We conducted the first large-scale MRI study of brain morphological differences and their relationship to cognitive or behavioural abnormalities in adults with NF1. We found that adults with NF1 have apparent enlargement of the corpus callosum and brainstem in comparison to unaffected adults. These 2D morphological enlargements are correlated to increased total white matter volume. In our companion study focused on brain volume, we also found an increase in total and regional white matter in the brains of adults with NF1 compared to control adults(28). In the current study, we did not find any obvious correlation between the brain morphological changes we observed and the clinical or neuropsychometric assessments in these individuals.
In the present study, we observed increased CC area, height, and anterior body and mid-body widths among adults with NF1. Our findings are consistent with those of previous studies in children and smaller groups of adults with NF1(23, 25). Also, previous brain MRI studies in people with NF1 have found evidence of increased total white matter volume, increased brain volume, and megalencephaly(6, 7, 9, 10, 19–22). Similarly, we found strong correlation between 2D CC size and total and CC white matter volume.
Loss of function mutations of NF1 cause dysregulation of proliferation in Schwann cells, which are responsible for the myelination of axons in the peripheral nervous system(16). In the brain, white matter is mainly comprised of myelinated axons(39). It is hypothesized that the increases in total white matter volume, total brain volume, and megalencephaly that occur in NF1 are related to the dysregulation of oligodendrocytes, the myelin-producing cells in the central nervous system(23, 40). If this interpretation is correct, enlargement of the CC, a structure largely composed of white matter, might be expected in people with NF1. The decrease in genu width and increase in the anterior and mid-body widths raises the possibility that the CC shape is also changed. Enlargement of the CC height and area without significant alteration of the length is consistent with a change in CC shape and volume.
The enlargement of the brainstem that we observed among adults with NF1 is a novel finding. The brainstem consists of both grey and white matter, with the MCP (which was clearly enlarged among adults with NF1) comprised mostly of white matter(41, 42). This is further evident by the strong correlation we found between increased brainstem 2D morphology measurements (specifically pons and MCP) and increased total white matter volume. Thus, the brainstem enlargement we observed is also consistent with dysregulated myelin proliferation in individuals with NF1.
Both age and sex are known to affect CC size, ocular globe size and position, and brainstem size(43–47). We carefully matched unaffected individuals by age and sex to avoid confounding by these factors in our analysis. However, the adult NF1 group was recruited from Hamburg, Germany, while the unaffected comparison group was obtained in Vancouver, Canada. Different MR imaging procedures and measurement software were used for the NF1 group and the unaffected comparison control group. The different imaging procedures resulted in an inability to detect differences between the two groups smaller than 0.05 cm, but all statistically significant differences in brain morphological measurements observed between NF1 and control subjects were greater than 0.05 cm.
Another limitation of our study is the lack of dedicated orbital MRIs. The asymmetry in the ONT we observed probably does not represent a true anatomical difference and may be a result of measurement error, as previous studies have not found ONT asymmetry(26, 27). The current study is less accurate than Ji et al.’s (2013), as we only measured ONT in one axial plane while Ji and associates used dedicated 3-dimensional magnetization-prepared rapid gradient echo sequences with 1 mm slices(27).
Our correlation analyses between structures measured on MRI and clinical/neuropsychometric assessments are limited by small sample sizes as only a subset of the adults with NF1 had psychometric testing. Some previous studies have found that increased CC volume or CC index correlated with decreased academic achievement and IQ in children with NF1(10, 24). In contrast, Kayl et al. (2000b) found that among children with NF1, smaller splenium size was correlated with increased attention problems as reported by teachers(29). Furthermore, a more recent study using diffusion tensor imaging to examine myelination of white matter specifically failed to find a significant relationship between total CC area and IQ scores in children with NF1(30). We did not find any robust correlations between the brain morphology and neuropsychometric measurements in adults with NF1.
Clinically, 2D MRI is used more often than 3D MRI. We conducted the largest study of 2D brain morphology in adults with NF1 reported to date to characterize the brain morphology alterations. The enlargement of the CC and brainstem and correlation to increased total white matter volume that we observed in adults with NF1 lends support to the hypothesis that neurofibromin haploinsufficiency causes dysregulation of myelin production in the brain. The relationship of this overgrowth of myelinated brain structures to the frequent occurrence of central nervous system gliomas and of benign and malignant peripheral nerve sheath tumours in individuals with NF1 is unknown but merits further study.