Indirect evidence for early widespread gray matter involvement in relapsing–remitting multiple sclerosis

Abstract

Multiple sclerosis (MS) has traditionally been viewed as an inflammatory demyelinating white matter (WM) disease of the central nervous system. However, recent pathology and MRI studies have shown lesions in the gray matter (GM) as well. To ascertain the extent of GM involvement, we obtained with nonlocalizing proton MR spectroscopy the concentration of N-acetylaspartate (NAA), a metabolite found almost exclusively in neuronal cells, T2-lesion loads, and GM and WM fractions in the entire brain of 71 relapsing–remitting (RR) MS patients (51 women, 20 men, 25–55 years old) and 41 healthy controls (27 women, 14 men, 23–55 years old). The average whole-brain NAA (WBNAA) difference between the patients and the controls was −2.9 mM (−22%, P < 0.0001); range: +1.2 to −7.8 mM (+8% to −63%). The patients' median T2 lesion volume was 5.5 (range: 0.140–28) cm³. GM and WM comprised 50.4 ± 3.8% and 30.4 ± 5.0% (mean ± standard deviation), respectively, of the total brain volume in the patients; 53.8 ± 3.7% and 35.4 ± 4.7% in the controls. Because WM and GM constitute approximately 40% and 60% of the brain parenchyma, respectively, and the NAA concentration in the former is 2/3 of the latter, WBNAA loss greater than 40% × 2/3 = 27% cannot be explained in terms of WM (axonal) pathology alone and must include widespread GM (neuronal) deficits. Therefore, the concept of MS, even at its earlier stages, as a WM disease might need to be reexamined.

Introduction

Multiple sclerosis (MS) has been regarded for more than a hundred years as an inflammatory demyelinating disease affecting the white matter (WM) of the central nervous system (Charcot, 1868). Only over the past decade has this perception been expanded slightly to accommodate mounting pathological and MRI evidence of axonal injury in WM lesions of the brain and cord Arnold et al., 1990, Davie et al., 1994, Trapp et al., 1998, van Walderveen et al., 1999. This damage was shown to extend beyond the macroscopic lesions into the so-called normal-appearing WM (NAWM) at varying degrees of severity and extent Allen and McKeown, 1979, Evangelou et al., 2000, Filippi et al., 2001, Fu et al., 1998, Loevner et al., 1995, Miki et al., 1999a.

Although some evidence that the gray matter (GM) is not spared has been available for more than 30 years Brownell and Hughes, 1962, Lumsden, 1970, only recently have pathology studies shown cortical and subcortical lesions, usually missed by conventional T2-weighted MR imaging (T2WI) to be relatively frequent at post-mortem exams of deceased MS patients Ge et al., 2001, Kapeller et al., 2001, Kidd et al., 1999, Peterson et al., 2001, Sharma et al., 2001. Furthermore, new specialized MR techniques, for example, magnetization transfer (MTI), diffusion tensor imaging (DTI), and proton magnetic resonance spectroscopy (¹H-MRS), frequently reveal abnormal physical properties of the MRI normal-appearing GM Bozzali et al., 2002, Ge et al., 2001, Kapeller et al., 2001, Sharma et al., 2001.

In vivo ¹H-MRS assessment of the amino acid derivative N-acetylaspartate (NAA), present almost exclusively in neuronal cells (Simmons et al., 1991), has reported declines in both lesions and WM Davie et al., 1994, De Stefano et al., 2000, Gonen et al., 2000, Matthews et al., 1996. Unfortunately, in GM ¹H-MRS encounters several limitations: first, because the cortex is merely 1–4-mm thick, voxels smaller than 1 cm³ must be used to reduce WM contamination (Li et al., 2002). Signal-to-noise and limited examination time considerations preclude such small voxels even in 2D and 3D ¹H-MRS studies. Second, with the exception of the midline and deep gray structures, the voxels must be placed away from the skull to avoid spectral contamination by intense lipid signals of subcutaneous adipose tissue and bone marrow (Gonen et al., 1998). Consequently, most cortical GM is difficult to access. Third, poor lesion or GM contrast complicates image guidance to MRI-visible pathology. Finally, combinations of the above obstacles render serial studies unreliable due to voxel registration uncertainties which cannot be fully corrected for in postprocessing in limited volumes-of-interest (VOI) of 2D or 3D ¹H-MRS.

These difficulties can be circumvented by whole-brain NAA (WBNAA) quantification Gonen et al., 1998, Gonen et al., 2000. Although its global coverage comes at a cost of all regional information, a previous comparison of relapsing–remitting (RR) MS patients with matched controls showed upwards of 10% WBNAA deficits as a function of age (Gonen et al., 2000). Because in RR MS, T₂ lesions usually account for less than 3% of the brain volume (Miki et al., 1999b), it was concluded that such losses could only be explained by normal-appearing WM (NAWM) involvement (Gonen et al., 2000). The extent of these deficits motivated the present study to ascertain whether the WM pool was large enough to absorb them. Because the brain's white and GM volume fractions in healthy adults are estimated at 30% and 60%, respectively (Courchesne et al., 2000), and the NAA concentration difference between them is approximately 30% (Wang and Li, 1998), WBNAA deficits exceeding 20% may indicate GM involvement in addition to all the MRI lesions and WM occult pathology.