Neuroimaging Spectrum of Intracranial Lipomas

Background: Intracranial lipomas are rare congenital malformations at characteristic sites. Though largely asymptomatic, some cause significant morbidity. We are studying this for the local population. Objectives: The purpose of this article is to analyze the spectrum of intracranial lipomas, determine their distribution at characteristic locations, and assess their associations. Method: A retrospective study of 21 patients diagnosed with intracranial lipomas detected on CT and MRI performed between September 2017 and May 2022 at Dr. D.Y. Patil Medical College, Hospital & Research Center, Pune. Results: Amongst the 21 patients with intracranial lipomas, the most common location was the pericallosal region seen in 11 patients (n = 11, 52.3%), of which nine were curvilinear type (n = 9), more common than tubular nodular type (n = 2). Other locations (n = 10, 47.7%) were found to be; quadrigeminal cistern in six patients (28.5%), cranial diploic space in two patients (9.5%), one each in interhemispheric falx (4.8%), and cribriform plate (4.8%). Associated anomalies were observed in three patients, which were fronto-ethmoidal encephalocele (n = 1), partial agenesis of the corpus callosum (n = 2), extension with a frontal subcutaneous lipoma (n = 1), and bilateral intraventricular extension (n = 1). Presenting symptoms were headache and dizziness (38.1%), seizures (19%), swelling on the forehead (9.5%), and other non-specific clinical symptoms (33.3%). Conclusion: Intracranial lipomas are rare congenital lesions that are usually asymptomatic and discovered incidentally. However, lipomas at interhemispheric locations can cause seizure disorders and some present with headaches and dizziness. Few have associations like communication with extracranial swellings and corpus callosum agenesis.


Introduction
The term intracranial lipomas are used more specifically to describe foci of adipose tissue seen within the intracranial cavity and CSF spaces [1].
There are a few studies that have assessed the prevalence and location, but these are done for the western population. Few rare case reports of intracranial lipomas [2] have been reported in India, but a systematic review to assess their prevalence was lacking. Previous autopsy series have shown intracranial lipomas to be very rare lesions. But in a study by J. Gossner, a small retrospective analysis of 50 patients who were undergoing CT for various reasons, nine patients were incidentally found to have small intracranial lipomas. Thus intracranial lipomas may be a more commoner finding lacking clinical relevance [3]. In this study, we want to assess the spectrum of intracranial lipomas, study their clinical presentations, and determine their distribution at characteristic locations in the local population.
Intracranial lipomas are not tumors but are thought to be malformations arising from the differentiation of the aberrant embryological meninx primitiva (the predecessor of the subarachnoid space). This explains why intracranial lipomas are seen in the subarachnoid space and why they have parenchymal abnormalities and traversing structures [4]. Most intracranial lipomas are asymptomatic, but they can present with symptoms like headache and seizures [5,6].
Pericallosal lipomas are divided into tubulonodular and curvilinear types. They are associated with agenesis of the corpus callosum in 50% of cases. Quadrigeminal lipomas are linked to the underdevelopment of the inferior colliculus. On CT, these appear as non-enhancing masses with fat density. It conforms to adjacent anatomy and has a soft lobulated appearance. Peripheral calcification may be present.
On MRI, they show high signal intensity on T1 and T2, low signal intensity on fat-saturated sequences, and no enhancement on T1 C+ (Gd), and can produce blooming on SWI due to susceptibility artifact. Data collection: Clinical information, including patient age, clinical symptoms, location of intracranial lipoma, and associated intracranial anomaly, were accessed from the hospital picture archiving and communication system (PACS).

Materials And Methods
Statistical analysis: Categorical variables were expressed as numbers and percentages, and results were presented in tables. Table 1 shows that of the 21 patients studied, eight were male (38.1%), and 13 were female (61.9%). The age at diagnosis ranged from three years to 76 years.

Sex Number
Male 8

TABLE 1: Sex distribution
Eleven patients (52.3%) had pericallosal lipomas, curvilinear type (n = 9) being more common than tubular nodular type (n = 2). The rest of the 10 patients had lipomas at other regions, quadrigeminal cistern lipomas being the most common (28.5%), followed by cranial diploic space (9.5%), interhemispheric falx (4.8%), and cribriform plate (4.8%). One case of curvilinear pericallosal lipoma was associated with the fronto-ethmoid encephalocele. Both cases of tubulo-nodular pericallosal lipomas were associated with partial agenesis of the corpus callosum. One case showed an extension with a frontal subcutaneous lipoma, and the other had a bilateral intraventricular extension. The location, frequency of occurrence, and associated anomalies are summarized in Table 2.    Figure 1 shows a case of tubulonodular pericallosal lipoma with associated anomalies. It showed partial agenesis of the corpus callosum. There is the communication of the intracranial lipoma with frontal subcutaneous lipoma. Lipoma appears bright on both T1WI and T2WI and is suppressed on T1FS images. On Diffusion tensor tractography, white matter tracts can be seen traversing through the lipoma (depicted by the black arrow in Figure 1D).

Discussion
Intracranial lipomas are uncommon congenital lesions [2]. They are a rare finding, found between 0.08% and 0.46% of the time in an autopsy series [3]. Though several theories have tried to explain their cause, the most favorable one is the concept of the subarachnoid space precursor (meninx primitiva), which demonstrates that lipomas arise when a persistent abnormal focus of the meninx primitiva is induced to differentiate into adipose tissue and mature into a lipoma. Lipomas are thus malformations rather than tumors. The meninx primitiva contains primitive perivascular reticuloendothelial, which becomes specialized in storing fat. This also explains the intralesional position of arteries and nerves, the cisternal, subarachnoid location of intracranial lipomas, and the absence of other mesodermal descendants such as muscle [4]. In our study, this explains why, on diffusion tensor tractography, white matter pathways can be seen passing through the lipoma ( Figure 1D).
Typically, intracranial lipomas are discovered incidentally. They are usually asymptomatic. However, they can be symptomatic, with epilepsy being the most common finding. By the age of 15, they develop epilepsy, which is nearly always partial. The pathophysiology of seizures appears to be interhemispheric disconnection [5]. Dementia, an elevated intracranial pressure, and hemiparesis are less frequent symptoms.
A biopsy is not required as CT and MR findings are characteristic of intracranial lipomas.
Limitation: As this is a retrospective study and only patients who underwent CT or MRI were considered, it may not reflect the true incidence of intracranial lipomas and their health burden. As intracranial lipomas are usually asymptomatic and many patients have not undergone CT or MRI, many cases go undetected. As the study was done in a single institution, it represents the local population of the Pune area and may not reflect the wider national population.
Intracranial lipomas are rare congenital lesions that are usually asymptomatic and incidental findings. However, lipomas at interhemispheric locations can cause seizure disorders, and some can present with headaches and dizziness. It is important to look for other associated anomalies as their detection significantly impacts patient management, as demonstrated in the case of intracranial extension of a frontal lipoma with tubulonodular corpus callosal lipoma. Other associations are also seen, like an intraventricular extension of lipomas and corpus callosum agenesis.