Title: Corpus callosum abnormalities: neuroradiological and clinical correlations.
Authors: Al-Hashim A. H., Blaser, S., Raybaud, C., & MacGregor, D.
Journal: Developmental Medicine and Child Neurology, 58, 427-436.
Published: 2015
The authors reviewed hospital records (from the Hospital for Sick Children, Toronto) of patients who had an MRI scan because concerns had been raised during the prenatal scan or there had been concerns about early development. In total 125 patients (aged 1 to 18 years) were identified who had abnormalities of the corpus callosum from scans conducted between 1999 and 2012.
In a typically developing brain, different regions of the corpus callosum are thought to be responsible for connecting distinct areas of the brain. The authors were interested in whether certain problems could be predicted by the part of the corpus callosum that was absent; for example, whether those without the front (anterior) region had problems with motors skills and those without the end (posterior) region had problems with vision. They were also interested in whether the presence of other brain malformations influenced outcome.
Most of the sample had complete agenesis of the corpus callosum, with no connecting fibres present (52%). A small proportion (2%) had the front regions (rostrum, genu, and body) absent, while it was more common (30%) to have the end regions (isthmus and splenium) absent when the diagnosis was partial agenesis of the corpus callosum. The remainder of the sample (16%) had an unusually thin corpus callosum (hypoplasia).
Their main finding was that cardiac anomalies were more common in the group with complete agenesis (25% of this group); however grouping patients by the type of agenesis was not associated with any particular outcome. For example, visual impairment occurred in 40% of the sample but was not more common in those with posterior agenesis as predicted. The authors suggest that perhaps more sophisticated measures of vision may detect differences.
Overall the sample had a high level of associated problems: developmental delay (77%), gross motor delay (74%), speech delay (74%), social delay (66%) and behavioural issues (49%). Epilepsy was diagnosed in 36% of patients and dysmorphic features were found in 61%. It should be noted that the sample had been referred to a tertiary healthcare centre and therefore would be expected to have more complex medical problems.
Probst bundles describe the fibres that fail to cross the midline between the two hemispheres and get rerouted. These were noted in 60% of the sample and often occurred alongside colpocephaly (enlargement of ventricles in the brain). Interestingly the presence of Probst bundles was associated with better adaptive and social functioning and may signify the important role of Probst bundles as compensatory fibres.
It was reported that 37% of the sample had an identifiable genetic cause of the corpus callosum abnormality such as a single gene disorder, copy number variants, or a recognisable syndrome. Interestingly, patients who had complex form of corpus callosum abnormalities (i.e., additional brain malformations) were no more likely to have an underlying genetic diagnosis (i.e., a positive result on a chromosomal microarray) than those with isolated forms. Additional brain abnormalities were fairly common (46% of the sample) and were associated with a higher risk of epilepsy, and delays in gross motor skills and speech.
The authors recommend that cardiac evaluations should be part of routine screens, in addition to the standard visual and auditory tests that are taken. Furthermore they stress the importance of genetic testing in all individuals with corpus callosum malformations, as it may not necessarily be the case that a high risk of chromosomal abnormalities is confined to complex forms of ACC.