Intraneural lipomas: institutional and literature review

Background: Adipose lesions of nerve can be envisioned as a spectrum ranging from intraneural/extraneural lipomas to lipomatosis of nerve (LN). We have noticed that intraneural lipomas are not as a homogenous group as previously thought and demonstrate differences which have clinical implications. To better understand intraneural lipomas, we conducted a search of cases at our institution and published cases in the world's literature.

Materials and methods: Mayo Clinic's database was searched between years 1994-2018. Published cases were identified using PubMed and Google Scholar databases. Following terms were used: intraneural lipoma, lipoma and nerve, lipoma and neuropathy, lipofibroma and nerve, fibrolipoma and nerve and neural lipoma as well as lipofibroma and fibrolipoma alone. Cases that could be clearly identified as intraneural lipomas by the location of the lipoma within the epineurium were included for analysis. These cases were then sub-classified as encapsulated intraneural lipomas or hybrid intraneural lipomas (demonstrating features of both intraneural/extraneural lipomas and LN) based on their characteristics.

Results: We identified 12 cases at our institution (8 encapsulated, 4 hybrid) and 24 published cases (21 encapsulated, 3 hybrid). The most commonly affected nerve was median both at our institution and in the published cases. Encapsulated cases were found to be relatively easy to resect. Hybrid cases demonstrated variable degree of interdigitating fat between the fascicles and were relatively difficult to resect.

Conclusion: Intraneural lipomas exist as two separate entities with distinct clinical implications. Although rare, this should be taken in account when planning surgery. Terminology should be clarified to prevent ambiguity and confusion.