Teaching Case

We present the case of an 8-year-old boy who underwent an above-knee amputation after chemotherapy for an osteosarcoma of the right distal femur. The patient was asymptomatic and had routine follow-up MRIs of the lower extremities at three and six months after amputation to rule out local tumour recurrence.

MRI (Avanto Siemens 1.5T) revealed a tubular hyperintense structure seen on STIR images, extending from the transection site to the level of the proximal femoral metaphysis, corresponding to a thickened right sciatic nerve. The enlargement was more significant distally and tapered proximally. No thickening was seen on the contralateral side and no focal enhancement of the nerve was noted on contrast-enhanced images. The sciatic nerve hypertrophy was absent on the preoperative MRI (Fig. 1a-b), appeared on the 3-month follow-up MRI (Fig. 2a-b) and seemed to have increased on the 6-month MRI (Fig. 3a-b). No evidence of tumour recurrence was observed.

Paradoxic sciatic nerve hypertrophy may be observed after leg amputation particularly in young patients [1] but also in adults [2]. It is considered paradoxic because one would expect the nerve proximal to the amputation site to become atrophic as demonstrated in investigations of nerve reactions to axotomy in humans and in animals [3]. The physiopathology of paradoxic hypertrophy is still not well understood due to the lack of histological evidence. One of the hypotheses suggested in the literature is that nerve enlargement could be due to an alteration in the normal cellular response in which neurofilaments continue to be synthesized and transported after nerve transection leading to their accumulation [2]. Osteosarcomas are known to synthesize trophic factors such as an insulin-like growth factor, which may play a role in nerve enlargement [4]. According to the literature, nerve hypertrophy seems to increase in relation with the time elapsed since amputation as observed in our patient [3]. A differential diagnosis to mention is a stump neuroma which is rather a more focal nodular enhancing mass at the transection site. In our patient the nerve was enlarged globally and there was no focal distal enhancing mass of the nerve.
It is important to recognize a paradoxic sciatic nerve hypertrophy and to distinguish it from neuroma or local tumoral recurrence, to avoid unnecessary biopsy. The patients with this pathology are asymptomatic and do not require treatment [3].