symptoms. However, as most research has been conducted with older children, there is limited knowledge about differences in neuropsychological functioning between ADHD only and the combination of ADHD and LD in preschoolers. We are only aware of one other study on this topic in preschool children. Beitchman, Tuckett, and Batth (1987) found that 4-year old boys with ADHD and delayed expressive language had lower scores on measures of IQ, expressive language, receptive language, and visual–motor integration than their peers with ADHD only. Because of the unexpected delay in visual–motor integration and the expected delays in language skills and IQ, the authors concluded that the combination of ADHD and LD reflects a general neurodevelopmental immaturity. In recent years, impaired motor function or developmental coordination disorder is no longer recognized as a necessary indicator of global developmental delay but regarded as a frequent comorbidity in preschoolers with LD (Gaines & Missiuna, 2007; Iverson & Braddock, 2011). The frequent co-occurrence may arise because of the close associations between oral motor difficulties in speech production and motor problems (Bishop, 2002) and/or because of delayed motor functions (e.g., delayed reaching or grasping in infancy or delayed crawling) limiting opportunities for exploring the environment and thereby for language learning (Campos et al., 2000; Iverson, 2010).

There has been little research on teacher reports of language in preschoolers. A recent study found significant associations between teacher-reported language skills and direct language assessment, even though the parent report of language skills was more accurate (Vagh, Pan, & Mancilla-Martinez, 2009). In a study of 6-year old children, Bishop, Laws, Adams, and Norbury (2006) concluded that teacher reports provide important information about language impairment and that they, despite their more subjective nature than direct assessment, give researchers information about behavior that may be challenging to elicit in a test situation and may be less influenced by attention or motivation. Such information may be particularly valuable in preschool samples, as an addition to direct assessment. Besides, in Norway, where the vast majority of children attend center-based child care from a young age, and where the quality of these centers/preschools is generally high and homogeneous (Winsvold & Guldbrandsen, 2009), preschool teacher-reported language may be a particularly valuable, low-cost addition to language assessment.

For more information about language problems in children with ADHD, we have to rely on the literature about school-aged children. In this age group, language problems are easier to diagnose and less transient than in preschoolers (Girolametto, Wiigs, Smyth, Weitzman, & Pearce, 2001); therefore, the main focus of this literature is on language impairment, instead of delay. Studies report that school-aged children with language impairment and ADHD show deficits related to both disorders and are therefore at considerable risk for cognitive impairment (Cohen et al., 2000). In particular, as working memory seems to be more deficient in children with language impairment than in children with ADHD, children with both disorders are more impaired in this domain than their peers with ADHD only (Cohen et al., 2000; Hutchinson, Bavin, Efron, & Sciberras, 2012; Jonsdottir, Bouma, Sergeant, & Scherder, 2005). Notably, most studies show that verbal and nonverbal working memory are more deficient in language impairment than in ADHD (Cohen et al., 2000; Hutchinson et al., 2012). A possible explanation for deficits in nonverbal working memory in language impairment is that working memory is assumed to be language-based even when nonverbal information is processed (Denckla, 1996).

ADHD and language impairment have been shown to be associated with impaired visual–motor integration in school children, but children with both disorders have so far not been found to perform more poorly on such tasks than their peers with ADHD only (Cohen et al., 2000). Impaired phonological awareness is associated with language impairment (Claessen & Leitao, 2012), not with ADHD (Gooch, Snowling, & Hulme, 2011). One would therefore expect children with ADHD to obtain higher scores on tests of phonological awareness than children with co-occurring language impairment. However, we do not know whether the research concerning older children with specific language impairment is transferable to the preliteracy skills of preschoolers with LD.

Co-occurring LD is thus associated with an array of negative consequences for children with ADHD, including commonly reported general cognitive immaturity, yet we lack up-to-date knowledge about identification of co-occurring LD. Moreover, neuropsychological functioning, neurodevelopmental disorders, and the interactions between the two need to be understood within a developmental perspective as they are age-dependent (Sowerby, Seal, & Tripp, 2011; Tillman, Eninger, Forssman, & Bohlin, 2011).

Aims

The main objective of the present study was to investigate how the co-occurrence of LD affected cognitive functions in preschool children with significant ADHD symptoms: whether children with co-occurring LD and ADHD symptoms had group-specific LDs, whether the co-occurrence of LD and ADHD symptoms accentuated the cognitive deficits related to ADHD or to LD, or whether the co-occurrence implied a global cognitive delay.

A follow-up analysis was included in the study to increase the practical value of the results. This second objective aimed at estimating to what degree the measures showing differences between ADHD only and ADHD + LD groups were usable for identifying individuals that were delayed on these measures.

Method

The present study is part of a longitudinal study of ADHD in preschool children which is a substudy to the Norwegian Mother and Child Cohort Study (MoBa), a population-based birth cohort at the Norwegian Institute of Public Health (Magnus et al., 2006). The present study was approved by the Norwegian Regional Ethics Committee for Medical and Health related Research. The parents of the participating children gave informed consent to participate in the research and publication of the results.

Participants

The present sample of children is a nonclinical sample recruited from the MoBa. A flowchart of the recruitment of the sample is presented in Figure 1. The participants were selected through a screening procedure based on responses to 11 questions in the parent questionnaire from MoBa at child age 36 months regarding hyperactivity, impulsivity, and attention problems from the child behavior checklist (Achenbach & Ruffle, 2000) and diagnostic criteria for ADHD (American Psychiatric Association [APA], 2000), or whether hyperactivity was reported as a health problem. To oversample children with relevant symptoms, about 80% of invited children were those scoring at or above the 90th percentile on these questions: 2,798 children with high scores on hyperactivity, impulsivity, and attention problems, or hyperactivity as a health problem, were invited, and 1,048 (37.5%) of these participated in the clinical assessments. In addition, 654 randomly drawn children from the MoBa cohort were invited and 147 (22.5%) of these participated, yielding a sample of 1,195 children who were clinically assessed at the mean age of 42 months. There were no statistically significant differences between clinically assessed children and invited nonparticipants with regard to background factors and pre- and perinatal risk factors except for a slightly higher level of maternal education in the assessed children (unpublished). Of the 1,195 clinically assessed children, the present study included two groups of preschoolers: children with significant symptoms of ADHD without LD (the ADHD group; n = 258) and children with significant symptoms of ADHD and LD (the ADHD + LD group; n = 119). Symptom identification is described below.

Figure 1.

Flowchart of recruitment.

In addition, 366 children (182 boys and 184 girls) from the same sample of assessed children showing neither significant psychiatric symptoms nor LD were used as a normative sample for cognitive tests to study Aim 2. The groups were defined based on information from the clinical assessment (see below).

Procedure

A questionnaire about expressive language (see below) was sent to the parents and preschool teachers of all the 1,195 children prior to clinical investigation. The clinical investigation consisted of a 1-day assessment including neuropsychological tests and a semi-structured interview with a parent. The neuropsychological assessment lasted for approximately 1.5 hr including breaks. Small incentives (stickers, small cookies, grapes, or raisins) were sometimes given to the child to improve motivation during the neuropsychological assessment. Each child was individually evaluated by a psychologist and one parent was present during the entire clinical assessment. The interviews were conducted by trained psychology students, psychologists, or psychiatrists. To ensure validity, the entire assessment was video-recorded and senior specialists in child neuropsychology reviewed the neuropsychological testing and the interviews. The examiners and senior specialists were blind as to the screening status of the child. None of the children in the sample received psychopharmacological treatment at the time of the assessment.

Measures

The measures used were selected to cover cognitive domains associated with LD, with ADHD, and with both. In addition, a measure was included for investigating nonverbal intellectual ability.

DSM-IV criteria for ADHD

The Preschool Age Psychiatric Assessment (PAPA; Egger et al., 2006) was used for the assessment of symptoms listed in the Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV; APA, 2000), including ADHD. The PAPA is a semi-structured, clinical interview conducted with a parent. It provides information about the scale and frequency of symptoms and about impairment, according to the diagnoses in DSM-IV. In the present study, this interview was used to assess the number of the following symptoms: inattention, hyperactivity, impulsivity (which add up to the total amount of ADHD symptoms), and symptoms of oppositional defiant disorder (ODD). The PAPA is the only comprehensive psychiatric interview to date with a demonstrated test–retest reliability and validity for assessing psychiatric symptoms and disorders in toddlers and preschool children. Interrater reliability of the number of DSM-IV symptoms and impairment scores were assessed by rescoring tapes of 79 randomly chosen PAPA interviews by raters blind to knowledge about the child. The average intraclass correlations (ICC) were .99 for number of inattention symptoms, .97 for hyperactivity symptoms, .96 for impulsivity symptoms, and .94 for total impairment score.

Children were included in the study if they displayed at or above threshold levels of ADHD symptoms (at least six of nine symptoms within inattentive and/or hyperactive/impulsive subdomains) with or without reported impairment, or subthreshold levels (three to five symptoms within at least one subdomain) of ADHD symptoms with reported impairment. Impairment was recorded present when parents reported that the child was significantly impaired by his or her ADHD symptoms in at least one area of functioning.

Language delay

Children were identified as showing LD based on a parent questionnaire about expressive language, the Child Development Inventory (CDI; Ireton & Glascoe, 1995). LD was defined as at least 1.25 standard deviations below the mean on the CDI parent questionnaire of expressive language; the mean and standard deviation were derived from the normative sample. This widely recognized cut point for language problems is the equivalent to the previously chosen cut point of 90th percentile in normal distributions (see section about participants).

The CDI is a questionnaire for assessing children aged 15 months to 6 years. The subscale measuring expressive language was included in the present study. It consists of 50 items assessing mainly expressive communication, from simple gestural (one item: “points to things”), vocal (three items about pronunciation) to complex language expression. It includes a few items about grammar (e.g., “Asks simple questions using correct grammar”), and a few items combine expressive language with verbal memory (e.g., “Retells short stories; tells what happens in correct order”). Each item was scored yes (1) or no (0) according to the parents’ view of the child’s skills, resulting in a possible maximum raw score of 50. CDI scores have been found to be consistent with children’s results on psychometric tests and the CDI has been found to have good sensitivity and specificity for identifying preschool children with delayed development (Doig, Macias, Saylor, Craver, & Ingram, 1999).

The CDI was also filled in by preschool teachers to obtain a teacher-reported measure of expressive language skills. Pearson’s correlation of teacher and parent report was .56 (p < .001) in the ADHD + LD group and .25 (p < .001) in the ADHD-only group. Small-to-moderate agreement between parent and teacher reports are frequent (Bishop et al., 2006).

Intellectual ability and working memory

The Stanford–Binet 5th revision (SB-5; Roid, 2003) is a widely used, standardized test battery with good psychometric properties (Strauss, Sherman, & Spreen, 2006). It was developed for assessing intellectual ability and other cognitive functions for ages 2 to 85 years. The stop rule for tests from the SB-5 test battery of discontinuing the test after four consecutive null scores was applied in all the tests from this battery.

Intellectual ability was estimated from a short version of the SB-5 revision; the “object matrices” task was used to measure nonverbal intellectual ability, and the “vocabulary” task was used to assess verbal intellectual ability.

The subtask “memory for sentences” in which the child is asked to repeat sentences of increasing length was included as a measure of verbal working memory. We suggest that the demands of this task exceed the verbal memory span of 3-year old children as the length of the sentences increases. The children are therefore forced to use their verbal working memory capacity to process the information.

A developmental neuropsychological assessment (NEPSY)

The NEPSY (Korkman, Kirk, & Kemp, 2000) is a standardized neuropsychological battery for 3- to 12-year-old children. Four tests from the NEPSY were used in the current study. All the NEPSY tests in the present study, except for “statue,” had a stop rule that consisted of discontinuation after four consecutive failed responses.

Receptive skills were assessed with the “comprehension of instructions” task. It requires the child to point to the correct picture in response to the test administrator’s commands (“point to . . .”). The instructions have increasing syntactic complexity. The items in this test are scored as either “correct” or “wrong”; the highest possible raw score was 20.

Phonological awareness was assessed with the “phonological processing” task, which requires the child to identify words based on presented word segments. The child identifies the word by pointing to one out of three possible pictures. Differentiating the words based on the segments becomes increasingly difficult as the test progresses. The items in this test are scored as either “correct” or “wrong”; the highest possible raw score was 14.

Response inhibition was assessed with the “statue” task. The child is asked to stand still (like a statue) with eyes closed and to be silent for 75 s, while the test administer produces several timed, distracting stimuli. Two points were awarded for every 5-s interval without utterances or significant movements and one point for every 5-s interval with one utterance or movement. More than one movement or utterance resulted in zero points for that interval. Smiling, slight finger movements, and involuntary coughing were allowed. Reminders of standing still and being silent could be given after utterances and movements. The highest possible raw score was 30.

Visual–motor integration was assessed with the “design copy” task in which the child is asked to copy the two-dimensional designs of increasing difficulty on paper. Points were rewarded according to the precision of the drawings with a maximum of four points for each drawing. The highest possible raw score was 72.

The NEPSY was chosen for this study because it is a well-known neuropsychological battery for preschoolers with tests of short duration that are appealing for young children and because it is one of the few test batteries for preschoolers that have been translated to Norwegian. The psychometric properties of the Swedish version of the NEPSY tests are reported as those of the Norwegian version have not yet been calculated. Phonological processing, comprehension of instructions, and design copy show adequate to high split-half reliability (.84, .82, and .78, respectively), which is the only measure of reliability that is reported in the Swedish manual of the NEPSY for these tests. For the “Statue” task, the Swedish manual only reports test–retest reliability from the American manual, which is low (.50; Korkman et al., 2000).¹

Data Management and Statistical Analyses

The majority of children completed all the tests, despite their young age. Participants were only included in the study if they had less than six missing items (less than 10% missing) on the CDI questionnaire from parents. The same rule applied to teacher scores on the CDI. Descriptive data were computed for variables known to affect cognitive functioning: age, gender, parental education, bilingualism, a history of hearing impairment, and the number of ADHD, and ODD symptoms. Independent samples t tests were conducted to assess whether there were significant differences between the groups on these characteristics.

A one-way between-groups multivariate analysis of variance was performed to investigate differences in cognitive functions depending on language status in ADHD, controlled for gender. Nine dependent variables were entered: verbal intellectual ability, nonverbal intellectual ability, verbal working memory, nonverbal working memory, receptive language, phonological skills, visual–motor integration, response inhibition, and teacher-reported expressive language. The independent variable was LD in ADHD. The data violated the assumption of the homogeneity of variance–covariance matrices; however, the Bow’s M, which indicated that this assumption was violated, tends to be too strict in large sample sizes (Tabachnick & Fidell, 2007).

To study the second aim, we investigated the percentage of the children in the ADHD + LD group who were delayed in the cognitive domains where group differences were found. This was done by establishing a cutoff score of −1.25 standard deviations in the normative sample. For illustrative purposes, z scores based on the performance of the normative sample were computed for the ADHD group and for the ADHD + LD group.

Results

Levels of ADHD symptoms at or above the threshold specified in DSM-IV or subthreshold levels of ADHD symptoms with impairment were found in 391 children. Of these 391 children, 14 were excluded from the study because of missing data on abbreviated IQ or on the CDI (Ireton & Glascoe, 1995) parent questionnaire about expressive language. Of the remaining 377 children, 119 had co-occurring LD and were included in the “ADHD + LD group”, whereas 258 comprised the “ADHD group” (see Figure 1).

Demographic characteristics of the two groups, descriptive data of hearing impairment and bilingualism, and groupwise comparisons of these measures are presented in Table 1. There were significant differences in the number of inattention symptoms and total ADHD symptoms between the groups. There were no significant differences between the groups in age, gender, parental education, bilingualism, hyperactivity symptoms, impulsivity symptoms, and the number of total symptoms of ODD. However, because gender bordered on significance, we chose to control for it in the multivariate analyses of variance. Differences between groups in inattention, the total number of ADHD symptoms, and gender were small.

Table 1.

Participant Characteristics (N = 377) and Group Differences on These Characteristics.

	ADHD group		ADHD + LD group					Cohen’s d
Characteristic	M	SD	M	SD	df	t	p	Cohen’s d
Age (years)	3.48	0.10	3.46	0.10	375	0.80	.428	0.08
P. edu. (years)	14.65	2.30	14.27	2.23	361	1.50	.134	0.16
ADHD symptoms	8.00	3.16	8.72	3.25	375	−2.06	.040	−0.21
Inattention	2.64	2.08	3.20	2.24	375	−2.37	.018	−0.24
Hyperactivity	4.18	1.49	4.24	1.40	375	−0.40	.687	−0.04
Impulsivity	1.17	0.95	1.28	0.92	375	−0.99	.324	−0.10
ODD symptoms	2.13	1.69	1.87	1.70	375	1.40	.162	0.14
Gender: % boys	52.71		63.03		236	1.90	.059	0.25
Hearing: % impaired	1.16		2.52		375	−0.98	.329	−0.10
Bilingualism (%)	15.5		18.49		375	−0.73	.469	−0.08

Note. LD = language delay; P. edu. = parental education (mean years of mother’s + father’s education divided by 2); ADHD symptoms = total number of ADHD symptoms; Inattention = number of inattention symptoms; Hyperactivity = number of hyperactivity symptoms; Impulsivity = number of impulsivity symptoms; ODD symptoms = number of oppositional defiant disorder symptoms; Hearing impairment = parent report at 36 months of age; bilingualism = mother or father have other mother tongue than Norwegian.

Table 2 shows mean scores and standard deviations for the teacher report of language skills and cognitive measures as a function of language status in ADHD. The children with LD had lower means on all tests except for nonverbal intellectual ability, where scores were almost identical for the two groups.

Table 2.

Mean Scores and Standard Deviations for Teacher-Reported Language Skills and Measures of Cognitive Skills as a Function of Language Status in ADHD.

	ADHD group		ADHD + LD group
Measure	M	SD	M	SD
1. Teacher report (language)	43.09	5.94	34.43	9.45
2. VIQ	9.99	2.03	8.44	1.47
3. NVIQ	10.90	1.97	10.80	1.98
4. VWM	11.01	2.60	9.14	3.07
5. NVWM	10.13	2.88	9.38	2.65
6. Receptive	11.33	2.85	9.34	3.08
7. Phonological	8.26	2.58	7.31	2.55
8. Visual–motor	12.90	5.90	10.51	5.82
9. Inhibition	13.47	8.48	11.58	8.16

Note. LD = language delay; Teacher report = teacher-reported expressive language (Child Development Inventory questionnaire); VIQ = verbal ability (scaled score; Stanford–Binet); NVIQ = nonverbal ability (scaled score; Stanford–Binet); VWM = verbal working memory (scaled score; Stanford–Binet); NVWM = nonverbal working memory (scaled score; Stanford–Binet); Receptive = receptive language (raw score; NEPSY); Phonological = phonological awareness (raw score; NEPSY); Visual–motor = visual–motor integration (raw score; NEPSY); Inhibition = response inhibition (raw score; NEPSY).

Table 3 shows the results of the multivariate and univariate analyses of variance. There were statistically significant differences between children with and without LD on the combined dependent variables. When the results for the dependent variables were considered separately, using a Bonferroni adjusted alpha level of .0056 (.05 divided by 9), verbal intellectual ability, verbal working memory, receptive language, expressive language, and visual–motor integration reached significance, whereas nonverbal intellectual ability, nonverbal working memory, phonological skills, and response inhibition did not.

Table 3.

Multivariate and Univariate Analyses of Variance for Teacher-Reported Language Skills and Measures of Cognitive Skills as a Function of Language Delay.

	Multivariate			Univariate
Variable	F ^a	p	Partial η²	F ^b	p	Partial η²
	11.96	.000	.29
Teacher report (language)				81.51	.000	.23
VIQ				36.84	.000	.12
NVIQ				0.89	.347	.00
VWM				29.01	.000	.10
NVWM				2.69	.103	.01
Receptive				15.89	.000	.06
Phonological				5.21	.023	.02
Visual–motor				8.71	.003	.03
Inhibition				2.45	.119	.01

Note. Controlled for gender. F ratios are Pillai’s approximation of F. Teacher report = teacher-reported expressive language (Child Development Inventory questionnaire); VIQ = verbal ability (Stanford–Binet); NVIQ = nonverbal ability (Stanford–Binet); VWM = verbal working memory (Stanford–Binet); NVWM = nonverbal working memory (Stanford–Binet); Receptive = receptive language (NEPSY); Phonological = phonological awareness (NEPSY); Visual–motor = visual–motor integration (NEPSY); Inhibition = response inhibition (NEPSY).

Multivariate df = 9, 260.

Univariate df = 1, 268. Bonferroni-corrected alpha value = .006.

Figure 2 presents mean z scores for the ADHD group and ADHD + LD group. The z scores were based on the scores of the children in the normative sample. There were significant differences between the groups on the first five measures in Figure 2, whereas there were no significant group differences on the last four measures.

Fig 2.

Z scores of group differences.

A visual representation of the results of the second aim is shown in Figure 3. The following percentages of children in the ADHD + LD group scored below the cutoff of 1.25 standard deviation below the mean of the normative sample (showing delayed functioning): 24% showed delayed verbal intellectual ability, 41% showed delayed verbal working memory, 38% were identified as having delayed receptive language, 27% showed delayed visual–motor integration, and 57% were identified as having delayed expressive language according to the teacher report.

Figure 3.

Percentages of children in ADHD + LD group with cognitive delays.

Discussion

The main objective of this study was to investigate how the co-occurrence of LD affected cognitive functions in preschoolers with significant ADHD symptoms. Children with ADHD symptoms and LD performed significantly worse than children with ADHD symptoms without LD on four out of five language-related measures (phonological awareness being the only exception) and with regard to visual–motor integration. There were no differences between the groups on other nonverbal measures, indicating that LD was not a marker of general developmental delay. Most effect sizes were moderate to large, and all measures taken together explained almost 30% of the variance in LD among children with ADHD symptoms. However, when the measures that had differentiated between the groups were analyzed individually, most identified less than half of the children in the ADHD + LD group as having delayed functioning.

Group differences were not accounted for by age, gender, parental education, hearing disability at 36 months of age, bilingualism, total number of ADHD symptoms, hyperactivity symptoms, impulsivity symptoms, and number of total symptoms of ODD. Inattention was significantly higher in the ADHD + LD group and could account for some of the differences in task performance between the groups. However, the increase in inattention symptoms in this group may, in part, reflect difficulties with receptive language and, in part, a misinterpretation of receptive language as inattention (Redmond, 2002). As hyperactivity and impulsivity were not significantly higher in the ADHD + LD group, the increase in the total number of ADHD symptoms in the ADHD + LD group is suggested to reflect the increase in inattention symptoms. Combining this information, the increases in inattention and in the total number of ADHD symptoms in the ADHD + LD group were not interpreted as limitations of the study but as natural consequences of the co-occurrence of ADHD and LD.

The results of the present study are in most part in accordance with the study of Beitchman et al. (1987), which showed that preschoolers with ADHD and LD had lower IQs, expressive language, receptive language, and visual–motor integration than their peers with ADHD only. In the present study, verbal, but not nonverbal, intellectual ability differentiated between the two groups. We did not include abbreviated IQ in the analyses, as this would have yielded less informative results compared with investigating verbal and nonverbal intellectual abilities separately. Furthermore, a lower full-scale IQ may be misinterpreted as a general neurocognitive delay, when it in fact mirrors a delay of verbal intellectual ability.

The same pattern as in intellectual ability was found in working memory, as only verbal working memory discriminated between the groups, whereas nonverbal working memory did not. This result is contrary to most literature on the subject but in accordance with Jonsdottir et al. (2005). The finding emphasizes the importance of assessing a child’s language when faced with impaired verbal working memory and the importance of screening for language problems when studying working memory and ADHD. This result further implies that even though nonverbal working memory may be language-based (Denckla, 1996), verbal working memory is language-based to a larger extent in young preschoolers.

Deficits in response inhibition are generally found to be strongly linked with ADHD and ODD (Albrecht, Banaschewski, Brandeis, Heinrich, & Rothenberger, 2005) and not with LD. As there were no group differences in total number of ADHD and ODD symptoms, it comes as no surprise that the response inhibition task did not discriminate between the two groups. The finding is nonetheless interesting because it shows that co-occurring LD is not necessarily associated with additionally decreased response inhibition in children with significant ADHD symptoms. It also indicates that response inhibition is more closely associated with hyperactivity and/or impulsivity than with inattention. However, caution is warranted when interpreting this result, as there is a floor effect on the “statue” task. We cannot rule out that a more sensitive test could have detected differences between the groups.

The “design copy” task is meant to measure eye–hand coordination, motor skills, nonverbal intellectual ability and planning (Strauss et al., 2006). Because planning has been shown to be associated with ADHD (Nigg, Blaskey, Huang-Pollock, & Rappley, 2002) and to a far lesser degree with specific language impairment, (for an overview, see Chapter 5 in Leonard, 1998) and nonverbal intellectual ability was equal between the groups in this study, the motor skills and eye–hand coordination assumably accounted for the group differences on this measure. Such deficits have been frequently reported in LD/specific language impairment and in ADHD (for an overview, see Zwicker, Missiuna, Harris, & Boyd, 2012), and our results indicate that children with both disorders have an added disadvantage regarding visual–motor integration. This result is not in accordance with findings in school aged children with language impairment (Cohen et al., 2000); however, motor dysfunction in ADHD has been found to increase with co-occurring disorders, especially reading disability (Kooistra, Crawford, Dewey, Cantell, & Kaplan, 2005). Our results could therefore be explained by comorbidity, or possibly by an interdependence of eye–hand coordination and language skills in young children.

No significant group difference in phonological awareness is also contrary to what we would have expected from the literature (Claessen & Leitao, 2012). This finding is neither likely to be due to a lack of statistical power (as the mean score of the ADHD + LD group on this test was less than half a standard deviation below the mean of the ADHD group) nor due to poor psychometric property, according to the Swedish manual of the NEPSY. The finding may, however, be explained by the young age of the participants, as group differences in phonological awareness may only be visible at a later age. However, the lack of a significant group difference on this measure may also be due to less phonological impairment in LD compared with SLI. Most of the literature focuses on phonological awareness in SLI in school-aged children and may not be transferable to preschool populations with LD, as not all preschool children with LD are expected to develop SLI when they get older.

Effect sizes showed that the presence of LD explained the most variance in verbal intellectual ability (a measure that mainly reflects expressive language and, only to some extent, receptive language) and verbal working memory, along with teacher-reported expressive language. The analysis of predicting receptive language based on expressive LD in the present study had a moderately large effect size. The result reflects a well-established association between expressive language and the other language-related domains. However, it is important to keep in mind that whereas all children with LD by this study’s definition had delayed expressive language based on parent report, not all had delayed receptive language. Therefore, the measures of expressive language are likely to detect more children with LD, whereas the co-occurring impairment of receptive language should be interpreted as a more serious LD (Whitehurst & Fischel, 1994).

The teacher ratings of expressive language identified more children with LD than the individual language tests. This may simply be due to the fact that the scale was identical to the parent questionnaire used for defining the LD group. Considering this, one might have expected the teacher questionnaire to identify more preschoolers with LD. However, teachers have been shown to identify children with the most severe and nonspecific language problems (Lundervold, Posserud, Sørensen, & Gillberg, 2008), and thus their ratings may not be sensitive for identifying children with mild LD. This could explain the small-to-moderate correlations of teacher and parent reports in the ADHD group, and the moderate-to-high correlations of the two in the ADHD + LD group; the more impaired the language of the child the higher the agreement between parents and teachers.

The other measures also had limited potential for differentiating between the groups when considered on their own. This indicates that there may be substantial individual differences and thereby highlights the complexity of LD. However, the combined measure in the multivariate analysis of variance supported the definition of LD based on the parent questionnaire. This finding is in line with several studies that have found parent reports to be good predictors of LD (e.g. Bishop et al., 2006; Bishop & McDonald, 2009).

Children in the ADHD + LD group did not have significantly lower scores than children in the ADHD group on tests of response inhibition, nonverbal working memory, and phonological awareness, which is inconsistent with findings of school-aged children. Because LD is often transient in preschool years (Girolametto et al., 2001), studies of older children may have groups with better defined and more stable LD. On the other hand, the present study is the only study on this topic with a purely nonclinical sample that we are aware of. Therefore, our sample is less affected by treatment seeking, which can cause studies with clinical samples to overestimate cognitive and clinical impairment in their samples. In addition, when regarding the findings of the present study from a developmental perspective and acknowledging cognitive functions to be age-dependent and interacting with psychiatric symptoms over time, neuropsychological functions in children with co-occurring LD may become increasingly delayed from preschool age to school age, explaining the findings in the literature.

Conclusion

The combination of ADHD symptoms and LD in preschoolers was characterized by cognitive deficits associated with both disorders and an added disadvantage in visual–motor integration, but not with general neurodevelopmental delay. Despite moderate-to-large effect sizes for the verbal measures, each measure had a limited potential of differentiating between the groups when used on its own.

Clinical Implications and Future Directions

Clinicians should be alert to parental concerns about LD and take into account that tests which are often used to assess LD may not be sufficiently sensitive. It is not advisory to base judgment about possible LD on a single or a few language-related measures in this age group.

Children with both disorders need interventions that target both disorders. However, the general understanding of children with significant ADHD symptoms and LD to be “worse functioning across domains” compared with their peers with significant ADHD symptoms only may not apply to young preschoolers. Some group differences may take years to increase to significant levels. For this reason, the preschool period could be the most promising period for interventions for children with significant ADHD symptoms and LD, before the gap between their skills and their peers’ skills may widen further. However, the results presented here need to be replicated using other neuropsychological measures before conclusions can be drawn with reasonable certainty. Longitudinal studies are needed to follow up preschoolers with significant ADHD symptoms and LD to study whether impairment across domains develops over time. The children in the present study will be reassessed at a later age.

Footnotes

Acknowledgements

We are grateful to all the children and their families who have participated in this study and to the enthusiastic staff at the Norwegian Longitudinal ADHD Study.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) declared receipt of the following financial support for the research, authorship, and/or publication of this article: The present study was supported by a grant from the Norwegian Research Council, Grant no. 185727/V50. The Norwegian Longitudinal ADHD Study, from which the present data were drawn, was supported by funds and grants from the Norwegian Ministry of Health, The Norwegian Health Directorate, The South Eastern Health Region, G & PJ Sorensen Fund for Scientific Research, and from The Norwegian Resource Centre for ADHD, Tourettes Syndrome and Narcolepsy. The Norwegian Mother and Child Cohort Study is supported by the Norwegian Ministry of Health and the Ministry of Education and Research, NIH/NIEHS (contract no NO-ES-75558), NIH/NINDS (Grant No. 1 UO1 NS 047537-01), and the Norwegian Research Council/FUGE (Grant No. 151918/S10).

Notes

Author Biographies

Nina Rohrer-Baumgartner, MSc, is a clinical psychologist and researcher at the Norwegian Institute of Public Health, Division of Mental Health.

Pål Zeiner, PhD, is a specialist in child and adolescent psychiatry, researcher at the Oslo University Hospital, and principal investigator of the Norwegian Longitudinal ADHD Study.

Patricia Eadie, PhD, is a speech pathologist and researcher with expertise in child language development and disorders. She is a senior lecturer in Speech Pathology at the University of Melbourne.

Jens Egeland, PhD, is director of research at the Division of Mental Health and Addiction in Vestfold Hospital Trust in Norway, as well as adjunct professor of neuropsychology at the University of Oslo.

Kristin Gustavson, PhD, is a specialist in clinical psychology and works as a researcher at the Norwegian Institute of Public Health, Division of Mental Health.

Ted Reichborn-Kjennerud, PhD, is professor of psychiatric epidemiology at the University of Oslo and department director at the Norwegian Institute of Public Health, Division of Mental Health. He was one of the founders of the Norwegian Longitudinal ADHD Cohort Study.

Heidi Aase, PhD, is department director at the Norwegian Institute of Public Health, Division of Mental Health, and principal investigator at the Norwegian Longitudinal ADHD Study.

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