The McDonald criteria are rules and suggestions for diagnosing multiple sclerosis.

The criteria give guidelines to doctors for using the clinical evidence of the signs and symptoms a person presents with, their MRI scans, and some other tests that can be used when trying to diagnose multiple sclerosis (MS).

Originally developed in 2001 by Professor Ian McDonald and a team of researchers, the criteria have gone through two revisions to help improve the accuracy of the testing and speed up the diagnosis process.

In this article, we look at what the criteria are and what is required for an MS diagnosis.

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The McDonald criteria help doctors to diagnose multiple sclerosis.

In essence, the McDonald criteria help doctors diagnose MS by outlining the clinical factors to look for, as well as any additional data that can help lead to a diagnosis. Whether or not a doctor will need additional data to diagnose MS using the McDonald criteria depends on the symptoms a person has.

A diagnosis needs no additional data if a person presents with the following symptoms:

  • two or more attacks of MS symptoms
  • evidence of two or more lesions on the brain and spinal cord or central nervous system
  • reasonable evidence of a previous MS flare

These symptoms collectively show dissemination in space and dissemination in time, both of which are terms that are described below.

The McDonald criteria for diagnosing MS look at the number of areas of the brain and spinal cord where lesions have formed. This spread is referred to as dissemination in space.

To be considered indicative for diagnosing MS, an MRI scan must show two or more bright lesions on the central nervous system.

The bright lesions are classified using the T2 scan, which is one of the most common MRI scans used to diagnose MS. The T2 scan detects both old and new damage caused to the brain and spinal cord.

Specifically, MRI scanning must reveal bright lesions on two or more of the following areas:

  • the spinal cord
  • the periventricular region of the brain
  • the infratentorial region of the brain
  • the justacortical region of the brain

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An MRI scan may be used to help diagnose MS.

Dissemination in time refers to how the number of lesions increases over time. Damage needs to occur on separate dates for an MS diagnosis.

The criteria for establishing dissemination in time include discovering:

  • any new bright lesion on the central nervous system that did not appear on the last MRI scan
  • an asymptomatic contrast-enhancing lesion and a non-contrast-enhancing T2 bright lesion on any one scan

An enhancing lesion is one that appears more clearly when a contrast substance is added to the area before scanning. These tend to be newer lesions. Gadolinium is a common contrast used in scanning. Non-enhancing lesions do not have such defined borders.

In general, each MS attack or flare that a person experiences occurs over a period of time. This period will vary from person to person. In some cases, flares are also only separated by a few days. In others, it may be several weeks or even months between flares.

The revised 2010 McDonald criteria can help speed up diagnosis as they include data beyond MRI scans. In some cases, MS can be diagnosed using the McDonald criteria with only one noted flare.

In most cases, the number of lesions will increase from the first MRI to the second or third, depending on the severity of the MS

If a person presents with two or more attacks and evidence of one lesion on the brain or spinal cord, an MS diagnosis based on the McDonald criteria cannot be made.

The doctor must first confirm dissemination in space, meaning an MRI scan must show lesions on more than one area of the brain and spinal cord. The doctor can also wait until the person experiences another attack that indicates a further affected area.

Similarly, in cases where a person presents with one attack and evidence of two or more lesions, a formal MS diagnosis cannot be made based on the McDonald criteria. The criteria require a dissemination in time for a diagnosis to be made, with attacks separated by different dates shown by MRI scanning, or a second attack with clinical symptoms.

When a person clinically presents with one attack and evidence of one lesion, more information is needed. A person also needs to show both dissemination in space and dissemination in time for a diagnosis to be made.

If a person clinically shows neurological progression suggestive of MS, a doctor needs more data to make a formal diagnosis.

In these cases, they will need to confirm 1 year of disease progression and dissemination in space, demonstrated by two or more of the following:

  • one or more T2 lesions in the brain, in regions characteristic of MS
  • two or more T2 focal lesions in the spinal cord
  • positive signs of MS in the fluid of the brain and spine

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The McDonald criteria have had two revisions since 2001.

There have been two revisions of the McDonald criteria since the original criteria were put forward in 2001.

Both revisions tried to address potential flaws in the criteria's method, speed, and the evidence needed to make an MS diagnosis.

The original guidelines for the McDonald diagnostic criteria are described here. As in the 2005 and 2010 revisions, the 2001 criteria required dissemination in both space and time.

For dissemination in space, three out of four of the following must be in place:

  • one or more infratentorial or spinal cord lesions
  • one gadolinium-enhancing lesion or nine T2 hyperintense brain and cord lesions
  • one or more lesions in the juxtacortical section of the brain
  • three or more lesions in the periventricular section of the brain

For dissemination in time, one out of two of the following conditions must be met:

  • one or more new gadolinium-enhancing lesions on a scan performed at a new area of the central nervous system at least 3 months after the initial symptoms occurred
  • one or more new bright T2 lesions on a scan 30 or more days after the initial symptoms and scan

Similarly to the most current revision, additional clinical criteria were also needed to make an MS diagnosis. A person also needed to have:

  • clinical symptoms
  • visual signs of MS
  • oligoclonal bands in the fluid of the brain and spine

Oligoclonal bands consist of protein and signal inflammation from a disease of the central nervous system.

MS has several different types, including relapsing, primary progressive, and secondary progressive.

The McDonald criteria can also identify a fourth condition known as clinically isolated syndrome.

Clinically isolated syndrome occurs when only one inflammatory disease attacks the brain or spinal cord. The syndrome may affect only one area, such as vision, or form many lesions on different parts of the nervous system.

The latest version of the McDonald criteria recognised a difference in diagnosing the progressive type of MS.

For progressive MS, a person now needs to have a full year of progression plus at least two of the three following criteria:

  • at least two T2 lesions on the spinal cord
  • evidence of at least one T2 lesion in the periventricular, juxtacortical, or infratentorial areas of the brain
  • positive signs of MS in the fluid of the brain and spine

Critics of the McDonald criteria have expressed several concerns. For example, McDonald and his team specify that the criteria should be applied after other diseases have been ruled out, to help prevent false positives.

Additonally, the McDonald criteria do not specifically define what a MS lesion is. Even after the 2010 revision, there is no definitive guide in the McDonald criteria to define what a lesion looks like.