Functional Neurological Disorder (FND) Explained

Functional neurological disorder, often shortened to FND, is diagnosed from clinical features that a trained clinician can identify. It is a genuine condition, and normal scans alone do not establish it. A person with FND can also have B12 deficiency or another medical condition, so new or changing symptoms still deserve a clinical review.

Many people with FND have lived with real neurological symptoms for months or years before receiving an explanation. Others have been told their tests are normal but are left unsure what that means. That can feel unsettling, especially when symptoms affect walking, speech, sensation, tremor, seizures, concentration or daily life.

As a GP, I can review general health, possible coexisting conditions and next steps. Specialist FND rehabilitation may require a dedicated team, but a careful GP review can still be important when symptoms are new, complex or have not been fully assessed.

What is functional neurological disorder?

Functional neurological disorder is a condition where the nervous system does not work as it should, even though scans or standard neurological tests may not show structural disease.

NHS Inform describes FND as a problem with how the brain receives and sends information to the rest of the body. It also states that FND should be diagnosed because a person has one or more clinical features that a specialist can recognise as typical of FND.

A simple way to understand it is that the “hardware” may look normal, but the signalling is not working properly. This is only an analogy, but it can help explain why symptoms can be very real even when tests do not show damage such as a stroke, tumour or multiple sclerosis.

FND can affect movement, sensation, speech, awareness and day-to-day function. It should always be explained respectfully and diagnosed carefully.

Is FND a real condition?

Yes. FND is real and recognised.

It is not imagined. It is not “put on”. It does not mean symptoms are voluntary or under conscious control.

This point matters because many people with FND have felt dismissed before. A good explanation should start by recognising that the symptoms are genuine, then helping the person understand what may be happening and what support may help.

FND sits at the boundary of neurology, rehabilitation, psychology and general medicine. Some people with FND also have anxiety, depression, post-traumatic stress, chronic pain or fatigue. Others do not. It is too simplistic to describe FND as “just psychological”.

What symptoms can FND cause?

Functional neurological disorder symptoms vary widely. Two people with FND may have very different experiences.

Common symptom groups include:

• functional limb weakness or heaviness
• tremor, jerking or abnormal movements
• walking problems or balance difficulty
• numbness, tingling or altered sensation
• episodes that look like seizures but are not caused by epileptic electrical activity
• speech symptoms, such as stuttering, slurred speech or difficulty finding words
• swallowing symptoms
• visual symptoms
• dizziness
• fatigue
• brain fog, poor concentration and memory difficulty

FND can involve movement, sensation, speech, swallowing and seizure-like symptoms. Symptoms can vary and may overlap with other neurological conditions.

Because the symptom range is broad, it is important not to assume that every neurological symptom is FND. The pattern, examination findings, timing and any new changes all matter.

What causes FND?

There is no single cause of FND.

Current understanding is that FND involves a problem with how the brain and nervous system process signals, attention, movement, sensation and prediction. In plain English, the nervous system can get stuck in a pattern where it produces symptoms even though there is no structural damage explaining them.

In some people, symptoms start after:

• a physical illness
• an injury
• surgery
• severe pain
• migraine or another neurological symptom
• an infection
• a period of intense stress
• trauma
• a panic episode
• another medical condition

In others, there is no obvious trigger.

This can be frustrating because people naturally want one clear reason. FND is often better understood in terms of contributing factors rather than one simple cause.

How is FND diagnosed?

FND should be diagnosed positively. This means the diagnosis is based on clinical features that are typical of FND, not simply because tests are normal.

That distinction is important. A proper FND diagnosis is not just “we cannot find anything else”.

NHS Inform says a diagnosis should be made because a person has one or more clinical features that a specialist recognises as typical of FND. NHS neurological guidance also describes positive diagnostic features such as Hoover’s sign in functional limb weakness and tremor entrainment in functional tremor.

Other medical issues may still need to be assessed. Depending on the symptoms, this may include considering neurological conditions, thyroid disease, anaemia, vitamin deficiency, inflammatory conditions, medication effects and metabolic problems.

The right assessment depends on the person. Some people may need urgent neurological review. Others may need blood tests, imaging, neurophysiology tests such as EEG or referral to a neurologist.

Assessing new or coexisting symptoms

FND can be diagnosed positively, but that does not mean every future symptom should automatically be attributed to FND.

Symptoms such as weakness, numbness, tingling, dizziness, fatigue and brain fog can occur in several different conditions. Some are neurological. Some are metabolic. Some are related to deficiency, inflammation, hormones, sleep or medication.

A person with FND can also have another condition at the same time. This is why new symptoms, worsening symptoms or symptoms that do not fit the usual pattern should still be reviewed clinically.

For patients, the practical question is not always “is this FND or something else?” Sometimes the answer may be: “FND may be present, but there may also be a treatable issue contributing to how you feel.”

Where B12 deficiency fits in

Vitamin B12 is important for healthy blood and nerve function. Low B12 can cause symptoms that overlap with neurological conditions, including fatigue, pins and needles, numbness, balance problems and cognitive symptoms such as brain fog.

NICE guidance on vitamin B12 deficiency lists symptoms and signs including unexplained fatigue, cognitive difficulties, pins and needles or numbness, impaired gait and neurological or mobility problems related to peripheral neuropathy or spinal cord disease. NICE also states that B12 deficiency should not be ruled out solely because anaemia or macrocytosis is absent.

This is why B12 deficiency should be considered when someone has unexplained neurological symptoms, particularly if there are risk factors such as:

• a vegan or restricted diet
• pernicious anaemia or autoimmune gastritis
• autoimmune disease
• long-term use of medicines that may affect absorption
• gut conditions such as coeliac disease or inflammatory bowel disease
• previous stomach or bowel surgery
• older age
• a family history of B12 deficiency or pernicious anaemia

B12 deficiency is not being presented here as a cause or cure for FND. It is one treatable condition that can produce overlapping neurological symptoms or coexist with another diagnosis.

Which symptoms can overlap with B12 deficiency?

Some B12 deficiency symptoms can overlap with symptoms reported by people with FND or other neurological conditions.

These may include:

• tingling or pins and needles
• numbness
• burning sensations
• balance difficulty
• walking problems
• fatigue
• brain fog
• mood changes

This does not mean B12 deficiency explains all FND symptoms. In particular, functional seizures should not be assumed to be a usual presentation of B12 deficiency. Seizure-like episodes, blackouts or altered awareness need appropriate medical assessment.

The key point is that symptoms alone are not enough. A clinician needs to look at the pattern, timing, examination findings, risk factors, medication history and blood results.

If B12 status has never been checked, or if results were borderline, affected by supplements or not clearly explained, a careful review may be useful.

What blood tests may be useful?

There is no single blood test that diagnoses FND. Blood tests are used to assess other conditions that may explain new symptoms or contribute to the overall picture.

At B12 Metabolic Clinic, blood tests and diagnostics are selected according to your symptoms, history and previous results. Depending on the situation, a review may include:

• full blood count
• vitamin B12
• folate
• ferritin and iron studies
• thyroid function
• vitamin D
• kidney and liver function
• blood glucose or HbA1c
• inflammatory markers such as CRP or ESR
• coeliac screening where appropriate
• pernicious anaemia-related testing where clinically indicated

Not everyone needs every test. More testing does not automatically mean better care. The value is in choosing tests that answer a clinical question, then interpreting them in the context of your symptoms.

A result in the reference range can be reassuring, but no single blood test rules out every possible explanation for symptoms.

What happens after an FND diagnosis?

If FND is diagnosed, management usually depends on the symptoms, severity and services available locally.

Care may involve:

• a GP
• a neurologist
• physiotherapy
• occupational therapy
• psychology or neuropsychology
• speech and language therapy
• pain or fatigue support
• rehabilitation services

Many patients benefit from a clear explanation of the diagnosis. Understanding that symptoms are real, recognised and potentially manageable can be an important first step.

Treatment and support may include:

• education about FND
• physiotherapy focused on retraining movement
• occupational therapy to support daily function
• speech and language therapy for speech or swallowing symptoms
• psychological therapy where relevant
• support for sleep, pain, fatigue and mental health
• review of coexisting medical problems

Specialist FND rehabilitation may require a dedicated team. My role as a GP is to review general health, consider possible coexisting conditions and advise on sensible next steps.

When should you seek urgent help?

Some neurological symptoms need urgent assessment.

Please seek urgent medical help through NHS 111, 999 or A&E if you develop:

• sudden weakness on one side of the body
• new facial drooping
• sudden speech difficulty
• severe sudden headache
• loss of consciousness
• new seizures
• chest pain or severe shortness of breath
• rapidly worsening weakness, numbness or walking difficulty
• new bladder or bowel problems with leg weakness or numbness

Do not assume new or severe symptoms are FND, even if you already have a diagnosis. New symptoms should be assessed on their own merits.

More about functional neurological disorder and deficiency

This guide explains the broad relationship between FND, neurological symptoms and treatable deficiencies. If you are trying to understand whether your symptoms may fit a deficiency pattern, you may find it helpful to read our guide to common deficiency-related signs and symptoms.

The main message is balanced: FND is a real diagnosis based on positive clinical features. At the same time, symptoms such as fatigue, tingling, numbness, brain fog or balance problems can have other causes or contributors, including B12 deficiency in the right clinical context.

How Dr Asim Naqvi can help

Patients often come to see me in Reading when they have been living with difficult, overlapping symptoms for some time. Many have symptoms that do not fit neatly into one simple diagnosis.

My approach is to listen carefully, review your history and look at whether treatable causes have been properly considered. This may include reviewing previous blood results, arranging further testing where appropriate and helping you understand what your results mean.

If you have been told you may have FND but have not had a full deficiency assessment, or if you have new symptoms that need review, a private GP consultation can help you decide on sensible next steps.

The aim is to bring clarity, check for treatable contributors and support you with calm, evidence-based advice.