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Multiple sclerosis

Multiple sclerosis (MS) is a relatively common acquired chronic relapsing demyelinating disease involving the central nervous system. It is by definition disseminated not only in space (i.e multiple lesions), but also in time (i.e. lesions are of different age).

A number of clinical variants are recognised, each with specific imaging findings and clinical presentation. They include: 

This articles concerns itself primarily with classic (Charcot type) multiple sclerosis. The other variants are discussed separately. 

Of note neuromyelitis optica (Devic disease) was considered a variant but is now recognised as a distinct entity. 

Epidemiology

Presentation is usually between adolescence and the sixth decade, with a peak at approximately 35 years of age 12. There is a strong, well recognised female predilection with a F:M ratio of 2-3:1. 

Multiple sclerosis has a fascinating geographic distribution: it is rarely found in equatorial regions, with incidence gradually increasing with distance from the equator 12

Clinical presentation

Clinical presentation is both highly variable acutely, as a result of varying plaque location as well as over time, with a number of patterns of longitudinal disease being described 11-12:

  1. relapsing–remitting
    • most common (70% of cases)
    • patients exhibit periodic symptoms with complete recovery (early on)
  2. secondary progressive
    • approximately 85% of patients with relapsing-remitting MS eventually enter a secondarily progressive phase
  3. primary progressive
    • uncommon (10% of cases) 
    • patients do not have remissions, with neurological deterioration being relentless
  4. progressive with relapses
  5. benign multiple sclerosis
    • 15-50% of cases
    • defined as patients who remain functionally active for over 15 years

As is evident from this list, there is overlap, and in some cases patients can drift from one pattern to another. 

Upon presentation patients often have evidence of multiple previous asymptomatic lesions, and the diagnosis of multiple sclerosis can be strongly inferred. In other instances patients present with the first plaque. This is known as clinically isolated syndrome (CIS) and not all patients go on to develop MS. 

Symptoms may be sensory or motor or mixed, including cranial nerve involvement, e.g. trigeminal neuralgia or optic neuritis

Pathology

The exact aetiology is poorly known although it is believed to have both genetic and acquired contributory components. An infectious agent or at least catalyst have long been suspected due to the geographic distribution and presence of clusters of cases, however no agent has as yet been firmly identified. 

MS is believed to result from a cellular mediated autoimmune response against ones own myelin components, with loss of oligodendrocytes, with little or no axonal degeneration. 

Demyelination occurs in discrete foci, termed plaques which range in size from a few millimetres to a few centimeters and are typically perivenular.

Each lesion goes through three pathological stages:

  • early acute stage (active plaques)
    • active myelin break down 
    • plaques appear pink and swollen
  • subacute stage
    • plaques become paler in colour ("chalky")
    • abundant macrophages
  • chronic stage (inactive plaques/gliosis)
    • little or no myelin breakdown
    • gliosis with associated volume loss
    • appear grey/translucent

Patients serum IgG levels tend to be elevated and CSF analysis commonly shows oligoclonal bands 

Associations

Radiographic features

Plaques can occur anywhere in the central nervous system. They are typically ovoid in shape and perivenular in distribution. 

CT

CT features are usually non-specific, and significant change may be seen on MRI with an essentially normal CT scan. Features that may be present include:

  • plaques can be homogeneously hypo attenuating 8, 11 
  • brain atrophy may be evident in with long standing chronic MS 5
  • some plaques may show contrast enhancement in the active phase 7,11
MRI

MRI has revolutionised the diagnosis and surveillance of patients with MS. Not only can an MRI confirm the diagnosis (see McDonald MRI criteria for multiple sclerosis), but follow-up scans can assess response to treatment and try and determine the disease pattern. 

  • T1
  • T2: lesions are typically hyperintense
  • FLAIR
    • lesions are typically hyperintense
    • when arranged perpendicular to lateral ventricles, extending radially outward (best seen on parasagittal images) they are termed Dawson fingers
  • T1 C+ (Gd)
    • active lesions show enhancement
    • enhancement is often incomplete around the periphery (open ring sign)
  • DWI/ADC: active plaques may demonstrate restricted diffusion 10-11
  • MR spectroscopy: may show reduced NAA peaks within plaques

Even on a single scan, some features are helpful in predicting relapsing-remitting vs progressive disease. Features favouring progressive disease include:

  • large numerous plaques
  • hypo intense T1 lesions

Treatment and prognosis

The aim of treatment is two-fold: to curtail progression (disease modifying agents) and symptomatic relief. 

Steroids, interferon, monoclonal antibodies are all used. Discussion of individual agents is beyond the scope of this article. It is important to remember from an imaging perspective that the use of steroids can make an active lesion reduce in size and reduce enhancement.

Prognosis is variable and depends on the pattern of disease a pattern has (e.g. primary progressive carries a worse prognosis than relapsing-remitting). 

In general, patients with relapsing-remitting MS will progress to secondary progressive disease in 10 years, and will require ambulatory aids (e.g. cane/wheelchair/frame) in another 5 to 15 years 12

Differential diagnosis

The differential diagnosis is dependent on the location and appearance of demyelination. For classic (Charcot type) MS the differential can be divided into intracranial and spinal involvement. 

For intra cranial disease the differential includes almost all other demyelinating disease as well as:

For spinal involvement the following should be considered:

The differential for multiple sclerosis variants (e.g. tumefactive MS, Devic disease etc... see above) are discussed separately. 


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