Axonal dysfunction and disability in a relapse of multiple sclerosis: Longitudinal study of a patient

Magnetic resonance imaging provides sensitive laboratory-supporting data for the clinical diagnosis of multiple sclerosis (MS), usually based on T2-weighted MRI.¹ However, although MS plaques defined in autopsy tissue give rise to the abnormal hyperintense signals on T2-weighted MRI, correlations between total MRI lesion volumes and disability are generally weak in established disease.¹ One possible explanation for this is that clinically significant pathologic changes extend into white matter appears normal on conventional MRI.^2,3 Anterograde and retrograde axonal changes may be expected around plaques and extend beyond the borders of hyperintense signal in acute, focal demyelinating lesions. Such abnormalities outside of lesions on conventional MRI may have importance for understanding mechanisms of functional impairment, as neuronal metabolic changes in acute lesions correlate strongly with disability.³ An aggressive, widespread pathologic process leading to both severe demyelination and axonal damage (with consequent degeneration of long fiber tracts) likely underlies the strong correlation found between spinal cord size and disability in patients with established MS.⁴

The hypothesis that axonal dysfunction can contribute to disability, and that it can be diffuse in patients with MS, can be tested by magnetic resonance spectroscopy (MRS), which allows quantitative monitoring of brain N-acetylaspartate (NAA) resonance intensity. NAA is found only in neurons and neuronal processes in the mature brain and appears to be a sensitive marker of reversible neuronal damage as well as of irreversible neuronal volume loss (i.e., loss of axonal diameter or wallerian degeneration).^3,5

Here we report results of a 6-year combined MRS, MRI, and clinical follow-up of a patient with MS designed to test the relationship between axonal dysfunction or loss and disability through periods of both relapse and partial remission of the disease.

Methods. Patient history. We studied a 24-year-old man who had a rather turbulent history of clinically definite relapsing progressive MS for 3 years at the time of entry into this study. During the same period, he was also entered into an interferon β-1b double-blind study and was subsequently shown to be in the placebo limb of the study. At the time of the first MR examination, neurologic evaluation revealed mild bilateral sensory deficits, generalized weakness, and mild to moderate cerebellar and pyramidal signs (Expanded Disability Status Scale score [EDSS]= 3). During the 6-year follow-up, the patient had one major relapse with increased sensory deficits, tetraparesis, cerebellar signs, optic neuritis, and bladder dysfunction (EDSS = 8). Following this severe attack, the patient terminated the interferon β-1b study and was treated with cyclophosphamide at month 26 (initial induction dose of 700 mg/m² for five doses over 10 days followed by single-dose boosters every 3 months until month 45). He had a partial remission of symptoms, and EDSS has remained stable through the subsequent period of study (EDSS = 6.5). He has been receiving interferon β-1b treatment since month 60.

MRI/MRS examinations. Eleven proton MRI and MRS examinations were obtained during the follow-up period as previously described.⁵ Proton MRIs were used to select an intracranial volume of interest (VOI) for spectroscopy (50 mm [left-right]× 70 mm [anteroposterior] × 20 mm [craniocaudal])(figure 1). The VOI was angled parallel to the anteroposterior commissural line and centered craniocaudally on the corpus callosum (to include the superior lateral periventricular regions). The VOI was kept constant in size and location after the first examination. Images with adequate a priori slice registration allowing comparable quantitative measurements of the total brain T2-weighted MRI lesion volumes were available from eight of the 11 MRI examinations for use in this study. MR spectra were obtained using a PRESS sequence (256 signal averages, TR 2,000, TE 272), and image segmentation was performed by one observer (S.N.), as previously described.⁵ Resonance intensities were determined from peak areas, and NAA was expressed relative to creatine as a resonance intensity ratio (NAA/Cr) to normalize the NAA resonance intensity between different examinations. Although some reports have suggested modest increases in creatine in the brains of MS patients,^2,6 the suspected increases are relatively small.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1. Sagittal (A) and transverse (B) MRI slices showing the position of the VOI used for spectroscopy.

Statistical analysis. Results were considered abnormal if they were more than 2 SD outside the normal mean. Values of brain MRI T2-weighted lesion volumes and NAA/Cr ratios were correlated with clinical disability using the Spearman rank correlation.

Results. The central brain NAA/Cr ratio was more than 2 SD lower than in the same region from normal brains (figure 2)(normal controls [n = 20] = 4.0 ± 0.3) on every examination. Over the course of the initial examinations, the patient was clinically stable and the central brain NAA/Cr ratio showed no significant changes. However, concurrent with a severe clinical relapse, which occurred 2 years after the start of the serial studies, there was a further approximately 30% decrease in brain NAA/Cr. This was associated with only a small (7%) increase in T2-weighted MRI lesion volume. The acute relapse was followed by partial remission of the new symptoms over the next few months. The brain NAA/Cr increased by approximately 25% over this period. Both the brain NAA/Cr and disability were subsequently stable. Over the 60 months of study, there was a strong correlation between NAA/Cr and disability scores (Spearman rank coefficient = -0.73, p< 0.001). In contrast, no correlation between T2-weighted MRI lesion volume and EDSS was apparent over the period of follow-up (Spearman rank coefficient= 0.19, p = 0.9). Although total brain T2-weighted MRI lesion volume decreased during the period of clinical remission, these changes were much smaller than those that were observed subsequently during a period of clinical stability.

• Download figure
• Open in new tab
• Download powerpoint

Figure 2. NAA/Cr resonance intensity ratio (top panel), clinical disability (EDSS) (middle panel), and total brain T2-weighted MRI lesion volumes (bottom panel) over 6 years. Patient's values of NAA/Cr ratio were more than 2 standard deviations lower than in normal brains on every examination (normal controls [n = 20] = 4.0 ± 0.3). The data show reciprocal changes in NAA/Cr and EDSS during the period of clinical relapse and the subsequent partial remission of symptoms. There was a strong negative correlation between NAA/Cr and EDSS during the period of follow-up (Spearman rank coefficient = -0.73, p < 0.001). In contrast, although total brain T2-weighted MRI lesion volumes showed a small increase during the relapse of the disease, there were also progressive increases during the period of stable EDSS and no general correlation with disability over the period of follow-up (Spearman rank coefficient = 0.19, p = 0.9).

Discussion. Experimental work with animal models has shown that there is not a simple relationship between demyelination and impairment of function in the CNS.⁷ Although acute inflammation may cause conduction block, with chronic demyelination secondary dysfunction or damage of axons passing through lesions may be a more important proximate mechanism of functional impairment. Recent proton MRS studies of focal lesions have supported this hypothesis with observations showing strong correlations between focal decreases in the neuronal marker NAA and functional impairments in patients with single, large cerebral demyelinating brain lesions³ and in MS patients with cerebellar symptoms.⁸ Here we have shown partially reversible changes in NAA/Cr in a large, central VOI during an acute relapse of the disease and its subsequent partial remission. Since the NAA/Cr signal from the VOI originated predominantly from white matter appearing normal on conventional MRI (lesions accounted for about 20 to 30% of the total VOI), and the large VOI was centered on the corpus callosum including brain regions where axonal projections converge after traversing large volumes of cerebral white matter, we interpret our results as an expression of a relatively diffuse axonal dysfunction associated with the severe acute relapse. These changes in NAA/Cr correlated strongly with clinical disability. In contrast, measurements of T2-weighted MRI lesion volumes failed to correlate strongly with disability in this follow-up study, as reported in previous studies of patients with established MS.¹

Evidence suggesting widespread reversible axonal dysfunction in the absence of overt inflammation has been reported in experimental allergic encephalomyelitis⁹ and after zidovudine treatment of human immunodeficiency syndrome.¹⁰ Diffuse increases in concentrations of cytokines or other inflammatory mediators could cause functional changes even in axons remote from foci of demyelination, as Moreau et al.¹¹ recently suggested to explain acute generalized symptom relapses in MS patients treated with lymphocytotoxic antibody therapy. Finally, a recent study¹² hypothesizes the importance of diffusible factors, such as a soluble neuronal Na⁺-channel inactivating factor, in CSF of MS patients during acute relapses. These factors may reduce neuronal excitability over an extensive part of the neural network and contribute to transient neurologic symptoms seen in patients with active MS.¹²

Further studies are needed to better define the extent to which axonal dysfunction or damage in acute relapses is preferentially localized to sites close to active lesions. However, considered more generally, our data support the hypothesis that axonal dysfunction or damage may have a significant role in determining functional impairment during MS relapses.

Acknowledgments

The authors are grateful to Ms. Arlene Cohen for coordinating the study and to Mr. Gilles Leroux and Mr. Andre Cormier for providing excellent technical support.