Serum Calcium After Tonic-Clonic Seizure
Serum calcium is elevated—not decreased—immediately after a tonic-clonic seizure, with post-ictal calcium levels significantly higher than inter-ictal levels measured 4 weeks after the seizure.
Post-Ictal Calcium Changes
The most direct evidence comes from a prospective study of 100 patients with idiopathic generalized tonic-clonic seizures, which demonstrated that post-ictal serum calcium levels (measured within 24 hours of seizure) were significantly higher compared to inter-ictal levels (measured 4 weeks after seizure) 1. This same study found that CSF calcium levels followed the same pattern, with significantly elevated post-ictal calcium compared to inter-ictal measurements 1.
Mechanism and Clinical Context
The elevation in serum calcium immediately after seizure represents an acute physiological response to the seizure event itself 1. This finding is counterintuitive because:
- Hypocalcemia is a well-recognized cause of seizures, not a consequence 2, 3
- Low calcium triggers seizures through increased neuromuscular irritability and lowered seizure threshold 2, 4
- Post-ictal hypercalcemia likely reflects acute stress response and cellular calcium flux during the seizure 1
Important Clinical Distinctions
When Calcium Causes Seizures (Pre-Ictal)
Hypocalcemia can trigger seizures at any age, particularly in patients with underlying parathyroid dysfunction such as 22q11.2 deletion syndrome 2, 4. Seizures may be the first manifestation of hypocalcemia, especially in patients with cancer, renal failure, or malnutrition 2, 3.
Laboratory Testing Strategy
Routine calcium measurement is not indicated in otherwise healthy adults with new-onset seizure who have returned to baseline neurologic status 2. The 2004 ACEP guidelines found that prospective studies showed very low yield for calcium testing, with only rare cases of unsuspected hypocalcemia (1 case in 247 patients in one study, 2 cases with known risk factors in another) 2.
Calcium should be measured when:
- Patient has altered mental status, fever, or new focal neurologic deficit 2
- Known risk factors exist: renal insufficiency, malnutrition, malignancy, prior neck surgery, or genetic syndromes 2
- Patient has not returned to baseline neurologic status 2
Magnesium Relationship
Post-ictal serum magnesium levels were significantly lower (not higher) compared to inter-ictal levels, showing an inverse relationship to calcium 1. Low CSF magnesium correlated with increased seizure frequency, poor control, and longer duration of epilepsy 1.
Clinical Pitfall
Do not confuse the post-ictal elevation in calcium with the pre-ictal hypocalcemia that can cause seizures. If you measure calcium immediately after a seizure and find it elevated or normal, this does not exclude hypocalcemia as the original trigger—you must repeat measurement after the acute post-ictal period (ideally 4+ weeks later) to assess baseline calcium status 1.