Discussion

Intracranial lipomas are typically asymptomatic lesions. However, when symptomatic they usually result in headaches, seizures, psychomotor retardation and/or cranial nerve deficits.4

These lesions are associated with brain malformations in 55% of cases5 and dysgenesis of the corpus callosum is most common. They are presumably of dysembryological rather than neoplastic origin and rare, with incidence estimated at 0.06%–0.46%1 and a high proportion of cases tending to be located in the midline cerebral structures.6 In Truwit et al’s study of 42 patients with intracranial lipomas,1 45% were classified as interhemispheric, 25% quadrigeminal/superior cerebellar and 14% suprasellar/interpeduncular. Yildiz et al’s study of 24 patients similarly observes the interpeduncular region to be the rarest location.7

Fusiform aneurysms are not rare in their association with intracranial lipomas, particularly those that are pericallosal. Eldevik and Gabrielsen8 reviewed 22 cases of pericallosal lipomas in the literature with 19 demonstrating fusiform aneurysmal dilatation on cerebral angiography. In our review of cases reported in English literature (table 1), there were eight cases of saccular aneurysms associated with lipomas, two of which were non-contiguous. Of these, six involved the middle cerebral artery, and four of these involved a lipoma in the sylvian fissure. None involved the posterior cerebral artery.

The embryological pathogenesis of intracranial lipomas may be the result of abnormal persistence and maldifferentiation of the meninx, as theorised by Verga in 19299 and supported by Truwit’s study of 42 patients with intracranial lipomas.1 Lipomas are, therefore, considered maldifferentiated subarachnoid space rather than true neoplasms. They may interfere with the growth of cortical tissue and result in local cortical dysplasia,1 10–12 explaining the high proportion of associated cerebral hypoplasias. Intracranial lipomas have been reported in association with accompanying cortical dysplasia and vascular malformations(ie, dilated abnormal feeder arteries with abnormal venous drainage surrounding the lipoma) that may represent tumour vasculature or cortical dysplasia.10 11 13

As an intracranial lipoma represents maldifferentiated subarachnoid space, it is not unexpected for it to be traversed by neurovasculature.1 Futami et al14 hypothesised several theories for the pathogenesis of a concurrent aneurysm. The development of a congenital lipoma may be concurrent with a congenital structural deficiency within the cerebral arterial wall, contributing to an increased likelihood of aneurysm formation. On the basis of Zervas et al’s15 theory that cerebrovascular smooth muscle relies on the diffusion of nutrients via the cerebrospinal fluid in the subarachnoid space due to the absence of vasa vasorum, it is possible that a contiguous lipoma impedes this process in the adjacent arterial wall.14 Lipomas may also secrete factors that weaken the arterial wall.14 It should be noted that as of yet there has not been a suitable theory presented to link intracranial lipomas to the formation of a non-contiguous aneurysm, although they have been reported.7 16

Our case involved the posterior cerebral artery, a rare site for non-lipoma associated aneurysms,5 which may be explained by variations in vessel size and haemodynamics.17 On the basis that lipomas may contribute to the development of a contiguous aneurysm15 18, the rarity of posterior cerebral aneurysms associated with lipomas may also be attributable to the low incidence of interpeduncular lipomas.

Small intracranial lipomas smaller than 5 mm may be difficult to differentiate from aneurysms in time-of-flight MR angiography and may need correlation with fat-suppressed imaging or CT imaging.19 The possibility of an underlying vascular abnormality should be evaluated given their association, and if doubt persists, further imaging or invasive digital subtraction angiography should be considered. In our case, the appearance of the lesion on MR angiography coupled with CT angiography was sufficient to make a definitive diagnosis.7 High-resolution vessel wall imaging is a promising tool to assess vessel wall enhancement, an emerging biomarker for unstable cerebral aneurysms,20 through ‘black blood’ techniques.18 Current evidence supports its use in ruling out aneurysm instability,21 finding high sensitivity but mixed specificity. While not performed in our case, its use should be considered in such complex cases where the decision to treat or not requires careful consideration. Further novel imaging tools emerging to characterise aneurysms include 7 Tesla MRI, intravascular imaging, and four-dimensional imaging18

Intracranial lipomas rarely justify surgical management given that they are non-malignant, and most cases are asymptomatic. Tahmouresie et al’s review of 21 patients showed postoperative improvement in 23.8% and death in 47.6%.22 In these cases, the substantial risk of operative complications may be owed to adhesions to surrounding structures,23 calcification of the lipoma or surrounding structures,24 and hypervascularisation.25 26 Due to these operative complexities, in cases where an adjacent or encasing lipoma is expected to significantly limit access to an aneurysm for the purpose of clipping and definitive management is indicated, we would recommend an endovascular approach. Biopsy may be considered when the diagnosis is uncertain from imaging alone, and if excision is deemed necessary, it should be limited to partial removal.23

Intracranial aneurysms in the posterior circulation measuring 7–12 mm have a 5-year rupture rate of 14.5%.27 Risk factors for lifetime rupture aside from the size of the aneurysm include current smoking status (OR 3.05), female sex (OR 2.50) and the age at diagnosis of the unruptured intracranial aneurysm being less than 50 years old (OR 7.50).28 In the presence of a lipoma, there is no clear evidence for increased rupture risk—from our review of the literature and including our case, one out of nine cases of saccular aneurysms associated with a lipoma within the anterior circulation ruptured on presentation, and no aneurysms that were contiguous with the lipoma ruptured. The incidence of rupture on follow-up could not be concluded from these cases, and the proportion of aneurysms that present with rupture is not well described in the literature. However, given that the prevailing theories on its pathogenesis may support an ongoing theoretical risk for further wall weakening, it is reasonable to exercise caution by performing vessel wall imaging and following up earlier than usual to establish whether the aneurysm is growing. Similarly, there is insufficient evidence to recommend earlier treatment compared with isolated aneurysms. The decision to treat an aneurysm, whether endovascularly or with microsurgical clipping, is a risk versus benefits decision usually made in conjunction with the patient. In this case, the aneurysm being encased within the lipoma would increase the operative complexity if being accessed by microsurgical means. On one hand, the patient had several predictors of poor endovascular outcome including an age greater than 50 and the location of the aneurysm being in the posterior circulation.27 On the other hand, it is important to acknowledge this patient’s age, the borderline size of the aneurysm and paucity of other risk factors for rupture. Additionally, the lipoma wrapping the aneurysm itself may provide a degree of protection against subarachnoid haemorrhage. On balance, it was deemed reasonable to conservatively manage his aneurysm.