Wallerian Degeneration And Axonal Loss

Objective
Wallerian Degeneration in Normal-Appearing White Matter in early Relapsing/Remitting Multiple Sclerosis (RRMS), and its correlation with the number of relapses and disease duration.

Background
Recent pathological studies have demonstrated Wallerian Degeneration in Normal-Appearing White Matter (NAWM) in Multiple Sclerosis (MS), in established RRMS, and in Chronic MS.

However, the presence of Wallerian Degeneration early in the disease and its correlation with relapse and with disease duration has not been studied.

Methods
We performed Proton Magnetic Resonance Spectroscopic Imaging in 21 MS patients, and 4 healthy controls, age and gender matched, aged under 45 years, with a maximum of 4 years since first bout, and an EDSS score of less than 3.0.

N-AcetylAspartate (NAA) (an index of Axonal integrity) was measured in the NAWM from the Pons and the Cerebellar Peduncles.

Results
We observed that the NAA levels were abnormally low in the NAWM in the early RRMS patients (p = 0.04, Student's t-test).

The decrease in the NAA concentration correlated with disease duration in the two areas studied (p = 0.03 for Pons and p = 0.04 for Cerebellar Peduncle); and with the number of previous relapses (Pearson's correlation = -0.582, p < 0.002).

Conclusion
Wallerian Degeneration measured by the NAA concentration at Pons and Cerebellar Peduncles is present early in the disease and correlates with the number of relapses and disease duration.

MRI methods are widely used to follow the pathological evolution of Multiple Sclerosis in life and its modification by treatment.

To date, measures of the number and volume of macroscopically visible lesions have been studied most often.

These MRI outcomes have demonstrated clear treatment effects but without a commensurate clinical benefit, suggesting that there are other aspects of Multiple Sclerosis pathology that warrant investigation.

In this context, there has been considerable interest in measuring Tissue Loss (Atrophy) as a more global marker of the adverse outcome of Multiple Sclerosis pathology, whether it arises in macroscopic lesions or in the Normal-Appearing tissues.

An International Workshop recently considered the measurement of Atrophy in Multiple Sclerosis and provided the basis for this review.

Brain White Matter bulk consists predominantly of Axons (46%) followed by Myelin (24%), and progressive Atrophy implies loss of these structures, especially Axons.

Although variable effects on tissue volumes may also arise from Glial Cell proliferation or loss, Gliosis, Inflammation and Edema.

Significant correlations found between Brain Volume and other putative MR Neuronal markers also indicate that Atrophy reflects Axonal Loss.

Numerous methods are available for the measurement of global and regional Brain Volumes and upper Cervical Cord Cross-Sectional Area that are highly reproducible and sensitive to changes within 6-12 months.

In general, 3D-T₁-weighted acquisitions and largely automated segmentation approaches are optimal.

Whereas normalized volumes are desirable for cross-sectional studies, absolute volume measures are adequate for serial investigation.

Atrophy is seen at all clinical stages of Multiple Sclerosis, developing gradually following the appearance of inflammatory lesions.

This probably reflects both inflammation-induced Axonal Loss followed by Wallerian Degeneration and PostInflammatory NeuroDegeneration that may be partly due to failure of ReMyelination.

One component of Atrophy appears to be independent of focal lesions. Existing ImmunoModulatory therapies have had limited effects on Progressive Atrophy, concordant with their modest effects on progressive disability.

Atrophy provides a sensitive measure of the NeuroDegenerative component of Multiple Sclerosis and should be measured in trials evaluating potential AntiInflammatory, ReMyelinating or NeuroProtective therapies.

Objectives
To investigate the extent of tissue damage in a region of Normal-Appearing Corpus Callosum (NACC) for different forms of Multiple Sclerosis (MS) using Diffusion Tensor and proton Magnetic Resonance (MR) Spectroscopic imaging.

Methods
A total of 47 patients with MS and 15 controls were included. Regions of interest from the NACC were manually segmented using high resolution anatomical images.

Diffusion Tensor eigenvalues and metabolite ratio of N-AcetylAspartate (NAA) to Creatine/Phosphocreatine (Cr) were calculated in the NACC region.

Results
Increased Apparent Diffusion Coefficients (ADCs) and decreased Anisotropy were observed in the NACC for patients with MS relative to the control subjects.

These resulted from increased Diffusion Tensor eigenvalues perpendicular to the maximum Diffusion direction.

The NAA:Cr ratio was decreased in the NACC for patients with MS relative to the control subjects.

Significant correlations between PeriCallosal T₁ lesion load and MR modalities in the NACC were observed for patients with Relapsing/Remitting, Secondary/Progressive MS (RR/SPMS), but not for patients with Primary/Progressive MS (PPMS).

Conclusion
This study provides further insight into changes in the ADC and Diffusion Anisotropy based on the Diffusion Tensor eigenvalues for patients with MS.

The changes in the Diffusion Tensor eigenvalues and NAA:Cr ratio in the NACC for patients with RR/SPMS suggest Axonal injury and/or dysfunction induced by Wallerian Degeneration.

The lack of correlation between these variables in the NACC and focal MS lesions for patients with PPMS further supports intrinsic differences related to tissue injury between these subtypes of MS.

Background
The progressive nature of Wallerian Degeneration has long been controversial.

Conflicting reports that distal stumps of injured Axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single Axon at two distant points.

As Axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating Axons.

We recently validated the study of Wallerian Degeneration using yellow fluorescent protein (YFP) in a small, representative population of Axons, which greatly improves longitudinal imaging.

Here, we apply this method to study the progressive nature of Wallerian Degeneration in both wild-type and slow Wallerian Degeneration (WldS) mutant mice.

Results
In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated Axons 37-42 h after transection and 40-44 h after crush injury.

Axons exist in this state only transiently, probably for less than one hour.

Surprisingly, Axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual Axons.

Indicating that Wallerian Degeneration progresses as a wave sequentially affecting adjacent regions of the Axon.

In contrast, most or all WldS Axons were partially fragmented 15-25 days after nerve lesion, WldS Axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly.

Furthermore, the first signs of degeneration were short constrictions, not complete breaks.

Conclusions
We conclude that Wallerian Degeneration progresses rapidly along individual wild-type Axons after a heterogeneous latent phase.

The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of Trophic or regulatory factors by Axonal transport triggers degeneration.

WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration.

These observations suggest that WldS axons undergo a slow anterograde decay as Axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type Axons at a slower rate.