Unruptured Posterior Cerebral Artery Aneurysm Presenting with Temporal Lobe Epilepsy

The International League Against Epilepsy (ILAE) proposed a new epilepsy classification scheme in 2013. ¹ The framework, which was formalized in 2017, builds upon several previous iterations while emphasizing the importance of seizure etiology at each level of classification. However, a recent systematic review reported that “epilepsy of unknown origin” continues to be the most prevalent diagnosis. ² The same study found structural/metabolic etiologies account for 40% of cases. ²

Unruptured intracranial aneurysms are often asymptomatic or accompany vague symptoms like headaches; however, they may also cause cranial neuropathies or lead to cerebral ischemia. ³ Giant aneurysms or those that project into cortical surfaces (i.e., the medial temporal lobe) can certainly lead to seizures. Overall, posterior cerebral artery (PCA) aneurysms are uncommon and hence they are very rarely implicated as the cause for seizures.

A search of the MEDLINE and EMBASE databases was performed with the specific Medical Subject Headings (MeSH) and keywords for (epilepsy or seizures) and (PCA and intracranial aneurysms). A total of 14 studies were initially identified. Titles and abstracts were screened; studies that described PCA aneurysms presenting with seizures were included. Of the seven remaining studies, two were excluded on further review. One did not have full text available and the other described a mycotic aneurysm located in the calcarine sulcus in the setting of metastatic choriocarcinoma. ^4,5 A summary of the cases is presented in Table 1.

Table 1: Summary of cases with PCA aneurysms presenting with seizures.

Here, we report a patient who experienced stereotypic spells consistent with temporal lobe epilepsy which was caused by an unruptured PCA aneurysm.

A 59-year-old left-handed man with a history of dyslipidemia and hypertension presented with episodes consistent with focal aware seizures. The patient described that these events began with a sense of panic and feeling like a curtain was coming down upon him with associated cognitive slowing as if he was wading through water. Soon thereafter, he would develop severe nausea with no emesis. These experiences lasted several minutes without loss of awareness, followed by an immediate return to his cognitive baseline. These events occurred two to three times per week with no identifiable triggers. His neurological examination did not yield focal deficits.

The patient was started on carbamazepine 200 mg BID empirically and referred to an epileptologist. A computed tomography angiogram and magnetic resonance imaging revealed a 7 mm aneurysm arising from the P2 segment of the left PCA that was embedded in the medial aspect of the left parahippocampal gyrus with increased signal suggesting inflammation or edema (Figure 1). A cerebral angiogram confirmed the presence of the superolaterally projecting saccular aneurysm (Figure 1).

Figure 1: Findings of diagnostic imaging. CTA slices in axial (A), coronal (B), and sagittal (C) views suggest a left PCA aneurysm which protrudes into the left parahippocampal gyrus. MRI slices in axial (D), sagittal (E), and coronal (F) views similarly suggest a left PCA aneurysm projecting into the left parahippocampal gyrus. Cerebral angiography demonstrating a left PCA aneurysm in coronal (G) and sagittal (H) views.

After considering endovascular and open surgical management options, as well as a conservative approach, microsurgical clip repair through a left subtemporal approach was recommended. This decision was based on the aneurysm size, distal location of the aneurysm, significant proximal vascular tortuosity, and small parent artery diameter. Furthermore, the surgical approach would also allow for resection of any local abnormal tissue. The patient underwent neuropsychological evaluation which suggested left hemisphere language dominance and low risk of significant cognitive decline with the proposed lesionectomy without hippocampal resection. An electroencephalogram showed moderate left temporal dysrhythmia seen maximal over the mid to posterior temporal derivations with a variable centroparietal field. These findings indicated focal cerebral dysfunction arising from the area most indicative of an underlying structural abnormality.

The surgery proceeded as planned with a left subtemporal craniotomy, clipping of the aneurysm base, and resection of gliotic tissue surrounding the aneurysm dome (Supplementary Figure 1). There was no evidence of adjacent hemosiderin deposition. The postoperative imaging confirmed satisfactory clipping, and the patient remains seizure free 5 months after surgery. He remains on carbamazepine which will be reassessed on his 6-month follow-up.

This case contributes to the literature describing PCA aneurysms as an epilepsy etiology. A review of the literature identified five cases of patients with PCA aneurysms presenting with seizures (Table 1). ^6–10 The median age was 30, with range between 7 weeks to 35 years. Unique to the present case, the patient is the oldest reported at 59 years of age. Males were more common in this series (4 males, 1 females), and aneurysms were more frequently present in the right hemisphere (3 right, 2 left). Notably, seizure onset was focal in four cases. ^7–10 Of these, awareness was described in two patients as being impaired and in one patient as intact.

Casey and Moore described a patient manifesting with generalized seizure. ⁶ This case was unique in that the 24-year-old male was believed to have developed the aneurysm following a closed head injury. The remaining cases do not identify an explicit etiology, this being consistent with common understanding that cerebral aneurysms are often multifactorial acquired lesions. The etiology of the seizures could be due to several causes including gliosis secondary to subclinical hemorrhage or mass effect and impingement into the adjacent medial temporal cortex.

Two of the five cases identified in the literature underwent endovascular treatment of their aneurysms. ^8,10 Posterior circulation aneurysms are often treated with an endovascular approach due to the challenging surgical exposure and the close proximity of other deep brain structures. Notably, outcomes in these cases were largely comparable to those treated with surgery, with no described follow-up morbidity or mortality in either group. ^6,7,9 One benefit to choosing microsurgical clipping is the direct visualization of the surrounding cortex allowing for concurrent lesionectomy. However, the choice of treatment modality ought to consider factors including patient characteristics, aneurysm morphology and location, and physician expertise. ¹¹

This case report adds to the limited literature describing unruptured PCA aneurysms presenting with seizures. This patient was significantly older than previous cases, and he experienced focal aware seizures consistent with temporal lobe epilepsy for some time before seeking medical care. His seizures have not recurred 5-month post-clipping and lesionectomy. Almost all previous cases also presented with focal aware seizures and their seizures were reduced post-intervention. Both endovascular and surgical therapies have demonstrated similar efficacy with no clear morbidity or mortality advantage to either modality in this small group of patients.