Lipomatosis of Nerve

Lipomatosis of nerve, formerly known as fibrolipomatous hamartoma, is the anomalous growth of fibrofatty tissue within a nerve, resulting in fusiform enlargement with or without associated nerve-territory oriented osseous overgrowth and proliferation of subcutaneous fat (1/3-2/3 of cases). Histological changes in the nerves are identical to those seen in cases of macrodystrophia lipomatosa, and the world health organization lists macrodystrophia lipomatosa as a synonym for lipomatosis of nerve. Other synonyms include: lipofibroma, fibrolipomatosis, intraneural lipoma, perineural lipoma, median nerve lipoma, and neural fibrolipoma.

While commonly first noted at birth or in early childhood, patients may not present for treatment until adulthood. Women and girls are more likely to have associated macrodactyly. Patients usually present with a gradually enlarging mass, with or without associated with motor or sensory deficits.

Lipomatosis of nerve most commonly affects the median nerve and its digital branches, followed by the ulnar nerves; however, it can affect any other nerve, including cranial nerves and the brachial plexus.

MRI findings are usually pathognomonic: Evenly distributed fat tissue splaying nerve fascicles, resulting in fusiform enlargement of the nerve. No significant edema is typically present within the nerve. The amount of fat varies and can be barely detectable.

In the face of pathognomonic imaging findings, biopsy of a major peripheral nerve in not needed, as it can result in neurologic deficit. Lipomatosis of nerve also cannot be surgically excised for the same reason; however, in the case of disease limited to cutaneous nerves the functional loss will be negligible. Decompression is helpful is tight spaces such as the median nerve in the carpal tunnel.

The images above show a case of lipomatosis of the median nerve, just deep to the plamaris longus tendon. The patient was treated with carpal tunnel release without significant improvement in symptoms.

References

• Nielsen GP. Lipomatosis of nerve. in Pathology and Genetics of Tumours of Soft Tissue and Bone. Fletcher CDM, Unni KK, Mertens F (eds). IARCPress Lyon, 2002. pp 24-25.
• Wong BZ, Amrami KK, Wenger DE, Dyck PJ, Scheithauer BW, Spinner RJ. Lipomatosis of the sciatic nerve: typical and atypical MRI features. Skeletal Radiol. 2006 Mar;35(3):180-4.