What is the likelihood that a small subarachnoid hemorrhage will cause seizures?

Likelihood of Seizures After Small Subarachnoid Hemorrhage

The likelihood of seizures after a small subarachnoid hemorrhage ranges from 6% to 18%, with most seizures occurring before medical presentation rather than during hospitalization. 1

Incidence and Timing of Seizures

Seizures following subarachnoid hemorrhage (SAH) can be categorized based on their timing:

• Early seizures (at onset): 7.9% of patients experience seizures at the time of hemorrhage 2
• Perioperative seizures: 6.2% of patients develop seizures during the acute treatment phase 2
• Delayed seizures: 3.1-7% of patients develop late epilepsy 1, 2

More recent retrospective reviews consistently show that the majority of seizures occur before medical presentation, with in-hospital seizures being relatively rare in patients receiving prophylactic anticonvulsants 1.

Risk Factors for Seizures After SAH

Several factors increase the likelihood of seizures following SAH:

Strong Risk Factors:

• Subarachnoid clot burden: Thicker hemorrhage significantly increases seizure risk (OR 2.76) 3
• Subdural hematoma: Presence of subdural hematoma dramatically increases risk (OR 5.67) 3
• Cortical infarction: Independent risk factor for late epilepsy 2

Additional Risk Factors:

• Aneurysm location: Middle cerebral artery aneurysms carry higher risk 1
• Intraparenchymal hematoma: Associated with increased seizure risk 1, 2
• Poor clinical grade: Higher World Federation of Neurosurgical Societies grade (IV-V) 2, 3
• Younger age: Patients under 40 years have higher risk 2
• Acute hydrocephalus: Associated with onset seizures 2
• Rebleeding: Increases seizure risk 2
• History of hypertension: Associated with higher seizure risk 1

Types of Seizures

Seizures after SAH can present in various forms:

• Convulsive seizures: Classic tonic-clonic activity
• Nonconvulsive seizures: Particularly concerning in comatose patients
  • 19% of stuporous or comatose patients may have nonconvulsive seizures (average 18 days after SAH) 1
  • Nonconvulsive status epilepticus has been reported in 8% of patients with unexplained coma or neurological deterioration after SAH 4
  • Nonconvulsive status epilepticus is associated with extremely poor prognosis 1, 4

Clinical Implications

The relationship between seizures and outcome remains uncertain:

• Some studies report no impact on outcome
• Others associate seizures with worse outcomes 1
• Nonconvulsive status epilepticus specifically is a strong predictor of poor outcome 1

Management Considerations

While not directly addressing your question about likelihood, it's worth noting that:

• Prophylactic anticonvulsants may be considered in the immediate post-hemorrhagic period 1
• Routine long-term use of anticonvulsants is not recommended 1
• Long-term anticonvulsant therapy should be considered for patients with specific risk factors (prior seizure, intracerebral hematoma, intractable hypertension, infarction, or middle cerebral artery aneurysm) 1

Pitfalls and Caveats

• Many seizure-like episodes associated with aneurysmal rupture may not be truly epileptic in origin 1
• Continuous EEG monitoring may be necessary to detect nonconvulsive seizures in comatose patients 4
• Anticonvulsant use has been associated with adverse effects in up to 23% of patients 1
• Phenytoin specifically has been associated with worse cognitive outcomes at 3 months 1

In summary, while the overall risk of seizures after small SAH is moderate (6-18%), certain patient and hemorrhage characteristics significantly increase this risk, and clinicians should be particularly vigilant in patients with thick hemorrhage, subdural hematoma, or cortical infarction.