Can an arachnoid cyst in the right hippocampal region cause seizures in a patient presenting with new‑onset seizures?

Can Arachnoid Cysts in the Right Hippocampal Region Cause Seizures?

Arachnoid cysts located in the temporal/hippocampal region can occasionally cause seizures, but the relationship is weak and inconsistent—most temporal arachnoid cysts are incidental findings unrelated to seizure activity. When evaluating a patient with new-onset seizures and an arachnoid cyst, you should assume the cyst is NOT the cause unless specific high-risk features are present.

Evidence Against a Causal Relationship

The strongest evidence suggests arachnoid cysts are usually incidental findings in epilepsy patients:

• In a retrospective study of 867 epilepsy patients, only 23.5% (4/17) of those with arachnoid cysts had seizure foci adjacent to the cyst location 1. Among 12 patients with temporal lobe cysts, only 3 had temporal lobe seizures 1.

• EEG abnormalities and seizure semiology typically do NOT correspond to arachnoid cyst location 2. In one series, only 1 of 8 patients had EEG findings matching the cyst location 2.

• Most patients with arachnoid cysts and epilepsy are classified as having idiopathic generalized epilepsy syndromes (such as juvenile myoclonic epilepsy or Rolandic epilepsy) rather than symptomatic focal epilepsy 2.

When Arachnoid Cysts MAY Cause Seizures

Despite the generally weak association, certain scenarios suggest a causal role:

• Large cysts causing mass effect with midline shift can present with acute seizure activity, including status epilepticus 3. These require urgent neurosurgical intervention 3.

• Temporal lobe cysts (including those near the hippocampus) that are moderate to large in size have been identified as potential causes of new-onset psychosis and seizures in rare cases 4.

• Sudden symptomatic deterioration in previously asymptomatic cyst patients may indicate the cyst has become clinically relevant 3.

Diagnostic Approach for New-Onset Seizures with Arachnoid Cyst

When encountering a patient with new-onset seizures and an incidentally discovered arachnoid cyst:

1. Obtain MRI brain without and with contrast as the definitive imaging study 4. MRI is superior to CT for characterizing the cyst, identifying alternative epileptogenic lesions (hippocampal sclerosis, cortical dysplasia, tumors), and assessing for mass effect 4.

2. Use high-resolution epilepsy protocol sequences: coronal T1-weighted (3mm) perpendicular to hippocampal long axis, 3D T1-weighted gradient echo (1mm isotropic voxels), and coronal/axial FLAIR to evaluate for hippocampal sclerosis—the most common cause of temporal lobe epilepsy 4.

3. Correlate EEG findings with cyst location 2, 1. If ictal or interictal epileptiform discharges localize to a region distant from the cyst, this strongly suggests the cyst is incidental 1.

4. Assess for mass effect: Look for midline shift, ventricular compression, or surrounding edema that would indicate the cyst is exerting pressure 3.

5. Consider alternative diagnoses aggressively: Hippocampal sclerosis, mesial temporal sclerosis, cortical dysplasia, low-grade tumors, and cavernous malformations are far more common causes of temporal lobe epilepsy than arachnoid cysts 4.

Treatment Algorithm

For seizure control, initiate antiepileptic drugs regardless of cyst presence 4. The choice of antiepileptic should follow standard epilepsy guidelines based on seizure type, not cyst presence 4.

Surgical intervention for the cyst is indicated ONLY if:

• The cyst is large with significant mass effect causing midline shift 3
• Seizures are refractory to multiple antiepileptic medications AND EEG clearly localizes seizure focus to the cyst region 3
• The patient develops acute neurological deterioration or status epilepticus attributed to cyst expansion 3

Endoscopic fenestration can achieve seizure freedom in highly selected cases, but complete seizure control is not guaranteed even after successful cyst decompression 5. Importantly, surgery itself carries risk of causing NEW seizures and cognitive deterioration 6, so the decision must weigh potential benefit against iatrogenic harm 6.

Critical Pitfalls to Avoid

• Do not assume the arachnoid cyst is causing seizures simply because it is present on imaging 2, 1. The burden of proof requires EEG localization matching cyst location and exclusion of other epileptogenic lesions 1.

• Do not rush to surgery for seizure control unless the cyst demonstrates clear mass effect or seizures are truly refractory 6. Medical management should be optimized first 5.

• Do not overlook hippocampal sclerosis or other subtle temporal lobe pathology that may coexist with or be mistaken for an arachnoid cyst 4. Dedicated epilepsy-protocol MRI is essential 4.

• Recognize that cyst fenestration can paradoxically CAUSE epilepsy and cognitive decline in previously asymptomatic patients 6. Presurgical neuropsychological testing should be performed 6.