MMN - How is Multifocal Motor Neuropathy diagnosed?

You join us for our third in our series of accessible, bite sized blogs all about MMN. In week one, we took a general overview of the condition, and last week we investigated some of MMN's classic symptoms and warning signs.

This week, let's look at what happens next.

Misdiagnosis and Timeline

We'll be honest, this part is hard. Officially, your time to diagnosis should be around one to two years. However - and it's important we say this is anecdotal from conversations with our community, your experience may vary - it can often be much longer, up to eight years in some cases.

MMN is very rare, and as we've previously mentioned, can mask itself as other conditions and even just simply aging. As such, you many not recognise an issue straight away, and your doctor may suggest some other conditions. Here's what we've heard:

ALS/MND: This is unfortunately one of the most frequent misdiagnoses in the MMN world. MND is not curable (MMN is also not curable, but it is treatable and should not impact on your life expectancy) and it can be tremendously upsetting to be informed you may have it.

CIDP: One of the other conditions we support - we may even have an existing relationship with you if you've been misdiagnosed, if that's the case, do reach out and we can look at more relevant Peer Support.

You may also be tested for Lewis-Sumner, Vasculitis, Progressive Muscular Atrophy (PMA), Radiculopathy (e.g. cervical radiculopathy), Ulnar Neuropathy or Other Entrapment Neuropathies. These share many similar symptoms, in some cases it can be very hard to distinguish between conditions, which is where testing comes in.

Tests

Let's skip forward in the timeline and assume you're now in front of a neurologist, they suspect you may have MMN. What happens now?

A clinical diagnosis of MMN usually relies on patient history, a physical exam, and some specialist tests. They're aiming to demonstrate the following*

*note: every person presents differently, as such, this list is not conclusive and may vary by MMN site in your body, your general health, and the neurologist treating you

The patient has a purely motor disorder affecting individual nerves

This means they need to be able to show that you have a condition that only (purely) impacts muscle movement/strength (motor). It should not impact sensation or feeling. This condition should also only impact individual nerves/areas rather than the full body.

Example: You could be having issue gripping a mug of tea with your left hand/arm, but the arm itself isn't sore, itchy, or numb. Your right arm could be absolutely fine at this point.

The patient shows no Upper Motor Neuron (UMN) signs - brisk reflexes/spasticity in the limbs

One of the key parts of diagnosing MMN is showing what isn’t there. MMN is a pure lower motor neuron (LMN) disorder, meaning it affects the peripheral motor nerves after they leave the spinal cord. It does not affect the upper motor neurons (the pathways that run from the brain down through the spinal cord).

Because of this, your neurologist will look for signs of UMN involvement such as:

• Brisk reflexes (stronger or more exaggerated responses): particularly around the knees and ankles.

• Spasticity (increased muscle tone causing stiffness) in the limbs.
  

To assess this, your neurologist can:

• Test your reflexes using a small tendon hammer (think of TV doctors gently tapping at a knee, it's only small and shouldn't hurt)

• Gently move your arms and legs to assess muscle tone

• Ask you to perform simple movements to evaluate coordination and stiffness

• Check the soles of your feet for abnormal reflex responses

  
  

Remember, MMN should have no Upper Motor Neuron involvement, so your neurologist should be looking to confirm your reflexes are normal, and your muscles/limbs aren't unusually stiff.

If UMN signs are present, it suggests another diagnosis.

The patient has no difficulty speaking or swallowing

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You shouldn't struggle to speak, and those around you haven't noticed a change in your pitch, volume, or tone. You should be able to eat, and drink, as normal. You will likely not be tested for anything in this area, but they may ask you about your speech or eating/drinking comfort.

The patient demonstrates no sensory deficits 

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While you may feel weakness, you should not feel any difference in sensation. Your neurologist may perform some simple tests:

• Light touch

• Pinprick sensation

• Temperature (hot and cold)

• Vibration

• Joint position sense

In MMN, the responses to these should be normal. Some MMN patients do feel a very slight tingle on occasion (possibly due to muscle twitches/spasms), but when tested clinically, sensation should be found to be normal.

The patient shows evidence of focal areas of nerves in which electrical impulses are slowed or blocked (conduction block) which can be detected on electrophysiology tests

 

Hold on, hold on - focal areas, electrical impulses, conduction blocks, electrophysiology.. let's rephrase.

In MMN, there'll be evidence that some sections of your nerves are acting like a damaged cable - the signal either slows down or gets partially blocked. Special nerve tests can measure this.

Your neurologist will likely carry out an electrodiagnostic study, which includes nerve conduction studies (NCS) and needle electromyography (EMG).

Needles and electrical tests can sound a little troubling. Let's break these down a bit so you know what's happening.

Nerve Conduction Studies

The NCS tests for how well your nerves carry electrical signals.

Small electrodes are placed on your skin over a nerve and muscle. Then a very brief electrical pulse is delivered to the nerve.

Your muscle should contract, and the machine will measure how fast and strongly that signal/pulse travelled.

Your neurologist will be looking for areas where this signal slows down, or drops off significantly between two points. That suggests a conduction block, and that, in turn, suggests MMN.

Does it hurt?

Not really, but it might be a bit uncomfortable or even startling, like a short, sharp, shock. Certain areas may be more sensitive than others, but as soon as the pulse stops, the sensation will also stop. Each pulse only lasts a fraction of a second (though it may be repeated a few times).

Needle Electromyography

The EMG tests the health of the muscles and the nerves supplying them.

A very thin needle electrode will be inserted into selected muscles, you'll then be urged to sit or lie nice and still. The electrode will record this electrical activity at rest.

Next, you'll likely be asked to gently contract the muscle by tensing/squeezing, or performing a small, gentle action. The electrode will record this electrical activity in action.

This helps determine whether the weakness is coming from the nerve or the muscle. In MMN, the EMG might show the muscle is not receiving messages from the nerves properly.

Does it hurt?

An EMG involves a fine needle being inserted into your muscle, as such, there may be a little soreness. Think of it like a vaccination. It feels like a sharp pinch, or scratch. There may be a dull ache when the needle is within the muscle.

You may experience a little soreness or even a small bruise over the following days.

It's usually described as uncomfortable, but manageable, and it should not last long.

 Blood Tests

And last but not least. Laboratory testing for IgM GM1 antibodies are also frequently done.

Doctors often test for IgM anti-GM1 antibodies because many people with MMN have them. These antibodies mistakenly target motor nerves, supporting the idea that MMN is an immune condition.

However, not everyone with MMN tests positive, so a negative result does not rule it out. It does however, lend more evidence to a diagnosis of MMN if you match some or all the diagnostic criteria on this page.

As we mentioned at the start of the testing section. Every person presents differently so this list is not conclusive and may vary by MMN site in your body, your general health, and the neurologist treating you. Please don't be concerned if your journey to diagnosis looks different.

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This was a bit of a longer blog than we usually write for these snapshots. It even took us by surprise writing it! However, some of the points are important, and we like to demystify some areas - especially tests which can sound a bit frightening without context - where we can.

Join us again next week!

MMN Support

Living with Multifocal Motor Neuropathy can be challenging, but you don’t have to face it alone. Inflammatory Neuropathies UK is here to support people and families affected by MMN. We provide trusted information, advice, and guidance; practical support where it’s needed; fund and undertake research; and raise awareness of rare Inflammatory Neuropathies. If you’d like to know more, you can read our blog series, explore our resources, or get in touch.

If you’re looking for support, to have a chat or ask questions, why not join our Facebook Group or read our Newsletter.