Investigating axonal damage in multiple sclerosis by diffusion tensor spectroscopy

Sensitive and specific in vivo measures of axonal damage, an important determinant of clinical status in multiple sclerosis (MS), might greatly benefit prognostication and therapy assessment. Diffusion tensor spectroscopy (DTS) combines features of diffusion tensor imaging and magnetic resonance spectroscopy, allowing measurement of the diffusion properties of intracellular, cell-type-specific metabolites. As such, it may be sensitive to disruption of tissue microstructure within neurons. In this cross-sectional pilot study, diffusion of the neuronal metabolite N-acetylaspartate (NAA) was measured in the human normal-appearing corpus callosum on a 7 tesla MRI scanner, comparing 15 MS patients and 14 healthy controls. We found that NAA parallel diffusivity is lower in MS (p = 0.030) and inversely correlated with both water parallel diffusivity (p = 0.020) and clinical severity (p = 0.015). Interpreted in the context of previous experiments, our findings provide preliminary evidence that DTS can distinguish axonopathy from other processes such as inflammation, edema, demyelination, and gliosis. By detecting reduced diffusion of NAA parallel to axons in white matter, DTS may thus be capable of distinguishing axonal disruption in MS in the setting of increased parallel diffusion of water, which is commonly observed in MS but pathologically nonspecific.     

Proton MR spectroscopy to assess axonal damage in multiple sclerosis and other white matter disorders

Proton MR spectroscopy allows in vivo measurement of N-acetylaspartate in white matter, providing a biochemical index of axonal integrity. Several recent studies of patients with multiple sclerosis and other white matter disorders have shown both transient and sustained decreases in N-acetylaspartate in white matter lesions and in brain regions appearing normal on conventional MRI. These data have emphasised that a substantial amount of axonal damage or loss (presumably secondary to myelin pathology) is consistently present in most of these disorders. Recent post-mortem studies support these results. In contrast to changes seen with conventional MR imaging, decreases in N-acetylaspartate have shown a close correlation with changes in neurological status. This suggests that axonal damage may be more relevant than demyelination for determining chronic functional impairments in primary white matter diseases. Thus, serial measurement of brain N-acetylaspartate with proton MR spectroscopy can provide a reliable and clinically-relevant monitor of disease evolution. As pathological changes responsible for long-term morbidity are logically important targets for therapeutic agents, early treatment directed at axonal protection should be useful in these disorders.     

Proton MR spectroscopy in multiple sclerosis

Axonal injury in multiple sclerosis (MS) is focal and diffuse, and is directly responsible for irreversible disability. Acute inflammatory events can be associated with reversible disability that may parallel reversible axonal injury. This in part accounts for the remission following relapses early in the disease. By the time there is clinical disability, substantial axonal injury already has occurred. This provides a strong rationale for the early limitation of inflammation and its consequences.     

Imaging neuronal and axonal degeneration in multiple sclerosis

Neurological Sciences - Neuronal and axonal damage has become an important issue in multiple sclerosis. This has been emphasised by recent magnetic resonance imaging (MRI) studies that have shown...     

Imaging of axonal damage in multiple sclerosis: Spatial distribution of magnetic resonance imaging lesions

We performed magnetic resonance imaging and magnetic resonance spectroscopic imaging on 28 patients with multiple sclerosis stratified for disability and clinical course (relapsing with at least partial remissions or secondary progressive disease). Lesions were segmented on the conventional proton density and T2-weighted magnetic resonance images, and lesion distribution images were generated for each patient. The conventional magnetic resonance and spectroscopic images were transformed into a standard brain-based stereotaxic coordinate space, allowing comparison of images from different patients on a voxel-by-voxel basis. The spatial distribution of lesions in the transformed magnetic resonance images did not differ significantly between the relapsing and the progressive disease groups. We then generated from the individual data sets, group lesion probability distribution images for the relapsing and the progressive disease groups. The spatial distribution of metabolites was characterized with respect to lesion distribution using the magnetic resonance spectroscopic images transformed into stereotaxic space and averaged. The neuronal marker N-acetylaspartate was diffusely lower in the multiple sclerosis patients than in normal control subjects. Comparison of the averaged metabolite and T2-weighted lesion probability images confirmed loss of N-acetylaspartate in regions of both high and low lesion probability. This suggests that diffuse axonal volume loss or dysfunction extends beyond the inflammatory lesions of multiple sclerosis, perhaps due to microscopic disease or wallerian degeneration along projection pathways of axons traversing the lesions.     

Imaging correlates of axonal swelling in chronic multiple sclerosis brains

OBJECTIVE: T2-weighted magnetic resonance imaging is a sensitive tool for monitoring progression of multiple sclerosis, but it does not provide information on the severity of the underlying tissue damage. Measurement of T1 hypointensities and magnetization transfer ratio (MTR) can potentially distinguish lesions with more severe tissue damage. The objective of this study was to use image-guided pathology to determine histological differences between lesions that are abnormal only on T2-weighted images versus lesions that are abnormal on T2-weighted, T1-weighted, and MTR images. METHODS: A total of 110 regions were selected from postmortem magnetic resonance images of 10 multiple sclerosis patients. Regions were classified into three magnetic resonance imaging-defined categories: normal-appearing white matter; abnormal on T2-weighted image only (T2-only); and abnormal on T2-weighted, T1-weighted, and MTR images (T2T1MTR). Myelin status, lesion activity, astrocytosis, serum protein distribution, axonal area, and axonal loss were evaluated histopathologically. RESULTS: Comparisons between groups showed that T2T1MTR regions were more likely to be demyelinated (83% compared with 55% of T2-only regions) and more likely to be chronic inactive lesions (68% compared with 0% of demyelinated T2-only regions). There was no difference between T2-only and T2T1MTR regions in axonal area, but there was a significant difference in axonal count, indicating that axons in the T2T1MTR regions were enlarged relative to those in T2-only regions. INTERPRETATION: Axonal swelling and axonal loss were major pathological features that distinguish T2T1MTR regions from T2-only regions.     

Metabolic differences between multiple sclerosis subtypes measured by quantitative MR spectroscopy

We used quantitative magnetic resonance (MR) spectroscopic imaging with T1-based image segmentation to evaluate the subtypes of multiple sclerosis (MS) (eight patients each group of relapsing-remitting [RR], secondary progressive [SP] and primary progressive [PP]). There was no significant difference in age between the PP group with the RP, SP or control group. We found that the metabolite ratio of choline/NA from the periventricular white matter region was not significantly different between the RR and SP groups. Using an ANOVA, the ratios of periventricular choline/NA or creatine/NA of these combined groups were significantly higher than the PP and control groups. Quantification of these data suggest that the major cause of the elevation of these parameters is due to an increase in choline and creatine in the RR group while NA is decreased in the SP group. Thus, early PP disease appears to be relatively intact with respect to neuronal loss.     

The relationship between diffuse axonal damage and fatigue in multiple sclerosis

BACKGROUND: Fatigue is a common and distressing symptom for patients with multiple sclerosis (MS). There is growing evidence that fatigue in MS has a central nervous system component. We hypothesized that diffuse cerebral axonal damage could be associated with fatigue and used proton magnetic resonance spectroscopy to noninvasively measure axonal damage or loss in the brains of patients with MS. OBJECTIVE: To assess the strength of the relationship between central brain N-acetylaspartate and fatigue. DESIGN: Data from 73 patients who had undergone proton magnetic resonance spectroscopy imaging and completed the Fatigue Severity Scale questionnaire were analyzed. RESULTS: The N-acetylaspartate-creatine ratio (NAA/Cr) was significantly lower in the high-fatigue group than the low-fatigue group (mean +/- SD, 2.69 +/- 0.29 and 2.99 +/- 0.33, respectively. P =.003). Independent of the Kurtzke Expanded Disability Status Scale, T2 lesion volume, age, and disease duration, NAA/Cr was significantly lower in the high-fatigue group as compared with the low-fatigue group. There was a statistically significant linear correlation between the Fatigue Severity Scale scores and NAA/Cr (Spearman rank rho = -0.361, P =.02). CONCLUSIONS: The results of this study, combined with those of others, suggest that widespread axonal dysfunction is associated with fatigue in MS. Increased recruitment of cortical areas and pathways in response to brain injury may be responsible for the patient's sense that the effort required to perform actions is disproportionately high.     

Acute axonal damage predicts clinical outcome in patients with multiple sclerosis

The objectives of this study were (1) to determine how cerebrospinal fluid (CSF) neurofilament heavy chain (NfH(SM134) and NfH(SM135)) levels relate to clinical outcome in optic neuritis (ON) and multiple sclerosis (MS) relapse patients treated with high dose oral methylprednisolone; and (2) to correlate neurofilament and myelin basic protein (MBP) concentrations, particularly as the latter was previously associated with clinical disability. Fifty subjects participated in two double-blind, randomized, placebo-controlled clinical trials. Eight/18 patients in the ON trial and 15/32 subjects in the MS attack trial were treated with oral methylprednisolone. In the MS attack trial group, CSF NfH(SM134) and NfH(SM135) measured at week 3 and deltaCSF NfH(SMI34) levels from baseline to week 3 were predictive of clinical outcome at week 8 and 52. In the ON group, no such association was seen. When both groups were combined, baseline CSF NfH(SHM134) and NfH(SM135) correlated positively with baseline enhancing lesion volume (ELV) (r(s) =0.50, P <0.01 and rS =0.53, P <0.01, respectively). Levels of NfH(SM135) at baseline and week 3 also strongly correlated with the MBP concentration. This study supports the view that acute inflammation in ON and MS results in axonal pathology and that the latter has a role in determining functional impairment.     

Demyelination and axonal damage in a non-human primate model of multiple sclerosis

The demyelinating plaque is the paradigmatic lesion of multiple sclerosis (MS), but only recently attention has been given to axonal damage and to its role in the pathophysiology of disease. Albeit the possible relevance of axonal loss in MS and its experimental models, the amount and timing of axonal sufferance has been addressed only in experimental autoimmune encephalomyelitis (EAE) of rodents. In this report we observed that, in the marmoset model of EAE, axonal damage occurs early during the demyelinating process as assessed by immunoreactivity for amyloid precursor protein (APP) and non-phosphorylated neurofilaments (SMI-32 positive) detected mostly in early active lesions compared to late active and normal appearing white matter. The rare occurrence of morphological features of axonal transection, such as APP or SMI-32 positive spheroids and swellings, as well as an increase of neurofilament density in the demyelinated axons without accumulation of electron dense organelles or osmiophilic bodies, at electron microscopy, suggests that early axonal damage may be, at least in part, a reversible process. These findings are of relevance for the development of therapies, which can protect axons and enhance their function and survival.     

Proton MR spectroscopy in clinically isolated syndromes suggestive of multiple sclerosis.

The concentration of the metabolite N-acetyl aspartate (NAA), thought to be a marker of axonal loss or damage, has been shown to be reduced in lesions, as demonstrated by high signal areas on T2-weighted MRI, and in normal-appearing white matter (NAWM) in established multiple sclerosis (MS). The stage of the disease when these changes first appear is not known. To try to determine this we studied 20 patients with clinically isolated syndromes, many of whom will be at the earliest clinical stages of MS, and 20 age- and sex-matched controls with single-voxel proton magnetic spectroscopy (MRS). MRS was performed using a General Electric 1.5T Signa EchoSpeed scanner (TR 3000 ms, TE 30 ms, PRESS). Absolute metabolite concentrations were determined using the LCModel fitting software. No significant reduction of NAA concentration was evident in the NAWM of the patients (patients: median 7.3 mM; controls: median 7.7 mM; P=0.19). There was, however, a significantly lower concentration of NAA in lesions (median 6.6 mM, P=0.015). Absolute values of choline-containing compounds, creatine and myo-inositol were significantly raised in the lesions (P=0.007, P=0.011 and P=0.002 respectively). The low NAA in lesions is consistent with axonal loss, damage or dysfunction occurring focally at the earliest clinical phase of the disease. The lack of any significant reduction in NAA in patient NAWM demonstrates that more widespread axonal changes are not yet detectable at this early clinical stage. A larger cohort and follow-up will be necessary to determine whether or not MRS findings have any prognostic significance for individual patients or sub-groups. This will also enable the clarification of the time course, pathogenesis and pathophysiological significance of the development of the low NAA, which is found in the NAWM of many patients with established MS.     

Imaging brain damage in first-degree relatives of sporadic and familial multiple sclerosis

OBJECTIVE: Our objective was to assess brain damage in first-degree relatives of patients with sporadic and familial multiple sclerosis (MS). METHODS: Asymptomatic first-degree relatives of sporadic (sMS, n = 152) and familial MS (fMS, n = 88) and healthy volunteers (NC, n = 56) underwent brain MRI and magnetization transfer (MT) imaging on a mobile MR scan. On MR examinations, we visually assessed white matter (WM) lesions and quantified WM lesion volumes, brain volumes, and MT ratio (MTr) in lesions and normal-appearing WM (NAWM). RESULTS: A lesional MR pattern similar to that of MS patients was found in 4% sMS and 10% fMS. In these WM lesions, MTr was lower (p < 0.0001) than in the WM of NC. In contrast, there was no difference in NAWM-MTr and brain volume values between the three groups. INTERPRETATION: Focal brain abnormalities indistinguishable from those of MS occur in asymptomatic first-degree relatives of MS patients. These are twice more frequent in fMS than in sMS but do not lead to the widespread tissue damage commonly found in MS patients. Although there is a genetic susceptibility to develop brain abnormalities suggestive of focal demyelination in first-degree relatives of MS patients, other factors are probably critical for the development of a diffuse, clinically relevant, pathology.     

Magnetic resonance spectroscopy as a measure of brain damage in multiple sclerosis

Recent MR studies have emphasised the importance of neuronal and axonal damage in multiple sclerosis. In this respect, proton MR spectroscopy (by monitoring levels of N-acetylaspartate, a putative marker of axonal integrity) has been particularly illuminating by showing indirect evidence of neurodegeneration in both lesional and non-lesional brain tissues from the earliest stages of the disease. The importance of these changes to patients' clinical disability argues for the primary role of neuronal pathology in the pathogenesis of the disease.