What is the next best investigation for a patient with a sudden onset tonic-clonic seizure, confusion, and no previous seizure history, with normal electrolytes and blood tests?

Next Investigation for New-Onset Tonic-Clonic Seizure

Brain imaging (CT or MRI) is the next investigation that should be performed for this patient with a sudden onset tonic-clonic seizure, confusion, and amnesia for the event (Option C).

Rationale for Brain Imaging

According to the American College of Radiology (ACR) Appropriateness Criteria for seizures and epilepsy, brain imaging is essential in the evaluation of new-onset seizures to identify potential structural causes 1. The guidelines specifically state:

• MRI of the brain without IV contrast is usually appropriate in the assessment of new-onset seizures unrelated to trauma
• In emergent situations, a noncontrast CT of the head may be a more appropriate initial choice 1

Key Factors Supporting Brain Imaging First:

• Unprovoked First Seizure: This patient has experienced a first unprovoked seizure with no history of previous episodes
• Post-ictal Confusion: The patient's confusion and amnesia for the event are classic post-ictal symptoms
• No Obvious Metabolic Cause: Normal electrolytes and blood tests rule out common metabolic causes
• No Infection History: The patient has no history of recent infection that might suggest an infectious etiology

Imaging Considerations

CT vs. MRI:

• CT Head: Appropriate for emergency evaluation to rapidly identify acute pathology such as hemorrhage, stroke, mass lesions, or hydrocephalus 1

  • Advantages: Quick, readily available, can detect most acute pathologies
  • Limitations: Less sensitive for detecting small cortical lesions, particularly in orbitofrontal and medial temporal regions 1

• MRI Head: Superior for detailed evaluation of brain parenchyma

  • Advantages: Higher sensitivity for detecting focal lesions (success rate of 70% vs. 30% for CT) 1
  • Should be considered if CT is negative but clinical suspicion for structural abnormality remains high

Alternative Investigations and Their Role

EEG (Option B):

• While EEG is important in the evaluation of seizures, it is typically performed after brain imaging in adults with new-onset seizures
• EEG helps classify the seizure type and determine epilepsy syndrome but is less critical for identifying the immediate cause of a first seizure 1

ECG (Option A):

• ECG would be prioritized if there were features suggesting a cardiac cause (e.g., syncope rather than seizure, cardiac history)
• Not the first-line investigation for a clear tonic-clonic seizure with post-ictal confusion 2

Lumbar Puncture (Option D):

• Reserved for cases where infection or subarachnoid hemorrhage is suspected
• Indicated when there are signs of meningeal irritation, fever, or after normal CT if subarachnoid hemorrhage is suspected 2
• Not indicated in this patient without fever or signs of infection 1

Clinical Pearls and Pitfalls

• Pitfall: Assuming a first seizure is idiopathic without ruling out structural causes
• Pitfall: Delaying brain imaging in favor of EEG may miss potentially life-threatening structural lesions
• Pearl: Up to 34% of patients with a normal neurological examination may have a focal lesion on CT 1
• Pearl: The incidence of structural lesions (stroke and tumors) increases with age 1

Conclusion

For this patient with a first unprovoked tonic-clonic seizure, brain imaging (CT or MRI) is the most appropriate next investigation to identify potential structural causes before proceeding with other diagnostic tests such as EEG.