Tag Archives: demyelinate

Causes and prevention of MS

What is multiple sclerosis?

This is the first of a series of basic lessons to help you understand multiple sclerosis (MS).

Key points

  • MS is an autoimmune disease in which the immune system attacks the central nervous system.
  • Its exact cause is unknown; some contributory environmental factors are outlined.
  • Common manifestations of MS include lesions, relapses and intermittent symptoms, which often worsen with fatigue.
  • Early treatment is important to help prevent the damage that occurs with MS.

Multiple sclerosis (MS) is an organ-specific autoimmune disease. Autoimmune simply means that the immune system, whose primary role is to fight infections and cancers, goes awry and attacks itself. Organ-specific means that a disease is limited to one organ. So, in the case of MS, the immune system attacks the central nervous system (CNS), which consists of the brain, spinal cord and optic nerves.

Every organ in the body has its specific autoimmune disease. For example:

  • joints: rheumatoid arthritis
  • skin: psoriasis 
  • insulin-producing cells of the pancreas: type 1 diabetes
  • intestines: inflammatory bowel disease
  • kidneys: autoimmune nephritis (interstitial or glomerulonephritis).

The cause of MS

At present, the exact cause of MS is unproven. MS is a complex disease that occurs due to the environment’s interaction with inherited or genetic factors.1 Some of the main environmental factors are:

  • low vitamin D levels or a lack of sunshine
  • smoking 
  • Epstein–Barr virus (EBV), the virus that causes infectious mononucleosis (glandular fever) 
  • obesity, particularly in adolescence.

What we don’t know is how these genetic and environmental factors interact to cause MS. There are many genetic variants that predispose someone to get MS, but only a minority of people who have these variants will get the disease. Similarly, only a minority of people exposed to environmental risk factors get the disease.

Mechanisms that underlie the common manifestations of MS

Lesions

MS is characterised by inflammatory lesions – areas of damage or scarring (sclerosis) in the CNS – that come and go. The clinical manifestations of MS depend on where these inflammatory lesions occur. If, for example, a lesion involves the optic nerve, it will cause impaired vision; if it involves the brain stem, it causes double vision, vertigo or unsteadiness of gait; a spinal cord lesion leads to loss of feeling, limb weakness or bladder and bowel problems.  

Relapses

A new MS lesion in a site that is eloquent will cause symptoms and neurological signs; if these last for at least a day, they are called an attack or a relapse. If a lesion occurs in a site not associated with overt symptoms, this is often referred to as a subclinical or asymptomatic relapse. Subclinical relapses can be detected using magnetic resonance imaging (MRI). It is said that for every clinical attack there are 10 or more sub-clinical attacks (new MRI lesions).2 

Damage frequently occurs at the site of MS lesions. The inflammation strips the myelin covering the nerve processes and may cut through axons. Axons are the nerve processes that transmit electrical impulses or signals. When the axons are stripped of their myelin sheath, and/or are cut, they can’t transmit electrical signals. This causes loss of function, which manifests with specific symptoms.

Demyelination: loss of the myelin sheath that insulates nerves, leading to disruption of electrical signals. Image courtesy of Timonina/shutterstock.com. 

Intermittent symptoms

Surviving axons that pass through the lesion are able to recover function, by synthesising and distributing so-called ion channels across the demyelinated segment or by being remyelinated. Both these processes are not perfect. For example, the new sodium channels may not function normally, so they sometimes fire spontaneously. The spontaneous firing of axons may cause positive symptoms, for example, pins and needles, pain or spasms. The new myelin is typically thinner and shorter than normal and is temperature, fatigue and stretch sensitive. 

Stretch sensitivity

If someone with MS has a lesion in their spinal cord, electric shock-like sensations may occur when they stretch the spinal cord by bending or flexing their neck; this is known as Lhermitte’s sign.  

Temperature sensitivity

Recurrent symptoms may occur when body temperature rises, for example following fever, exercise or a hot bath. The MS symptoms (which may vary among individuals) disappear when the fever resolves or the body cools down. The temperature sensitivity is often referred to as Uhtoff’s phenomenon. 

Fatigue

Symptoms tend to worsen with physical and/or mental fatigue; for example, someone with MS may begin dragging a leg or dropping their foot after 20–30 minutes of walking. This is because the transmission in the functioning nerves, which have been previously damaged, begins to fail. This failure may be related to a lack of energy and/or to temperature changes that occur with exercise. 

Worsening MS (also called progressive MS)

If the axons, or nerve processes, above and below an MS lesion die off, the surviving axons may sprout to take over the function of the axons below the lesion. This puts an unnecessary strain on the surviving axons, which makes them vulnerable to die off in the future. A reduction in the number of nerves in a neuronal system reduces the neurological reserve of that system, making it more vulnerable to future attacks. In other words, the ability to recover from future attacks is reduced, and the neuronal pathway is susceptible to delayed degeneration and premature ageing. Clearly, if no treatment is given and focal inflammatory lesions continue to come and go, this will cause worsening of the disease. If enough damage is allowed to accrue, even switching off new inflammatory lesions may not prevent the so-called delayed neurodegeneration. This is why one of the primary principles of managing MS is early treatment to prevent damage from occurring in the first place. We have also discovered that the neuronal systems with the longest nerve fibres, in particular the bladder and legs, are much more susceptible to damage. We think this is simply because the longest pathways provide the greatest scope to be hit by multiple MS lesions.

Ageing and MS

As we get older our nervous systems degenerate. If we live long enough, we will all develop age-related neurological problems, such as unsteadiness of gait, loss of memory, reduced vision, loss of hearing, and poor coordination. 

What protects people with MS from becoming disabled and developing age-related neurodegeneration are brain reserve and cognitive reserve. Brain reserve is simply the size of your brain or the number of nerve cells you have. Cognitive reserve, in comparison, relates to how well these nerves function; it is associated with your level of education and how well you enrich your life by using your brain. From about 35 years of age, our brains start to shrink. In MS, this brain shrinkage is in general much greater than normal, and the resulting reduction in brain and cognitive reserve almost certainly primes the nervous system to age earlier. This is one of the reasons why people with MS continue to develop worsening disability later in the course of their disease. This insight is one of the main reasons why we promote early effective treatment of MS to protect and maintain brain and cognitive reserves.  

References

  1. Olsson T, et al. Interactions between genetic, lifestyle and environmental risk factors for multiple sclerosis. Nat Rev Neurol 2017;13:25–36.
  2. Gafson A, et al. The diagnostic criteria for multiple sclerosis: From Charcot to McDonald. Mult Scler Relat Disord 2012;1:9–14

Diagnosis, Stages of MS

Do I have active MS?

Before deciding to start a disease-modifying therapy you need to know if you have active MS.

Key points

  • To qualify for a disease-modifying treatment for MS you must have active disease.
  • Active MS is characterised by relapses (new symptomatic or asymptomatic lesions); the clinical diagnosis of relapse may be supported by MRI or CSF evidence of activity.
  • Different levels of disease activity qualify for different types of DMT.
  • Diagnostic criteria for MS have evolved considerably over the past two decades; this has helped to make treatment decisions earlier and easier, both for MS neurologists and for people with MS.

To be eligible for disease-modifying therapy (DMT) you must have ‘active MS’. This term is increasingly used to refer to current or recent evidence of focal inflammatory activity, i.e. new lesions on magnetic resonance imaging (MRI) or a relapse. Inflammation damages axons, or nerve processes. When a lesion develops, the effects of inflammatory mediators can cut (transect) axons, demyelinate them or stop them from working.

By contrast, the gradual worsening of disability that occurs in people with more advanced MS (which may, or may not, occur in the presence of focal inflammatory activity) has many potential causes, only one of which is focal inflammation.

Signs of active MS

Relapses

When a new MS lesion occurs in an eloquent part of the central nervous system it causes new symptoms or exacerbates old ones – this is usually interpreted as a relapse. Relapses, by definition, last at least 24 hours in the absence of infection or fever.

Criteria for ‘active’ MS accepted by many MS health professionals. CSF, cerebrospinal fluid; NFL, neurofilament light.
*Some neurologists accept 24 months, 36 months or even more when assessing MRI activity. There is no international consensus on the gap between the baseline and new MRI scan to define active disease.

Asymptomatic lesions

Most focal MS disease activity does not cause any overt symptoms because the brain has a way of compensating for damage. For every clinical relapse, there are at least 10 or more lesions on MRI. Therefore, what we see clinically in terms of relapses is the tip of the iceberg. Even standard MRI is relatively insensitive in detecting and monitoring MS disease activity; it misses new lesions that are smaller than 3 ̶ 4 mm in size and does not detect most lesions that occur in the grey matter of the brain (cortex and deep grey matter nuclei, e.g. thalamus and basal ganglia). Therefore, MRI scans also reveal just the tip of the iceberg. This is one of the reasons we also use cerebrospinal fluid (CSF) neurofilament levels as a marker of this microscopic activity.

Disease activity levels

Inactive MS

Many people with MS experience frequent intermittent symptoms or ‘pseudorelapses’ that come on when they are tired, after exercise or have a raised body temperature from a fever, exercise, hot bath or a warm environment. These intermittent symptoms are usually quite stereotyped and last minutes to hours. They are indicative of a previously damaged pathway but do not represent a relapse or disease activity.

Active MS

Most neurologists require evidence of disease activity in the last 12 months, with some of us accepting a 24-month or 36-month window if there is no serial or regular MRI support. However, if you have had no relapses or MRI evidence of new lesions in the last 24 months, then your MS is defined as inactive. (This does not mean your MS is necessarily stable; you could have worsening disability as part of the progressive or smouldering phase of the disease.) Inactive MS needs to be monitored in case it reactivates, in which case you could become eligible for treatment.

Schematic showing different levels of MS disease activity.
*Some neurologists accept MRI activity in the last 24 months, 36 months or even longer as a criterion for active MS.

Highly active MS and rapidly evolving severe MS

Active MS has been divided into an additional two categories that have implications for DMT prescribing (depending on where you live).

  • Highly active MS describes MS with unchanged or increased relapse rates, or ongoing severe relapses compared with the previous year, despite treatment with beta-interferon or another so-called first-line therapy. In England, patients in this subgroup are eligible for natalizumab, alemtuzumab, fingolimod and cladribine.
  • Rapidly evolving severe MS (RES) is defined as two disabling relapses and MRI evidence of activity within a 12-month period. In England, patients in this subgroup are eligible for natalizumab, alemtuzumab and cladribine.

Evolution of diagnostic criteria

In the early 2000s, disease activity was defined using clinical criteria only; you needed at least two documented relapses in the last 2 years to be eligible for DMT.1 This meant that a neurologist had to examine you to confirm abnormalities compatible with a relapse. However, many people with MS without rapid access to a neurologist would recover before being assessed, meaning that their relapses often could not be documented. This was very frustrating for someone wanting to start a DMT. If patients had MRI evidence to support recent disease activity, how could we deny them access to a DMT because they were not seen in a timely way to have their relapse documented in the clinical notes?

In 2009, our criteria incorporated MRI into the definition to allow us to treat so-called high-risk patients with CIS (clinically isolated syndromes compatible with demyelination). These criteria required patients with CIS to have nine or more T2 lesions on MRI or at least one gadolinium-enhancing lesion. These MRI criteria were based on the McDonald diagnostic criteria at the time.2 These eligibility criteria evolved further in 2014, once alemtuzumab was licensed, to include clinical or MRI activity.

References

  1. McDonald WI, et al. Recommended diagnostic criteria for multiple sclerosis: guidelines from the International Panel on the diagnosis of multiple sclerosis. Ann Neurol 2001;50:121–7.
  2. Polman CH, et al. Diagnostic criteria for multiple sclerosis: 2010 revisions to the McDonald criteria. Ann Neurol 2011;69:292–302.