BERLIN – Demyelinating diseases appear to disrupt white matter development in children, slowing the trajectory of brain growth to almost unmeasurable levels, based on results from a single-center comparison study of 213 individuals.
Dr. Robert A. Brown
While children with multiple sclerosis (MS) showed the most severe slowing, even a single demyelinating event slowed white matter growth, Robert A. Brown, PhD, said at the annual congress of the European Committees for Treatment and Research in Multiple Sclerosis.
Dr. Brown of the Montreal Neurological Institute at McGill University, Montreal, employed a signal mass correction of consecutive brain MRIs enhanced with magnetization transfer. Magnetization transfer ratio (MTR) quantifies myelin more effectively than does other imaging enhancement modalities, Dr. Brown said.
“It labels the macromolecules of myelin and correlates almost perfectly with Luxol fast blue stain on histology,” he said. And by measuring myelin instead of whole-brain volume, MTR sidesteps the confounders of inflammation and edema. “When tissue swells, the water dilutes the myelin. MRI is really sensitive to density, so dilution with water lowers that signal.”
But MTR isn’t failsafe either, he said, especially in teens. “A cautionary note: In healthy adolescents, white matter MTR can actually decrease, not increase, not because they are losing myelin but because the axons in brain tissue are growing so fast that they outstrip the production of new myelin. So, we can get another dilution effect here, except that instead of water, axons are diluting the myelin. We have to take that into account when using MTR.”
A volume-corrected MTR calculates both mass and volume to give what Dr. Brown termed signal mass. “We have demonstrated previously that signal mass is about twice as powerful as volume change alone for measuring the differences [in brain volume] between adults with MS and healthy controls.”
The study he presented at ECTRIMS used this technique to examine the trajectory of white matter change in a cohort of children from the Canadian Pediatric Demyelinating Disease Study who were all scanned at the same site in the same center. He compared brain volume at baseline and 1 year in 102 children with a monophasic demyelinating disease, 87 with MS, and 24 healthy, age-matched controls.
The children with MS were a median of about 17 years old at baseline, while those with a monophasic event and healthy controls were a median of about 12 years old. Median follow-up was 1 year in the healthy controls, 2 years in the MS cohort, and 4 years in the monophasic group. The investigators adjusted their comparisons for sex, since both bioavailable testosterone and androgen-receptor activity correlate with decreased MTR in young men. This doesn’t mean, though, that testosterone decreases myelination. Rather, it’s postulated that testosterone increases axonal caliber, which would decrease the number of neurons in each imaging voxel and, thus, the MTR signal (J Neurosci. 2008 Sep 17;28[38]:9519-24).
In the volume-only assessment, white matter in healthy controls increased at a rate of about 0.5% per year. White matter growth was about 0.2% per year in children with monophasic demyelination, which was significantly lower than in healthy controls.
“The MS children had no white matter growth that we could measure,” with an annual change of about 0.01%, Dr. Brown said. “It looks like a failure of normal development and was significantly lower than what we saw in the children with a demyelination event.”
MTR showed the expected age-associated decreases, which were highest among those with MS: –0.8% per year in healthy controls, –0.6% per year in those with a monophasic event, and –0.9% per year in those with MS.
The signal mass change showed the whole picture, Dr. Brown said. Signal mass declined 0.3% per year in healthy controls, 0.5% per year in the monophasic group, and 0.9% per year in the MS group – a significantly worse trajectory than either the control subjects or those with a monophasic event.
“Signal mass puts it all together and gives us the total picture of tissue loss, with quite severe loss in children with MS. It seems as though both monophasic insult and pediatric-onset MS disrupt brain development.”
Dr. Brown has been a consultant for NeuroRx Research and Biogen.
SOURCE: Brown RA et al. Mult Scler. 2018;24(S2):27-8, Abstract 63.