To investigate differences in total water content within normal appearing white matter (NAWM) and lesions of subjects with multiple sclerosis (MS), neuromyelitis optica (NMO) and healthy controls.MS is characterized by demyelination and axonal damage in the central nervous system (CNS). NMO is an immune mediated inflammatory and demyelinating disorder of the CNS, preferentially affecting spinal cord and optic nerves.¹ While MS and NMO have overlapping clinical features, recent discovery of an antibody (NMO-IgG) in the blood of individuals with NMO, indicate that they are distinct diseases.² NAWM is known to be affected by diffuse tissue damage in MS while damage in NMO is thought to be restricted to acute lesions. The NMO-IgG antibody targets the protein aquaporin-4 in the cell membranes of astrocytes which acts as the most abundant channel in the CNS for the transport of water across the cell membrane. Changes in water content (WC) in the CNS of subjects with NMO seem probable due to the involvement of the water channel in this disease. Therefore, the pattern of water content changes in MS and NMO compared to controls was explored.10 subjects with relapsing remitting MS (mean age=40y, median EDSS=2.5), 9 subjects with NMO (mean age=43y, median EDSS=2.5, 7 NMO-IgG positive) and 10 healthy controls (mean age=41y) were scanned on a 3.0T Philips Achieva system (Best, The Netherlands). Scans included a 32 echo GRASE sequence (TE/TR=10/1000ms, 1x1x5mm³, 20 slices, EPI factor=3),³ DESPOT1-HIFI T1 sequence (TR=6.5, SPGR flip angles=2,3,4,6,9,13,18^o/TE=3.6ms, IR-SPGR flip angle=5^o/TE=3.2ms, 1.7x1.7x1.7mm³),⁴ a 3D-T1 turbo field echo sequence (TR/TE=28/4ms) and a FLAIR sequence (TR/TI/TE=900/2500/80ms). Water content was calculated using the cerebrospinal fluid reference method outlined in Figure 1.^5,6 Lesions were segmented using a semi-automated in-house method.⁷ Segmentation of NAWM was performed using FAST (part of the FSL library) on the 3DT1 image with lesions removed. Individual white matter tracts were segmented by inverse registration of atlas-based regions of interest from standard space to subject space using FNIRT (part of the FSL library). These regions of interest (ROI) were then overlaid on the WC images to extract the mean WC for each ROI.On average, NAWM WC was higher in MS (72.1±3.0%,p=0.02) and NMO (71.8±1.6%,p=0.05) compared to controls (69.4±2.3%). Patterns of results were similar across most white matter tracts (Figure 2). NMO lesions had non-significant higher WC than MS lesions (84.6±5.6% vs 81.4±6.3%, p=0.3). There was a large range in lesion WC for different subjects as well as different lesions within subjects (Figure 2).

The NAWM of MS and NMO subjects had a higher water content than healthy controls likely due to low levels of ongoing inflammation and edema. Although unexpected in NAWM, the diffusely increased water content in NMO was similar to WC measured in previous studies of MS white matter.⁸ This increase in NMO WC did not seem to be driven by particular white matter tracts or locations of high aquaporin-4 expression, but was rather diffuse throughout the brain. Thus the widespread changes in water content within the NAWM of subjects with NMO may indicate a more widespread process, which will require further studies. NMO tissue studies do suggests there may be ongoing disease in the white matter away from focal plaques including small cuffs of inflammatory cells and astrocyte fragmentation in the form of small non-foamy macrophages.⁹

Visible lesions had more water than NAWM due to the focal nature of their inflammation and edema. However, lesions also had a wide range in WC likely indicating different stages of the underlying pathologies and degree of tissue damage. In general, NMO lesions had a much higher water content than NAWM whereas about half the MS lesions had only slightly elevated WC compared to NAWM. This likely reflects the more highly destructive nature of acute NMO lesions which are more destructive than MS lesions¹⁰ and characterized by a pronounced humoral inflammatory response, completely demyelinated tissue and the presence of many foamy macrophages.^9,11

NMO NAWM shows diffusely increased water content similar to MS. Lesions show a much larger increase in WC suggesting more extensive damage.