FND: Definition, Diagnosis, and why it is incorrect in most cases

FND: Definition, Diagnosis, and why it is incorrect in most cases

Oct 24, 2026 · @Pradeep

Be very careful when you are diagnosed with FND without your doctor looking for other explanations or consulting other specialists. If I was diagnosed with FND right off the bat, I would leave. We in Rhode Island have a hospital that diagnosis almost everyone with FND, even if they have a diagnosed severe medical condition.

What is FND?

FND stands for Functional Neurological Disorder. Here is a simple way to think about it.

Think of the body like a computer. The hardware is the parts you can touch: the brain, nerves, spine, and muscles. The software is the programs that tell those parts what to do.

In many illnesses, the hardware is broken. A nerve is pinched. A part of the brain is hurt. In FND, the hardware is not damaged. The problem is in the software. The brain is not sending or getting messages the right way.

This means a person with FND can have very real problems, such as:

•      Weak arms or legs

•      Shaking or jerking

•      Trouble walking

•      Spells that look like seizures

•      Numbness

•      Trouble talking or swallowing

•      Trouble seeing

FND is not faking. The person is not doing it on purpose. Their symptoms are real, and they can be very hard to live with.

FND used to be called conversion disorder. In 2013, doctors changed the name. They also stopped requiring a stressful event before making the diagnosis. Today, the official name in the big book doctors use for these diagnoses (called the DSM-5-TR) is Functional Neurological Symptom Disorder.

How Doctors Are Supposed to Diagnose FND

Doctors use a list of four rules. All four must be true. Here they are in plain words.

Rule 1: The person has at least one symptom that affects movement or feeling. For example: weakness, shaking, numbness, seizure-like spells, or trouble talking, swallowing, or seeing.

Rule 2: The doctor finds signs on the exam that do not match a known disease. This is the most important rule. The doctor must see something during the exam that points to FND. It is not enough for the tests to be normal.

Rule 3: Another illness does not explain the symptom better. The doctor must think about other conditions first. There are doctors who do not look for other conditions and brand patients with FND. Be careful.

Rule 4: The symptom causes real trouble in the person's life, or it is serious enough that it needs a doctor to check it.

Doctors also write down:

•      What kind of symptom it is (weakness, shaking, seizure-like spells, and so on)

•      How long it has lasted (less than 6 months, or 6 months or more)

•      Whether there was a stressful event before it started (but this is no longer required)

What signs do doctors look for?

Here are some signs doctors use to find FND:

•      Hoover sign. The doctor tests leg strength two ways. If the leg is weak when the person tries to push it down, but strong when the person lifts the other leg, that can point to FND.

•      Shaking that changes with a beat. The doctor asks the person to tap a rhythm with the other hand. If the shaking changes to match the new beat, or stops, that can point to FND.

•      Symptoms that get better when the person is distracted.

•      Weakness that gives way suddenly instead of staying steady.

•      For seizure-like spells: The best test is a video EEG. This watches the brain waves while the spell happens. In FND spells, the brain waves look normal during the spell. Other clues are eyes staying shut during the spell, side-to-side head shaking, and long spells.

Why Rule 2 and Rule 3 matter so much

Here is the hidden problem. Rule 2 says the symptom must "not match a known disease." Rule 3 says another illness must not "explain it better."

But a doctor can only rule out the diseases they know about.

If a doctor does not know much about Ehlers-Danlos syndromes, POTS, MCAS, or tethered cord syndrome, the symptoms may look like they "do not match" anything. The doctor may then say "FND" when the real answer is a condition they did not think of.

So the rules are good. They only work well when the doctor knows a lot of different conditions.

What If I Have FND and Another Illness?

Some people have both. For example, a person with EDS or POTS can also have some FND symptoms. This is confusing. How can you tell which symptom comes from which?

First, an honest word: you cannot always tell on your own. Even doctors find this hard. But there are clues. Watching your body closely can help you and your doctor sort things out.

Clues that a symptom may come from a body condition (hardware)

•      It has a clear trigger you can repeat. It happens every time you stand up (this may be POTS). It happens after certain foods, heat, or smells (this may be MCAS). It happens when you bend your neck forward (this may be a neck or spine problem).

•      It can be measured. Your heart rate jumps when you stand. Your blood pressure drops. You get a rash or flushing. A test shows a change.

•      It follows a nerve path. Numbness covers the area one nerve supplies, not a whole arm or a straight line down the middle of the body.

•      It comes with other body changes. Muscles shrink. Reflexes change. You lose bladder or bowel control. Joints slip out of place.

•      It gets better with treatment for that condition. For example, fluids and salt help POTS symptoms, or a mast cell medicine helps flushing.

Clues that a symptom may be functional (software)

•      It gets better when your mind is busy with something else. For example, your walking is better when you are talking or not thinking about it.

•      It can switch on or off very quickly without a clear trigger.

•      It changes in strange ways during the exam, like the Hoover sign or shaking that follows a new beat.

•      The pattern does not follow nerve paths.

An important warning

Many body conditions also go up and down. EDS, POTS, and MCAS can make you feel fine one hour and very sick the next. Symptoms that change are not proof of FND. A good doctor knows this.

Also, pain can make a muscle "give way" during testing. A weak leg that gives way because it hurts is not the same as FND.

Tools that can help you sort it out

1.     Keep a symptom diary. Write down what happened, what time, what you were doing, and what you ate. Look for patterns.

2.     Take videos. Record spells, shaking, or walking problems. This can help a doctor more than words.

3.     Check your heart rate and blood pressure lying down and then standing. Write down the numbers.

4.     Write a list for your doctor with two columns: "Symptoms with clear triggers" and "Symptoms with no clear trigger."

5.     Ask your doctor: "Which of my symptoms do you think are from my body condition, and which do you think are functional? What signs show you that?"

Why sorting this out matters

If you have both, you may need both kinds of care. You may need medicine, fluids, or surgery for your body condition. You may also need therapy that helps retrain the brain for the functional part. Getting only one kind of care will leave part of the problem untreated.

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