No signs are pathognomonic for psychogenic seizures, but the bedside differentiation is important because people with psychogenic seizures are often exposed to potentially harmful anti-epileptic treatments.
These signs only apply to patients with generalized convulsions. We are looking for clues of volition, inconsistent with a generalized seizure:
Responsiveness – a patient with psychogenic seizures can sometimes be suggestible enough to cooperate with the exam during the event. For example, I once held a bedside teaching session during a psychogenic event, explained the importance of tongue biting, then asked the patient to stick out the tongue so we could see whether she was biting her tongue. She stuck the tongue out and earnestly moved her lips away to demonstrate tongue-biting. Most cases are harder to diagnose.
Eyes closed – During an epileptic seizure the eyes are usually open. During a psychogenic seizure they tend to be shut, often forcefully.
Nonrhythmic motor movements – During an epileptic seizure, motor movements go through a certain progression, the classic being tonic then clonic. These are rhythmic and symmetrical. A psychogenic seizure is variable, intermittent, sometimes nonrhythmic, with an emphasis on pelvic thrusting (Elvis sign). Side to side movements are generally seen only with psychogenic seizures.
Vocalization – Before an epileptic seizure there might be stereotyped vocalization or a loud pitched cry, but this should not happen during the generalized convulsion. In psychogenic seizures, there might be vocalizations of crying and moaning during the convulsion.
Immediate return of consciousness – In psychogenic seizures we see immediate arousal. Postictal confusion indicates an epileptic seizure.
Specific signs of epileptic convulsion include cyanosis and stertorous (snoring) breathing. The patient with a seizure does not breathe during the event. The physician can feel under the nostril for signs of breathing in the patient suspected of having a psychogenic seizure. The back of the hand is sensitive for feeling a breath. Epileptic seizures often occur during sleep, whereas psychogenic seizures presumably would not.
None of these signs are considered 100% sensitive and specific. But put together, the diagnosis can probably be made with a reasonable degree of certainty.