Factors associated with lowered intelligence in homozygous sickle cell disease

The intelligence quotient (IQ) of 60 patients with homozygous sickle cell (SS) disease and 60 age and sex matched controls with a normal haemoglobin (AA) genotype aged 15-18 years, followed up in a cohort study from birth, was assessed by the Wechsler intelligence scales for children or for adults. IQ appeared to be normally distributed in both genotypes but mean values in SS disease were 5.6 points (95% confidence interval (CI) 1.0 to 10.2) lower than in AA controls (p = 0.016). The difference occurred in both verbal (5.5 points, p = 0.017) and performance (5.0 points, p = 0.044) subscales of the IQ score and the IQ defect in SS disease was associated with a significantly lower attention factor score (p = 0.005) but not with other factor scores. The genotype difference in IQ was not accounted for by differences in parental occupational level, school absenteeism, or school drop out, or reported activity level. In SS disease, IQ was not related to mean steady state haemoglobin, fetal haemoglobin, or mean cell haemoglobin concentration, or clinical severity as judged by the frequency of painful crises, hospital admission, or sick visits. IQ, at age 15-18 years, correlated with the patients' height at all ages from 1 to 10 years (partial correlations increasing from 0.14 (p = 0.15) at age 1 to 0.27 (p = 0.004) at age 10). Adjusting for height reduced the mean genotype difference in IQ to 5.5 (95% CI 0.6 to 10.3) points at age 1 and to 2.6 points (95% CI to -2.3, 7.5) at age 10. Prepubertal height therefore accounted for much of the genotype difference in IQ. It is speculated that early factors, possible nutritional, contribute to both impaired growth and mental development in sickle cell disease.