2 October - Case of the Week #158


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Thanks to Geoffrey A. Talmon, M.D., University of Nebraska Medical Center, Omaha, Nebraska (USA), for contributing this case and the discussion. To contribute a Case of the Week, email khpathology@gmail.com with the clinical history and your diagnosis.  Please also email us diagnostic microscopic images (textbook quality) in JPG, GIF or TIFF format (send as attachments, we will shrink if necessary), and include any other images (gross, immunostains, etc.) that may be helpful or interesting.  We will write the discussion (unless you want to), list you as the contributor, and send you $35 (US dollars) by check or PayPal for your time after we post the case. Please only send cases with high quality images and a diagnosis that is somewhat unusual (or a case with unusual features).


Case of the Week #158


Clinical History


A 26-year-old man presented with a soft tissue mass on the palm between the second and third metacarpal heads that originated from the ulnar aspect of the second digital nerve.  The lesion was present since birth and recently began increasing in size.  


The resected nerve had a 2.0 cm fusiform dilation near one end.  The cut surfaces of the dilated portion were vaguely encapsulated, white and laminated with interposed yellow adipose tissue.


Micro images:



What is your diagnosis?



























Lipomatosis of nerve (fibrolipomatous hamartoma)




Lipomatosis of nerve (LN) is a benign lesion consisting of infiltration of nerve bundles by collagen and mature adipose tissue.  It is also known as fibrolipomatous hamartoma, macrodystrophia lipomatosa and neural fibrolipoma.  It was first described in 1952, and is considered a hamartomatous lesion without any syndromic association (Fletcher: Pathology and Genetics of Tumours of Soft Tissue and Bone (WHO, Vol 5), 2002).

Lipomatosis of nerve may be noted at birth, but typically presents at ages 11 to 39 years with no history of antecedent trauma.  For unclear reasons, it has a predilection for the median and less frequently, the ulnar nerves (Histopathology 1994;24:391), although cases have been seen in the toes, cranial nerves and brachial plexus.


It commonly presents as an asymptomatic or gradually enlarging, painless mass.  It may be associated with distal sensory or motor deficits if compression atrophy occurs, and is part of the differential diagnosis for carpal tunnel syndrome.  One-third of cases are associated with macrodactyly of the digit innervated by the affected nerve, a finding that is more common in females.  A case of bilateral LN has been reported (Muscle Nerve 1998;21:656).


The appearance of LN by magnetic resonance is pathognomonic.  T1 weighted images reveal a fatty mass that is evenly distributed between nerve bundles and seen running along individual nerves.  It is often described as having a “coaxial cable-like” appearance on axial scans (Acta Radiol 2003;44:326).


Histologically, the mass is composed of a proliferation of mature adipose tissue and collagen that is confined by the epineurium and separates nerve bundles.  Laminated perineural fibrosis, septation of the nerve fascicles, and pseudo-onion bulb formation are also described as salient features.  Occasionally, metaplastic bone is seen.


The differential diagnosis includes other fatty lesions affecting the nerve, notably intraneural lipomas, traumatic neuromas and diffuse lipomatosis.  Intraneural lipomas are characterized by fatty tissue that displaces nerve bundles, but does not separate them.  Traumatic neuromas have onion bulb formation, but usually lack concentric perineural fibrosis, and have a high T2 signal density on magnetic resonance imaging.  Diffuse lipomatosis is not confined to the epineurium.


As a hamartoma, lipomatosis of nerve has benign behavior.  Treatment is somewhat controversial (J Neurosurg 1998;89:683), ranging from excision (which many consider the mainstay of therapy) to simple division of the transverse carpal ligament to relieve symptoms.  Excision often requires the sacrifice of the involved nerve.  Recurrence due to incomplete resection occurs in 33-60% of cases.


Additional references: Orthopedics 2001;24:836, Histopathology 1994;24:391 



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