Multiple sclerosis

Multiple sclerosis (MS) is a relatively common acquired chronic relapsing demyelinating disease involving the central nervous system, and is the second most common cause of neurological impairment in young adults, after trauma ¹⁹. Characteristically, and by definition, multiple sclerosis is disseminated not only in space (i.e multiple lesions in different regions of the brain) but also in time (i.e. lesions occur at different times).

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

• classic multiple sclerosis (Charcot type)
• tumefactive multiple sclerosis
• Marburg type (acute malignant)
• Schilder type (diffuse cerebral sclerosis)
• Balo concentric sclerosis

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

Importantly, neuromyelitis optica (Devic disease) was considered a variant of multiple sclerosis, but is now recognised as a distinct entity, and is therefore also discussed separately. 

Epidemiology

Presentation is usually between adolescence and the sixth decade, with a peak at approximately 35 years of age ^12,19. There is a strong, well recognised female predilection with a F:M ratio of approximately 2:1 ¹⁹. 

Multiple sclerosis has a fascinating geographic distribution: it is rarely found in equatorial regions (e.g. 15 per 100,000), with incidence gradually increasing with distance from the equator (e.g. 250 per 100,000) ^{12, 19}. 

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 secondary 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 multiple sclerosis. 

Radiologically isolated syndrome (RIS) is another entity based on MRI brain findings which described as incidental white matter lesions suggestive of MS on imaging in a patient without associated clinical symptoms ¹⁷.

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

Diagnosis

The diagnosis of multiple sclerosis requires the constellation of clinical findings and various investigations, including ¹⁹:

• typical history
• oligoclonal bands in CSF
• immunoglobulin G in serum
• abnormal visual evoked potential
• MR imaging
• lack of viable alternative diagnosis

Pathology

The exact aetiology is poorly known although it is believed to have both genetic and acquired contributory components. An infectious agent (i.e., EBV), or at least a catalyst, has long been suspected due to the geographic distribution and presence of clusters of cases; however, no agent has yet been firmly confirmed. Some authors also suggested that "chronic cerebrospinal venous insufficiency" can cause or exacerbate MS but this theory has not been proven by further investigations ¹⁵.

Multiple sclerosis is believed to result from a cell-mediated autoimmune response against one's own myelin components, with loss of oligodendrocytes, with little or no axonal degeneration in the acute phase; however, in later stages, loss of oligodendrocytes results in axonal degeneration. 

Demyelination occurs in discrete perivenular foci, termed plaques, which range in size from a few millimetres to a few centimetres ¹⁹.

Each lesion goes through three pathological stages:

• early acute stage (active plaques)
  • active myelin breakdown 
  • 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

Associations

• a strong association with HLA-DR2 class II has been identified ¹¹
• Melkersson-Rosenthal syndrome: postulated

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 hypoattenuating ^{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 help determine the disease pattern. 

• T1
  • lesions are typically iso- to hypointense (T1 black holes) 
  • callososeptal interface may have multiple small hypointense lesions (Venus necklace) or the corpus callosum may merely appear thinned ¹¹ 
  • hyperintense lesions are associated with brain atrophy and advancing disease ¹⁸
• T2
  • lesions are typically hyperintense
  • acute lesions often have surrounding oedema
• T2* (SWI)
  • at higher field strengths most plaques have been shown to be perivenular (at 3T, 45% of lesions; at 7T, 87% of lesions) ¹⁹
• FLAIR
  • lesions are typically hyperintense
  • a very early sign is called "ependymal dot-dash sign": alternating small foci of hyperintensity along the callososeptal interface ¹⁶
  • when these propagate centrifugally along the medullary venules and arranged perpendicular to the lateral ventricles in a triangular configuration (extending radially outward - best seen on parasagittal images), they are termed Dawson's fingers
  • FLAIR is more sensitive than T2 in detection of juxtacortical and periventricular plaques, while T2 is more sensitive to infratentorial lesions
• T1 C+ (Gd)
  • active lesions show enhancement
  • enhancement is often incomplete around the periphery (open ring sign)
• DWI/ADC
  • active plaques may demonstrate high or low ADC (increased or decreased diffusion) ^10-11-22
  • also typically open ring in morphology
• MR spectroscopy 
  • NAA peaks may be reduced within plaques, which is the most common and remarkable finding
  • Cholinecholine and lactate are found to be increased in the acute pathologic phase
• double inversion recovery DIR: a new sequence that suppresses both CSF and white matter signal and offers better delineation of the plaques.

Location of the plaques can be infratentorial, in the deep white matter, periventricular, juxtacortical or mixed white matter-grey matter lesions.

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

• large numerous plaques
• hyperintense T1 lesions

Treatment and prognosis

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

Steroids, interferon, monoclonal antibodies and autologous hematopoietic stem cell transplantation are all used. Although discussion of individual agents and therapies is well beyond the scope of this article, it is worth being aware of the main agents available and how their mechanism of action ²⁰:

• interferon beta: inhibition of T-lymphocyte proliferation
• glatiramer acetate: immunomodulation
• teriflunomide (Aubagio): reduces both T-cell and B-cell activation and proliferation
• dimethyl fumarate (Tecfidera): immunomodulation
• fingolimod (Gilenya): prevents lymphocyte migration out of lymph nodes and into CNS
• natalizumab (Tysabri): inhibits binding of lymphocytes to endothelium
• alemtuzumab (Lemtrada): immunomodulation of T-cell and B-cell function
• mitoxantrone: reduces T-cell and B-cell proliferation and reduces T-cell activation

Complications

In addition to the potential for disease progression resulting in progressive neurological impairment, a number of specific complications need to be considered. These include ^20,21: 

• progressive multifocal leukoencephalopathy (PML)
  • particularly in patients treated with natalizumab with positive JC virus serology
• PML-IRIS
  • a complication of cessation of natalizumab or treatment for natalizumab-related PML with plasma exchange or immunoabsoption ²¹
• primary CNS lymphoma
  • rarely lymphoma appears to arise from previously identified demyelinating lesions

Prognosis

Prognosis is variable and depends on the pattern of disease a patient 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. Approximately half of affected individuals will no longer be independently ambulatory after 20 years ¹⁹. 

Overall life expectancy is also reduced, by 7 to 14 years ¹⁹. 

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 intracranial disease, the differential includes almost all other demyelinating diseases as well as:

• CNS fungal infection (e.g. Cryptococcus neoformans) - patients tend to be immunocompromised
• mucopolysaccharidosis (e.g. Hurler disease) - congenital and occurs in a younger age group
• Marchiafava-Bignami disease (for callosal lesions) 
• Susac syndrome
• CNS manifestations of primary antiphospholipid syndrome ¹³

For spinal involvement, the following should be considered:

• transverse myelitis
• infection
• spinal cord tumours (e.g. astrocytomas)

Multiple sclerosis variants (e.g. tumefactive MS, Devic disease) are discussed separately.