Fig. 28.1
MRI left elbow – T1w (a), T2w (b) and T1 post-gadolinium (c, d). A 10 mm, nodular, minimally enhancing mass is seen to arise from the left posterior interosseous nerve (white arrow), just distal to the radiocapitellar joint, as it courses between the brachioradialis and the supinator. The mass appears homogenous and hypointense on T1 and T2 weighted images (red arrow)
The left PIN was explored and the peripheral nerve surgeon noted the possibility of ‘hypertrophic neuropathy’ at the level of the supinator.
Posterior interosseous nerve biopsy revealed findings consistent with an intraneural perineurioma (A representative biopsy of a median nerve perineurioma is illustrated in Fig. 28.2).
Fig. 28.2
Representative biopsy of a median nerve perineurioma is illustrated. A haematoxylin and eosin stained longitudinal section through the nerve shows an expansile lesion at one end (a) comprising cytologically bland spindle cells proliferating and expanding the fascicle, effacing the axons. Myelinated nerve fibres (arrows) are seen coursing along the fascicle at the end distant to the lesion (b). Epithelial membrane antigen, a marker of perineurial cells strongly labels the spindle cell proliferation (c) whereas occasional surviving axons are highlighted by immunolabeling for neurofilaments (d). A resin semi-thin transverse section through the lesion stained with MBA-BF reveals tightly packed ‘onion bulb’ formations of concentric layers of proliferating perineurial cells. Some of these structures contain myelinated fibres at their centre (arrows) (e).
Diagnosis
Left posterior interosseous intraneural perineurioma.
Discussion
The initial consideration in this case is the differential diagnosis of a subacute, painless, posterior interosseous mononeuropathy.
PIN lesions can be classified into non-compressive and compressive. In terms of the former, inflammatory processes should be considered. These include nerve ischaemia or infarction due to vasculitis though isolated, painless PIN involvement over this extended time period would be unusual. In addition there are no systemic features and blood tests are normal. Brachial neuritis may present with an isolated mononeuropathy however the history here is not in keeping with that diagnosis.
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