Evidence for subacute fat embolism as the cause of multiple sclerosis

Understanding a role for hypoxia in lesion formation and location in the deep and periventricular white matter in small vessel disease and multiple sclerosis

The deep and periventricular white matter is preferentially affected in several neurological disorders, including cerebral small vessel disease (SVD) and multiple sclerosis (MS), suggesting that common pathogenic mechanisms may be involved in this injury. Here we consider the potential pathogenic role of tissue hypoxia in lesion development, arising partly from the vascular anatomy of the affected white matter. Specifically, these regions are supplied by a sparse vasculature fed by long, narrow end arteries/arterioles that are vulnerable to oxygen desaturation if perfusion is reduced (as in SVD, MS and diabetes) or if the surrounding tissue is hypoxic (as in MS, at least). The oxygen crisis is exacerbated by a local preponderance of veins, as these can become highly desaturated 'sinks' for oxygen that deplete it from surrounding tissues. Additional haemodynamic deficiencies, including sluggish flow and impaired vasomotor reactivity and vessel compliance, further exacerbate oxygen insufficiency. The cells most vulnerable to hypoxic damage, including oligodendrocytes, die first, resulting in demyelination. Indeed, in preclinical models, demyelination is prevented if adequate oxygenation is maintained by raising inspired oxygen concentrations. In agreement with this interpretation, there is a predilection of lesions for the anterior and occipital horns of the lateral ventricles, namely regions located at arterial watersheds, or border zones, known to be especially susceptible to hypoperfusion and hypoxia. Finally, mitochondrial dysfunction due to genetic causes, as occurs in leucodystrophies or due to free radical damage, as occurs in MS, will compound any energy insufficiency resulting from hypoxia. Viewing lesion formation from the standpoint of tissue oxygenation not only reveals that lesion distribution is partly predictable, but may also inform new therapeutic strategies.

Hyperbaric oxygen therapy for multiple sclerosis

Multiple sclerosis (MS) is a chronic, inflammatory, and degenerative neurological illness with no cure. It has been suggested that Hyperbaric Oxygen Therapy (HBO(2)T) may slow or reverse the progress of the disease. This article summarizes the clinical evidence for the use of HBO(2)T in the treatment of MS. We conducted a literature review focused on the interaction of hyperbaric oxygenation and MS. In particular, we appraised the clinical data regarding treatment and performed a meta-analysis of the randomized evidence using the methodology of the Cochrane Collaboration. We found 12 randomized studies in the area, all of which were performed between 1983 and 1987. A meta-analysis of this evidence suggests there is no clinically significant benefit from the administration of HBO(2)T. The great majority of randomized trials investigated a course of 20 treatments at pressures between 1.75ATA and 2.5ATA daily for 60-120 min over 4 weeks against a placebo regimen. None have tested the efficacy of HBO(2)T against alternative current best practice. No plausible benefit of HBO(2)T on the clinical course of MS was identified in this review. It remains possible that HBO(2)T is effective in a subgroup of individuals not clearly identified in the trials to date, but any benefit is unlikely to be of great clinical significance. There is some case for further human trials in selected subgroups and for prolonged courses of HBO(2)T at modest pressures, but the case is not strong. At this time, the routine treatment of MS with HBO(2)T is not recommended.

Hyperbaric oxygenation in fluid microembolism

Because clinicians require objectively demonstrable neurological deficits to confirm a diagnosis, the recognition of embolic events in the nervous system is generally restricted to the effects of ischemic necrosis produced by arterial occlusion. However, magnetic resonance imaging (MRI) has shown that lesser degrees of damage associated with small emboli are common, especially in the mid brain, and are usually clinically silent. They are frequently associated with atheromatous embolism in the elderly, but microembolic debris, such as fat, is common in the systemic venous return of healthy people and generally trapped in the microcirculation of the lung being removed by phagocytosis. However, pulmonary filtration may fail and microemboli may also pass through an atrial septal defect in so-called 'paradoxical' embolism. Studies of bubbles formed on decompression in diving have demonstrated the importance of pulmonary filtration in the protection of the nervous system and that filtration is size dependant, as small bubbles may escape entrapment. Fluid and even small solid emboli, arresting in or passing through the cerebral circulation, do not cause infarction, but disturb the blood-brain barrier inducing what has been termed the 'perivenous syndrome'. The nutrition of areas of the white matter of both the cerebral medulla and the spinal cord depends on long draining veins which have been shown to have surrounding capillary free zones. Because of the high oxygen extraction in the microcirculation of the gray matter of the central nervous system, the venous blood has low oxygen content. When this is reduced further by embolic events, tissue oxygenation may fall to critically low levels, leading to blood-brain barrier dysfunction, inflammation, demyelination and eventually, axonal damage. These are the hallmarks of the early lesions of multiple sclerosis where MR spectroscopy has also shown the presence of lactic acid. Significant elevation of the venous oxygen tension requires oxygen to be provided under hyperbaric conditions. Arterial tension is typically increased ten-fold breathing oxygen at 2 atmospheres absolute (ATA), but this results in only a 1.5-fold increase in the cerebral venous oxygen tension. The treatment of decompression sickness, and both animal and clinical studies, have confirmed the value of oxygen provided under hyperbaric conditions in the restoration and preservation of neurological function in the 'perivenous' syndrome.

Hyperbaric oxygen therapy for multiple sclerosis

BACKGROUND

Multiple Sclerosis (MS) is a chronic, recurrent and progressive illness with no cure. On the basis of speculative pathophysiology, it has been suggested that Hyperbaric Oxygen Therapy (HBOT) may slow or reverse the progress of the disease.

OBJECTIVES

The object of this review was to evaluate the efficacy and safety of HBOT in the treatment of MS.

SEARCH STRATEGY

We searched the Cochrane MS Group trials register (July 2002), the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 2, 2002), MEDLINE (January 1966 to October 2002) and the National Library of Medicine (NLM) database (July 2002), along with specialised hyperbaric resources and handsearching of relevant journals and proceedings.

SELECTION CRITERIA

All randomised, controlled trials involving a comparison between HBOT and a sham therapy in MS were evaluated.

DATA COLLECTION AND ANALYSIS

Two reviewers independently appraised all comparative trials identified, extracted data and scored them for methodological quality.

MAIN RESULTS

We identified ten reports of nine trials that satisfied selection criteria (504 participants in total). Two trials produced generally positive results, while the remaining seven reported generally no evidence of a treatment effect. None of our three a priori subgroup analyses placed these two trials in the same group and were therefore unable to account for this difference. Three analyses (of 21) did indicate some benefit. For example, the mean Expanded Disability Status Scale (EDSS) at 12 months was improved in the HBOT group (group mean reduction in EDSS compared to sham -0.85 of a point, 95% confidence interval -1.28 to -0.42, P = 0.0001). Only the two generally positive trials reported on this outcome at this time (16% of the total participants in this review).

REVIEWER'S CONCLUSIONS

We found no consistent evidence to confirm a beneficial effect of hyperbaric oxygen therapy for the treatment of multiple sclerosis and do not believe routine use is justified. The small number of analyses suggestive of benefit are isolated, difficult to ascribe with biological plausibility and would need to be confirmed in future well-designed trials. Such trials are not, in our view, justified by this review.

Spinal cord myelin is vulnerable to decompression

Spinal cord white matter is the major site of tissue damage resulting from decompression sickness (DCS or "the bends"). Damage is thought to result from bubble nucleation within the tissue. Why DCS occurs predominantly in the spinal cord and not in the brain is not known; neither is the exact pathological mechanism by which the spinal cord is damaged, nor how multiple sclerosis (MS)-like symptoms may ensue. To investigate the molecular basis of white matter damage, we subjected myelinated mouse tissues to varying durations of decompression, and then after recompression to one atmosphere, examined them for changes in myelin structure and composition. X-ray diffraction showed that the myelin period in spinal cord decreased by 4%, whereas those of optic and sciatic nerves were stable. The change in period was accompanied by a change in membrane bilayer profile--i.e., relative to control, the width of the bilayer decreased by approximately 6 A, whereas the interbilayer spaces each increased by approximately 3 A. The changes in electron density levels suggested a redistribution of matter from the interbilayer spaces into the lipid headgroup layers. By contrast with these structural changes, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance thin layer chromatography (HPTLC) revealed no noticeable change in myelin composition--i.e., there was no release of myelin-specific proteins or lipids. Our findings indicate that spinal cord myelin has an inherent structural vulnerability that may facilitate the targeting of this tissue during pressure changes.

The etiology of multiple sclerosis: a new and extended vascular-ischemic model

It is hypothesized that multiple sclerosis is a disease of the cerebro-vascular system. The basic defect is visualized as a wound in the CNS due to a focal hypertension of genetically susceptible vessels which results in vascular injury and the initiation of a series of biochemical and physiological events culminating in an ischemic hypoxia leading to demyelination and a secondary damaging process associated with the immune system.

Possible role of adsorbed surfactant in controlling membrane permeability and function

It is well established in the physical sciences that the adsorption of a monolayer of certain surfactants onto the surface of a synthetic membrane used for ultrafiltration can greatly modify its permeability to water and its ability to transmit small solute molecules and ions of physiological interest. In this hypothesis, it is proposed that, when indigenous surfactant is adsorbed to certain membranes in the body, it can similarly modify their permeability. Since adsorption can be a rapidly reversible process, this would provide a simple physical means of controlling the overall level of physiological activity of the membrane and, possibly, an additional means of differentiating membranes according to function. The hypothesis raises many questions concerning its applicability to the general structure of biological membranes, the nature of the surfactant, its ability to adsorb to solid surfaces and the reasons why such a coating may have been missed. There are then the questions of which membranes might benefit most and what happens if the coating is too sparse or is removed unintentionally.

Is it any good? The evaluation of therapy by participants in a clinical trial

Research into patients' perspectives on treatments, in concentrating on their compliance with medically prescribed regimens, have taken bio-medicine's evaluation of therapeutic efficacy and benefit for granted. This paper suggests that the clinical trial, the predominant method of evaluation, should become the object of research attention. Clinical trials of hyperbaric oxygen therapy for multiple sclerosis have failed to show any therapeutic benefit: people who themselves have the disease have however continued to employ it, arranging its delivery for themselves. This study focuses on the decision by a small number of participants in one such clinical trial whether or not to continue using the therapy afterwards. On the basis of in-depth interviews, concerning how the participants evaluated the therapy and came to their decision, it is suggested that at least in the case of this therapy and this condition, the assumptions inherent in the trial method, and its concept of genuine therapeutic benefit, structures the conclusions of the trial in a way that is profoundly at variance with the participants' own methodological assumptions and concept of benefit. Inter alia, the study challenges the view of patients as being inevitably driven by their disregard of proper scientific method to an unreasoning optimism in their assessments of possible treatments.

Multiple sclerosis: a survey of alternative hypotheses concerning aetiology, pathogenesis and predisposing factors

There is abundant evidence that both environmental and genetic factors play a role in the aetiology and pathogenesis of multiple sclerosis. Currently, a virus infection and an abnormality of T-cell function are favoured candidates for the environmental and genetic factors, respectively. However, as long as convincing evidence for a specific viral infection and for a pathogenetic role for T-cells in multiple sclerosis is lacking, it may be worthwhile to consider alternative hypotheses that have been proposed in recent years. These have been classified in this review under the headings Toxins, Dietary and Metabolic Abnormalities, Embolism, Infection, and Immunological Mechanisms.

Erythrocyte antioxidant enzymes in multiple sclerosis and the effect of hyperbaric oxygen

The activities of catalase, glutathione peroxidase, and glutathione reductase, were not significantly different from normal whereas that of superoxide dismutase was decreased (P less than 0.05) in erythrocytes from patients with multiple sclerosis. Assay of the lipid peroxidation product, malondialdehyde, after incubation of erythrocytes with 10 mM H2O2 under carefully controlled conditions (peroxide stress test) demonstrated that MS erythrocytes are significantly (P less than 0.001) less susceptible to H2O2-induced lipid peroxidation in vitro. This finding suggests that the level of an endogenous antioxidant, possibly vitamin E, may be elevated in MS red cells. After treatment with hyperbaric O2, the activity of MS erythrocyte catalase is significantly (P less than 0.01) elevated by 2-6-fold.