Functional Neurological Disorder
FND involves neurological symptoms (weakness, tremor, seizures, sensory loss) not explained by neurological disease. Diagnosis is based on positive clinical signs, not exclusion alone.
Key Facts
Positive diagnostic signs distinguish FND from neurological disease — diagnosis is NOT one of exclusion Hoover's sign: Involuntary extension of 'weak' leg when flexing contralateral hip — positive sign for functional weakness Prevalence: Second most common reason for neurology outpatient referral (~16% of new neurology referrals) Dissociative (non-epileptic) seizures: Account for ~20-30% of patients referred to epilepsy clinics Diagnosis requires internally inconsistent neurological examination (symptoms not conforming to neuroanatomy) CBT and physiotherapy are the mainstays of treatment — NICE recommends MDT approach Comorbidity with depression (~50%), anxiety (~50%), and childhood adversity (~30-50%) Previously called 'conversion disorder' — term 'functional' is preferred as less stigmatising
Overview
Key Facts
FND is characterised by neurological symptoms that are inconsistent with recognised neurological disease but are genuine and distressing. Modern understanding focuses on abnormal nervous system functioning rather than psychological 'conversion'.
Epidemiology
FND is the second most common reason for new neurology referrals (~16%). Incidence ~4-12 per 100,000/year. F:M ~2-3:1. Peak onset 30-40 years. Dissociative seizures affect ~2-33 per 100,000.
Aetiology
- Predisposing: Childhood adversity, personality traits, prior neurological disease
- Precipitating: Physical injury, illness, psychological stress (but trigger not always identifiable)
- Perpetuating: Illness beliefs, avoidance, deconditioning, iatrogenic factors
Pathophysiology
- Abnormal self-agency: Disruption in sense of voluntary control over movement/sensation
- Predictive coding model: Brain's predictive processing generates symptoms based on erroneous predictions about body state
- fMRI: Altered activation in supplementary motor area, prefrontal cortex, and limbic regions during functional symptoms
- Not simply a manifestation of psychological distress — many patients have no identifiable stressor
Clinical Presentation
Motor Symptoms
- Functional weakness: Often affecting whole limb; Hoover's sign positive; hip abductor sign positive
- Functional tremor: Variable frequency, entrainable (changes with distraction/contralateral movement), increased with attention
- Functional gait disorder: Dramatic, inconsistent patterns (dragging leg, 'walking on ice')
- Functional dystonia: Fixed posture, often inverted ankle
Dissociative (Non-Epileptic) Seizures
- Prolonged events (>2 minutes), fluctuating course, eyes closed (epileptic seizures usually eyes open)
- Pelvic thrusting, side-to-side head movement, asynchronous limb movements
- No post-ictal confusion/raised prolactin (though these are not definitive)
- Can coexist with epilepsy (~10-20%)
Sensory Symptoms
- Non-dermatomal sensory loss, splitting at the midline (anatomically impossible)
- Tubular visual field loss (non-expanding with distance)
Red Flags
- Do NOT assume all atypical neurological presentations are functional — thorough neurological assessment required
- Comorbid epilepsy in ~10-20% of patients with dissociative seizures
- Suicide risk if severe functional impairment
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Multiple sclerosis | Relapsing-remitting neurological deficits, MRI lesions | MRI brain/spine, CSF |
| Epilepsy | Stereotyped events, post-ictal features, EEG abnormalities | EEG, video-EEG telemetry |
| Myasthenia gravis | Fatigable weakness, ptosis, diplopia | Anti-AChR antibodies, EMG |
| Stroke/TIA | Acute onset, vascular territory, risk factors | CT/MRI, CT angiography |
| Peripheral neuropathy | Dermatomal/nerve distribution, objective NCS changes | NCS/EMG |
| Malingering | Intentional symptom production for external gain | Clinical judgment, context |
Diagnosis / Investigation
Bedside
- Neurological examination: Identify positive functional signs
- Hoover's sign: Functional weakness
- Tremor entrainment: Functional tremor
- Dragging monoplegic gait: Functional gait
- Video-EEG telemetry: Gold standard for distinguishing dissociative seizures from epilepsy
Bloods
- Prolactin: May rise post-epileptic seizure (15-20 min) but NOT after dissociative seizure (limited utility)
- Standard bloods to exclude metabolic/toxic causes
Imaging
- MRI brain/spine: To exclude structural neurological disease — often normal in FND
Special Tests
- NCS/EMG: Normal in functional weakness
- EEG: Normal during dissociative seizure (vs epileptiform activity in epilepsy)
Management
Non-pharmacological (First-line)
- Explanation: Clear, compassionate delivery of diagnosis with education about FND mechanism
- Physiotherapy: Consensus-based physiotherapy for functional motor symptoms (the Nielsen et al. approach)
- CBT: Addresses maintaining factors (illness beliefs, avoidance, attention to symptoms)
- Occupational therapy: Functional rehabilitation
- MDT approach: Neurology + psychiatry/psychology + physiotherapy
- Self-help resources: neurosymptoms.org (endorsed by neurology societies)
Pharmacological
- Treat comorbidities: SSRIs for depression/anxiety
- No medication treats core FND symptoms
- Avoid: Antiepileptic drugs for dissociative seizures (unless coexisting epilepsy)
Referral Criteria
- Neurology for diagnosis confirmation and positive clinical signs
- Specialist FND service where available
- Liaison psychiatry if significant psychiatric comorbidity
- Physiotherapy with FND expertise
Prognosis
- Variable: ~50% improve or resolve, ~20% remain the same, ~30% worsen
- Dissociative seizures: ~70% continue to have seizures at 10-year follow-up (many also develop additional functional symptoms)
- Good prognostic factors: Shorter duration, identifiable trigger, acceptance of diagnosis, engagement with treatment
- Poor prognostic factors: Delayed diagnosis, ongoing litigation, comorbid personality disorder, fixed beliefs about organic cause
- Diagnostic delay is associated with worse outcomes — early accurate diagnosis improves prognosis
Other Relevant Information
Positive Signs of FND
| Symptom | Positive Sign |
|---|---|
| Functional leg weakness | Hoover's sign (involuntary extension during contralateral hip flexion) |
| Functional arm weakness | Drift without pronation (organic UMN weakness → pronator drift) |
| Functional tremor | Entrainment (frequency changes with contralateral tapping) |
| Functional sensory loss | Midline splitting, non-dermatomal |
| Dissociative seizures | Eyes closed, prolonged, fluctuating, preserved awareness |