Differentiating FND from Epileptic Seizures
The diagnosis of functional/dissociative seizures (FDS) versus epileptic seizures should be made using video-EEG monitoring as the gold standard, combined with identification of positive clinical signs during both ictal and inter-ictal examination.
Diagnostic Approach
Video-EEG Monitoring (Gold Standard)
Video-EEG long-term monitoring in a seizure monitoring unit allows trained neurologists/epileptologists to reliably differentiate FDS from epileptic seizures 1. This is the definitive diagnostic test and should be pursued when the diagnosis is uncertain.
Clinical Semiological Features During Events
Key distinguishing features of FDS include:
- Event duration: FDS typically last >7 minutes (reported in 27% of FDS cases vs 11% in epilepsy), while syncope presents with very brief events of seconds (27% vs 9% in epilepsy) 2
- Preserved awareness during apparent unresponsiveness: Patients may hear and understand during FDS even when unable to respond 3
- Specific motor patterns: Look for side-to-side head movements, pelvic thrusting, eye closure during events, and asynchronous limb movements 4
- Gradual onset and fluctuating course: FDS often have longer build-up and waxing-waning intensity
- Emotional symptoms: Panic, anxiety, and hyperarousal are frequently reported before and during FDS 5
Important caveat: Brief events (<1 minute) were still reported by 23% of people with FDS, and long seizures (>7 minutes) occurred in almost 10% of epilepsy patients, so duration alone is insufficient 2.
Inter-ictal Physical Examination
This is critically important and often overlooked. 79% of FDS patients demonstrate at least one positive sign of FND on inter-ictal examination, compared to only 16.6% of epilepsy patients 6. More specifically:
- Three or more FND signs: Present in 66.6% of FDS patients vs only 4.1% of epilepsy patients (specificity 83.3%, sensitivity 79.2%) 6
- Positive signs to examine for: Hoover's sign, tremor entrainment, distractibility, variability during functional tasks 3
- Median number of signs: FDS patients typically show 4 positive FND signs 6
Pre-ictal Symptoms and Triggers
- Psychological triggers: FDS onset is more commonly preceded by depression or stress reactions 7
- Warning signs: Contrary to common assumptions, pre-seizure warnings (auras) are similar for focal and generalized epilepsy, so their presence doesn't rule out FDS 2
- Sensory grounding: If patients can abort events using sensory grounding techniques (noticing environmental details, cognitive distractions, sensory-based distractors), this suggests FDS 3
Critical Diagnostic Pitfalls
Comorbidity exists: Some patients have both FDS and epilepsy concurrently. Document each seizure type separately and teach patients to differentiate their events 1
FND is a "rule-in" diagnosis: It requires positive examination signs, not exclusion of epilepsy. Multiple consistent features should be present for high diagnostic certainty 8
Brief training improves accuracy: A 15-minute educational session on FDS semiological features improved clinician accuracy from 77% to 89% in distinguishing FDS from epileptic seizures 4
Age and demographics: FDS patients are significantly younger at onset than FMD patients, more likely to be Black and less likely to be Asian 7
Imaging Considerations
For new-onset seizures, non-contrast CT head is appropriate for emergent evaluation to identify hemorrhage, stroke, mass lesions, or structural abnormalities requiring immediate intervention 9. However, MRI is superior for detecting subtle cortical lesions, temporal lobe pathology, and small structural abnormalities that may cause epilepsy 9.
Management Implications
Once FDS is diagnosed:
- Evidence-based psychotherapy (cognitive behavioral therapy, neurobehavioral therapy) administered by trained seizure counselors 1
- Multidisciplinary rehabilitation including occupational therapy focused on biopsychosocial framework, education, functional activity rehabilitation, and self-management strategies 3
- Avoid antiepileptic drugs unless concurrent epilepsy is documented
The key is performing a thorough inter-ictal neurological examination looking for positive FND signs—this simple step is frequently omitted but provides crucial diagnostic information 6.