Slower Pace of Intellectual Development Is Common in Children With Cerebral Palsy–A Population‐Based Study

ABSTRACT

Aim

To describe the intellectual level and changes during development in children with cerebral palsy (CP) and to investigate if there are factors associated with the pace of intellectual development.

Method

Population‐based study of all 264 children (141 boys, 123 girls) with CP born 1999–2006 in the region of Västra Götaland. Information was retrieved from the CP Register of western Sweden and all medical records to the age of 18 years. For the 171 children assessed more than once, results from first and last assessments were further compared.

Results

Intellectual disability became more common with increasing age. Forty percent of the 264 children were diagnosed with intellectual disability at age 7, 50% at age 12% and 54% at age 18. Of the 171 children assessed more than once, 55% showed a lower intellectual level over time. Epilepsy, predominant grey matter brain injury, impaired communication and fine motor function were associated with increased risk of slower pace of intellectual development.

Conclusion

Intellectual disability as well as slower pace of intellectual development over time are common in children with CP, highlighting the importance of repeated intellectual assessments, especially in children with epilepsy, predominant grey matter brain injury, impaired communication or fine motor function.

Summary

• Intellectual disability was present in 54% of 18‐year‐old with cerebral palsy.

• Repeated assessments showed a lower intellectual level over time in more than half.

• Slower pace of intellectual development was associated with epilepsy, grey matter brain injury, impaired communication, and impaired fine motor function.

Abbreviations

ADHD

attention‐deficit/hyperactivity disorder

BFMF

bimanual fine motor function

CFCS

communication function classification system

CP

cerebral palsy

DSM

diagnostic and statistical manual of mental disorders

GMFCS

gross motor function classification system

ICD

International Statistical Classification of Diseases and Related Health Problems

ID

intellectual disability

IQ

intelligence quotient

IQR

interquartile range

MACS

manual ability classification system

MRI

magnetic resonance imaging

MRICS

mri classification system

SCPE

Surveillance of Cerebral Palsy in Europe

VSS

Viking Speech Scale

1. Introduction

Cerebral palsy (CP) is an umbrella term for non‐progressive movement and posture disorders caused by inborn or early acquired brain lesions [1, 2, 3]. It is the most common motor disability in childhood and occurs in around two per 1000 live births in high‐income countries and is estimated to be more than three in low‐ and middle‐income countries. The motor disorders are often accompanied by other impairments, for example, perception, communication, cognition, epilepsy, autism, and attention‐deficit/hyperactivity disorder (ADHD) [4, 5, 6].

The cognitive impairments vary between and within different types of CP, and the interplay between original brain lesions, co‐morbid impairments and life experiences is complex [7]. Intellectual disability (ID) is present in approximately 30%–50% of children with CP [8, 9].

Even though the original brain damage is non‐progressive the effects may vary over time as life situations and external demands change [10]. Intellectual level is an important factor of functioning in life with implications for, for example, health, education and employment. Longitudinal studies of intellectual abilities in children with CP sometimes show declining results with age while the results remain stable in others [11, 12, 13].

The aim of this study was to describe the intellectual development in a large and population‐based group of children with all types of CP from the CP Register of western Sweden and how it changed during development. An additional aim was to investigate if there were factors associated with increased intellectual difficulties over time.

2. Method

2.1. Participants

The study population comprised eight birth‐year cohorts of children and adolescents with CP born 1999–2006 from the region of Västra Götaland, covered by the CP Register of western Sweden. This group of 264 children (see Table 1) has been the focus of several previous studies. They were first described in the CP Register at 4–8 years of age [14, 15] and later screened and assessed for neuropsychiatric impairments at 10–17 years of age [5, 16, 17, 18]. In the present study all 264 children were studied retrospectively up to the age of 18 years focusing on intellectual assessments and evaluations. The 171 children assessed at least twice were then analysed in more detail.

TABLE 1.

Intellectual assessments and development over time in a total population of 264 children with cerebral palsy (CP) born 1999–2006 from the CP Register of western Sweden. Intellectual development is presented in relation to background characteristics, CP type, classification of gross motor and fine motor functioning, communication, MRI findings and associated impairments.

	Total	Number of assessments		Intellectual level over time for the 171 children assessed at least twice
		0–1	2–6	Higher	Same	Lower
	n	n	n	n (%)	n (%)	n (%)
n	264	93	171	8 (5)	69 (40)	94 (55)
Sex
Male	141	42	99	5 (5)	42 (42)	52 (53)
Female	123	51	72	3 (4)	27 (38)	42 (58)
Gestational age
Week 23–27	26	3	23	4 (17)	7 (30)	12 (52)
Week 28–31	30	11	19	0 (0)	11 (48)	18 (52)
Week 32–36	45	16	29	1 (3)	8 (28)	20 (69)
Week 37–42	163	63	100	3 (3)	43 (43)	54 (54)
CP type
Unilateral spastic CP	103	33	70	3 (4)	33 (47)	34 (49)
Bilateral spastic CP	98	34	64	5 (8)	22 (34)	37 (58)
Dyskinetic CP	45	24	21	0 (0)	6 (29)	15 (71)
Ataxic CP	18	2	16	0 (0)	8 (50)	8 (50)
GMFCS
I	127	43	84	4 (5)	40 (48)	40 (48)
II	40	13	27	1 (4)	12 (44)	14 (52)
III	20	2	18	0 (0)	3 (17)	15 (83)
IV	35	6	29	3 (10)	10 (34)	16 (55)
V	42	29	13	0 (0)	4 (31)	9 (69)
BFMF
I	97	35	62	3 (5)	33 (53)	26 (42)
II	67	17	50	2 (4)	18 (36)	30 (60)
III	41	8	33	2 (6)	12 (36)	19 (58)
IV	24	9	15	1 (7)	4 (27)	10 (67)
V	35	24	11	0 (0)	2 (18)	9 (82)
MACS
I	93	34	59	3 (5)	30 (51)	26 (44)
II	59	15	44	2 (5)	21 (48)	21 (48)
III	35	9	26	1 (4)	6 (23)	19 (73)
IV	33	7	26	2 (8)	9 (35)	15 (58)
V	44	28	16	0 (0)	3 (19)	13 (81)
CFCS
I	116	49	67	4 (6)	38 (58)	25 (37)
II	45	3	42	1 (2)	12 (29)	29 (69)
III	30	5	25	0 (0)	8 (32)	17 (68)
IV	30	5	25	3 (12)	7 (28)	15 (60)
V	43	31	12	0 (0)	4 (33)	8 (67)
Viking Speech Scale (VSS)
I	122	44	78	4 (5)	35 (45)	39 (50)
II	58	10	48	3 (6)	21 (44)	24 (50)
III	17	3	14	0 (0)	4 (29)	10 (71)
IV	67	36	31	1 (3)	9 (29)	21 (68)
Visual impairment
No	213	67	146	6 (4)	62 (42)	78 (53)
Yes	51	26	25	2 (8)	7 (28)	16 (64)
Hearing impairment
No	243	88	155	8 (5)	63 (41)	84 (54)
Yes	21	5	16	0 (0)	6 (38)	10 (63)
Epilepsy
No	155	57	98	5 (5)	49 (50)	44 (45)
Active	109	36	73	3 (4)	20 (27)	50 (68)
MRICS
A	23	8	15	0 (0)	7 (47)	8 (53)
B	82	22	60	4 (7)	24 (40)	32 (53)
C	82	32	50	2 (4)	14 (28)	34 (68)
D	29	8	21	2 (10)	8 (38)	11 (52)
E	25	10	15	0 (0)	8 (53)	7 (47)
Not performed	23	13	10	0 (0)	8 (80)	2 (20)
Autism
No	193	84	109	5 (5)	50 (46)	54 (50)
Yes	71	9	62	3 (5)	19 (31)	40 (65)
ADHD
No	194	86	108	5 (5)	45 (42)	58 (54)
Yes	70	7	63	3 (5)	24 (38)	26 (57)

Abbreviations: ADHD, attention‐deficit/hyperactivity disorder; BFMF, bimanual fine motor function; CFCS, communication function classification system; GMFCS, gross motor function classification system; MACS, manual ability classification system; MRICS, magnetic resonance imaging classification system; VSS, Viking Speech Scale.

2.2. Data Collection

All available intellectual assessments were retrieved from the medical records and reviewed and verified by authors M.W.S. (licensed psychologist) and M.P. (paediatric neurologist). The 264 children had been assessed 562 times during the years 2001–2024 at different healthcare providers and assessment centres but in similar ways. The assessments had sometimes been based on concerns but mostly made as part of regular follow‐ups, the latter also becoming more common over time [19].

Most assessments had been made by a multidisciplinary team including at least a psychologist and a paediatric neurologist. Speech and language pathologist, occupational therapist, physiotherapist and special education teacher had often been involved, especially in the children with more severe impairments. Information had been gathered using a combination of different methods: clinical assessments, neuropsychological tests, questionnaires and interviews with parents and teaching staff. The collected information had been discussed and a decision about intellectual level made by the assessment team.

The most used tests had been different forms and versions of the Wechsler scales (Wechsler Preschool and Primary Scale of Intelligence, Wechsler Intelligence Scale for Children and Wechsler Adult Intelligence Scale, Wechsler Nonverbal Scale of Ability), used in more than three‐quarters of the assessments. The Griffiths Scales of Child Development, the Bayley Scales of Infant Development, and the Merrill‐Palmer Scales of Development had been used for children at lower levels of development. Tests had often been administered in part, or several different tests had been combined.

When necessary, adaptations of test procedures had been made to compensate for impairments of, for example, hearing, vision, movement or communication [20]. Several different alternative response modes had been used, such as gaze pointing, sometimes using frames, partner‐assisted responses, and different forms of augmentative and alternative communication. Over time the use of tablets and computers had become more common. Other adaptations included the use of enlarged visual materials, allowing extra time for problem‐solving tasks or excluding tests demanding advanced fine motor skills.

The most used questionnaires had been different versions of the Adaptive Behavior Assessment System and the Vineland Adaptive Behavior Scales, both focusing on everyday adaptive functioning and completed by parents and teaching staff.

The results were categorised according to intellectual level and not as a continuous variable, that is, intelligence quotient scores (IQ). The reasons were both to emphasise the comprehensive nature of the assessments as well as to accommodate the differences in used methods and in how detailed the results were described in the available medical records. Six intellectual levels were defined according to International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD‐10) and Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM‐5): average intellectual functioning (IQ 85–115), borderline intellectual functioning (IQ 70–84), mild ID (IQ 50–69), moderate ID (IQ 35–49), severe ID (IQ 20–34) and profound ID (IQ < 20).

Thirty‐six of the 264 children had not been formally assessed using tests, but instead decisions about intellectual level had been made before school start based on clinical evaluation and information from parents and preschool teaching staff.

2.3. Definitions

CP was classified according to Surveillance of Cerebral Palsy in Europe (SCPE) into unilateral spastic, bilateral spastic, dyskinetic, or ataxic [2]. Gross motor function was classified using Gross Motor Function Classification System (GMFCS) from I to V [21]. Fine motor functioning was classified using Bimanual Fine Motor Function (BFMF) [22] and Manual Ability Classification System (MACS) [23] from I to V. Everyday communication was classified according to Communication Function Classification System (CFCS) [24] from I to V and speech production according to Viking Speech Scale (VSS) [25] from I to IV.

Gestational age was described as extremely preterm (≤ 27 weeks), very preterm (28–31 weeks), moderately preterm (32–36 weeks) or term (≥ 37 weeks). Visual impairment was defined as an acuity of no more than 0.3 in the best eye with correction. Hearing impairment included sensorineural impairment or deafness (unilateral or bilateral). Epilepsy was defined as epilepsy under treatment according to medical records. MRI findings were classified using the MRI classification system (MRICS) into maldevelopments (A), predominant white matter injury (B), predominant grey matter injury (C), miscellaneous (D) or normal (E) [26]. Clinical diagnoses of autism and ADHD had been made based on DSM‐5 criteria.

2.4. Analysis

All assessments and evaluations were compiled. Children assessed at least twice (n = 171) were analysed further. The results from the first and the last assessment were compared, and the outcome categorised as higher, same, or lower intellectual level over time. Background characteristics, CP type, motor functioning, communication, MRI findings, and associated impairments were analysed for factors associated with a lower intellectual level over time.

Descriptive statistics were used to compare groups (see Table 1). For the association between categorical variables, the χ ² test of independence was used, and if there were more than two categories within an ordinal scale, the Cochran‐Armitage χ ² test for trends was used. A p < 0.05 was considered statistically significant.

2.5. Ethics

The study was approved by The Regional Ethics Review Board in Gothenburg, ref. 145‐07 and 398‐12. Consent was not required in this register‐based research.

3. Results

Two hundred twenty‐eight children had been assessed in altogether 562 assessments. The 36 children not formally tested were considered as having either an average intellectual level (n = 28), severe ID (n = 2) or profound ID (n = 6) based on clinical evaluation and information from parents and teaching staff. Fifty‐seven children had been assessed once, 24 of whom had an average intellectual level, two borderline intellectual functioning and 31 ID (one mild, four moderate, five severe and 21 profound ID).

In the total group of 264 children, diagnosis of ID became more common with increasing age.

A few children had been diagnosed with ID at 2 years of age, increasing to 40% at age 7, 50% at age 12 and 54% at age 18. Four children had died before 18 years of age, one at age 12 and three at age 14; one of these had a severe ID and three a profound ID. At the end point, 141 out of 260 children were diagnosed with ID. Results from all intellectual assessments and clinical evaluations are presented by intellectual level at each year from 1 to 18 years of age in Figure 1. The combined proportion of severe and profound ID was stable from age 7, while mild and moderate ID increased to 18 years of age.

FIGURE 1.

Intellectual levels by age in a total population of 264 children with cerebral palsy (CP) born 1999–2006 from the CP Register of western Sweden. Intellectual levels were results from either intellectual assessments or clinical evaluations and are presented for each year of age based on the most current assessment. All children had been assessed or evaluated before school starting age. Four children had died before the age of 18 years.

In addition, 108 of the 264 children (42%) had been diagnosed with autism and/or ADHD; 38 (15%) with autism, 37 (14%) with ADHD, and 33 (13%) with both. Children with ID more often had autism and/or ADHD (72/141, 51%) than children with average or borderline functioning (36/119, 30%).

The 171 children assessed at least twice were analysed further. They had been assessed two to six times each (median three times) in total 505 assessments. The assessments had been administered at different ages and intervals. Most children (n = 150) had been assessed before starting school at the age of 7. The median age at first assessment was 5 years 3 months (interquartile range IQR 2 years 0 months). The majority (n = 111) had been assessed for the last time after the age of 10, although with a greater age variation. The median age at last assessment was 12 years 11 months (IQR 5 years 6 months). The median time between the first and last assessment was 7 years 2 months (IQR 5 years 9 months).

As shown in Table 1 and illustrated in Figure 2 eight children (5%) were found at a higher intellectual level, 69 (40%) at the same intellectual level, and 94 (55%) at a lower intellectual level at the last assessment compared to the first assessment.

FIGURE 2.

Changes in intellectual level from the first to the last assessment in 171 children with cerebral palsy (CP) born 1999–2006 from the CP Register of western Sweden who had been assessed at least twice during development. Changes are presented in a Sankey diagram made with SankeyMATIC.

Most of the 94 children with a slower pace of intellectual development were found one level below first assessment (n = 69). The remaining changed two levels (n = 25), most of them from average intellectual functioning to mild ID (n = 18).

At the first assessment 105 children (61%) had an average or borderline intellectual functioning, while 66 (39%) had some level of ID. At the last assessment, the opposite pattern was found, with 106 (62%) having an ID.

The risk of a lower intellectual level over time was not associated with age at first or last assessment or with the intellectual level at first assessment, except for the children with a profound ID at first assessment.

3.1. Associated Factors

Background characteristics, CP type, classification of gross motor and fine motor functioning, communication, MRI findings and associated impairments are presented in Table 1.

A higher risk of a lower intellectual level over time was associated with three factors: (1) epilepsy (χ ² = 9.41; p = 0.002), (2) impaired fine motor functions in both BFMF and MACS classifications (χ ² trend = 7.42; p = 0.006 and χ ² trend = 8.34; p = 0.004), (3) impaired communication expressed as having a speech not understandable to unfamiliar listeners or being without speech, VSS level III–IV (χ ² = 4.78; p = 0.044). In addition, independent and effective communication expressed as CFCS level I was associated with a lower risk compared to CFCS levels II–V (χ ² = 13.88; p < 0.001).

No association with a lower intellectual level over time was found regarding sex, gestational age or CP type, nor with gross motor function classified as GMFCS levels (χ ² trend = 3.53; p = 0.06), or with visual or hearing impairments, autism or ADHD.

The underlying brain lesions were analysed for patterns with a clear pathology. Children with predominant grey matter injury (MRICS C) more often had a lower intellectual level over time (χ ² = 4.85; p = 0.028), but no association was found for malformations (MRICS A) or predominant white matter injury (MRICS B). Epilepsy was also more common in the children with predominant grey matter injuries.

The eight children with a higher intellectual level over time were more often at higher levels of all the functional scales (see Table 1).

4. Discussion

ID was common in this population‐based group of children with CP and repeated assessments over time indicated that more than half were found at a lower intellectual level at the last assessment compared to the first. The proportion of individuals with ID (54%) is higher than most previous reports [9]. This difference is most likely attributable to the fact that our population was followed until 18 years of age in contrast to lower proportions often reported at younger ages, a key point of our study. Importantly, our findings indicate a slower pace of intellectual development compared to the general child population, with no evidence suggesting loss of abilities.

No obvious conclusions can be drawn from the present study regarding at what age the intellectual development started to slow down, but some previous studies have suggested around the age of 5 years [10]. This is paralleled by gross motor functioning as previous studies have shown gross motor function curves reaching plateaus around the same age, depending somewhat on the children's GMFCS levels [27]. The age when the pace of intellectual development starts to slow may also differ and depend on the intellectual level or other impairments, making it harder to draw conclusions.

Assessing younger children for the first time can be challenging and, in clinical practice, may result in a milder ID level than the results indicate, especially in children with more severe impairments. However, the proportion of combined severe and profound ID remained stable from 7 years of age, when all children had been assessed or evaluated. There may also be a need for at least two assessments to confirm an ID diagnosis if the child's IQ is close to 70.

For the children possible to test with the Wechsler scales, full scale IQ was used as the measure of intellectual functioning in this study due to the nature of the available data. Since children with CP often display uneven cognitive profiles, for example, regarding verbal and non‐verbal IQ, it is important to keep in mind that this composite measure may mask changes, strengths, or weaknesses still affecting everyday functioning in significant ways [28, 29]. However, it is also important to note that the assessments as described above most often were based on several different sources of information discussed in the assessment teams, thus also considering the everyday adaptive functioning and not only focusing on the IQ or changes to IQ as measured by tests.

The reasons behind the slower pace of intellectual development displayed by many of the children in this study population remain unclear but, as has previously been suggested, it is probably related to a complex interaction of many different factors that may also have cumulative impacts [7, 9, 10, 11, 12, 13]. The original brain lesion resulting in less remaining brain tissue may prevent the pruning of excess synapses necessary for later intellectual development or, in other ways, prevent the development of more complex intellectual abilities required at later ages. The lack of intact neural capacity may also mean that not all abilities can develop, as is seen in the general population, resulting in uneven intellectual abilities and development. Impairments may also be present from birth but become apparent only with increasing age or when demands from the environment exceed the abilities. The often‐restricted life experiences of impaired children, with fewer opportunities for exploring and learning, may also be a relevant factor, as well as suboptimal nutrition [10, 12].

Significant associations were found in this study between lower intellectual level over time and epilepsy, predominant grey matter injuries, impaired fine motor skills, and impaired communication. Epilepsy is a widely known adverse factor for intellectual abilities and is associated with more widespread brain injuries and a greater burden of life, both in children with and without CP [30]. Grey matter injuries have in previous studies been found to be more common in children with CP and intellectual disabilities [9, 31].

Severe fine motor impairments were associated with a higher risk of a slower pace of intellectual development. Fine motor skills and intellectual abilities involve some of the same brain areas and share similar developmental timescales, suggesting a relationship. Previous studies have proposed that the association may be explained both by the patterns of the brain lesions and the interplay between motor and intellectual development [32, 33].

Finally, children with severe speech impairments more often had a lower intellectual level over time while children with independent and effective communication had a more positive intellectual development. There are well‐known connections between expressive language, receptive language, and intellectual abilities, and previous studies have also shown relationships between fine motor skills, communication, and intellectual functioning [34]. A severely impaired communication probably also contributes to more restricted interactions and life experiences, which may in turn affect the intellectual development [12].

Further studies may aim to describe and analyse results from intellectual assessments in greater detail or have a prospective design and use a study protocol, thus making it possible to, for example, analyse changes to intellectual abilities over time in greater detail. It would also be interesting to explore the interactions between intellectual development, adaptive behavior, participation in everyday activities, and environmental factors.

4.1. Strengths and Limitations

This large group of children with CP is population‐based and emanates from a well‐maintained CP‐register with high coverage of children with all types of CP and levels of impairments. There was good access to medical records over the long timespan. Other factors that may be considered as strengths are that assessments were made by clinicians, often working in multidisciplinary teams, with the use of several different sources and types of information, and that adjustments were made to testing procedures to facilitate the assessments.

A limitation of this study may be the retrospective approach and lack of a study protocol, making it difficult to compare results in greater detail, investigate how different aspects of cognitive functioning develop over time or investigate at what age the pace of intellectual development started to slow down. Another limitation may be that some of the children from the original study group were clinically evaluated without formal neuropsychological testing or assessed only once, even though most of these children were considered as either not intellectually impaired or very impaired. The lack of standardised intellectual measures for people with severe impairments of hearing, vision, movement or communication may also be seen as a limitation.

5. Conclusion

ID is common in children with CP and the prevalence increases with age. More than half of the children assessed at least twice showed a slower pace of intellectual development. Epilepsy, predominant grey matter brain lesions, impaired communication and fine motor function were associated with increased risk of a slower than expected intellectual development. The results highlight the importance of intellectual assessments in children with CP as well as the need for repeated assessments over time.

Author Contributions

Mattias Wicke Selvén: validation, formal analysis, writing – original draft, writing – review and editing, data curation, investigation. Kate Himmelmann: conceptualization, methodology, validation, supervision, resources, writing – review and editing. Magnus Påhlman: validation, formal analysis, supervision, visualization, writing – review and editing, writing – original draft, methodology, conceptualization, data curation, investigation.

Funding

This work was supported by the Gothenburg Society of Medicine; Linnea and Josef Carlsson Foundation; Foundation Sunnerdahl Disability Fund.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.