Summary A 14 year old girl on multidrug treatment for borderline tuberculoid leprosy presented with a swelling in her left arm and soon thereafter developed ulnar claw hand. MRI showed a well-defined ovoid lesion arising from the left ulnar nerve, isointense to muscle on T1W images and hyperintense

on T2W and STIR images. On post gadolinium T1W sequence the lesion showed peripheral rim enhancement with central necrosis suggestive of abscess. The ulnar nerve proximal and distal to the lesion was thickened and showed mild contrast enhancement. On aspiration the swelling yielded frank pus which was positive for acid-fast bacilli.

Introduction

Leprosy is a chronic granulomatous infection caused by Mycobacterium leprae, which involves superficial structures, especially skin and peripheral nerves. The clinical manifestations of leprosy form a continuum extending from polar tuberculoid to polar lepromatous leprosy. In all forms of leprosy peripheral nerve involvement is a common feature. The generally indolent course of leprosy may be interrupted by two types of reaction--erythema nodosum leprosum (ENL) and reversal reaction. ENL is characterised by the appearance of tender, inflamed subcutaneous nodules, fever and malaise. Reversal reaction is characterised by inflammation in previous skin lesions, appearance of new skin lesion and neuritis. (1) Nerve abscess may occur in various forms of leprosy (1) due to caseation of nerve fasciculi leading to cold abscess formation. Although any cutaneous nerve may be involved, ulnar nerve is most commonly affected. (1) In this report we present the MRI appearance of ulnar nerve abscess in a patient with leprosy.

Case report

A 14 year old girl presented with two hypopigmented macules on the face and dorsal aspect of left hand with loss of sensation over these lesions. On clinical examination the peripheral nerves were not thickened. A slit skin smear examination revealed presence of acid fast bacilli. She was diagnosed to have borderline tuberculoid leprosy and she was put on multibacillary multidrug therapy. During the eleventh month of treatment, she noticed a small nodule on the medial aspect of her left arm. It was a well defined subcutaneous nodule, mildly tender on palpation. There was no fever, joint pain or visual symptoms. Over a period of 2 weeks, the patient also developed ulnar claw hand and complained of paraesthesia, so clinically possibility of ulnar nerve abscess and reversal reaction were considered. Ultrasound examination showed a well-defined, cystic lesion with internal debris (Figure 1).

Necrotic supratrochlear lymph node and ulnar nerve abscess were suggested as the diagnostic possibilities. Aspirate from the swelling was positive for acid-fast bacilli. MRI was done to establish the relationship of swelling to the ulnar nerve.

The following sequences were done; T1W TSE(TE = 18, TR = 520) in axial and sagittal planes, T2W TSE(TE = 104, TR = 4080) in axial plane, STIR(TI = 130, TR = 5000) in sagittal plane, T2W TSE(TE = 57, TR = 4470) in sagittal plane with fat saturation and post gadolinium T1W TSE(TE = 18, TR = 520) in axial and sagittal planes, using Siemens Sonata 1.5 T MRI scanner (Erlangen, Germany).

MRI showed a well-defined ovoid lesion in the course of the left ulnar nerve at midarm level (Figure 2).

The sagittal sequences delineated the continuity of the lesion with the thickened nerve. The lesion was isointense to muscle on T1W images. The wall of the lesion was slightly hyperintense to muscle. On fat saturated T2W sequences the lesion was hyperintense. The nerve above and below the level of lesion was thickened and showed hyperintensity on T2W and STIR images. On post gadolinium T1W sequences, the lesion showed peripheral rim enhancement of the wall with central non-enhancing region suggestive of necrosis. The thickened nerve both proximal and distal to the lesion also showed mild enhancement.

Patient continued on multidrug therapy and steroids were added based on the clinical findings. Prednisolone 30 mg daily was initially given for 2 weeks and it was reduced to 20 mg daily for 1 month and gradually tapered. As the subcutaneous swelling had become fluctuant during follow up, it was aspirated, leading to a significant decrease in its size. At the 2 month follow up visit, there was significant improvement in the ulnar claw hand.

Discussion

Leprosy is a unique infectious disease with a prolonged incubation period and a predilection for skin and nerves. The involvement of nerves can be caused by the primary infection and by immunologically mediated reactions. Reversal reaction is one of them.

Although the contribution of diagnostic imaging has been limited in leprosy, the improved soft-tissue definition afforded by MRI may be useful in evaluating neural involvement. Descriptions of MRI features of peripheral nerve involvement in leprosy are sparse in literature. MRI may show diffuse oedema and swelling of the involved nerve due to neuritis. However these findings are quite non-specific and the differential diagnosis includes other hypertrophic neuropathies like Refsum's disease, amyloid infiltration, chronic relapsing polyneuritis and Guillain-Barre syndrome. (2) Presence of nodules or nerve sheath granulomas is suggestive of leprosy. (3) Martinoli et al. (4) analysed peripheral nerves with ultrasonography and magnetic resonance imaging in leprosy and classified leprosy nerves into three groups based on imaging appearance; group I consisted of normal appearing nerves, group II included enlarged nerves with fascicular abnormalities and group III included nerves with absent fascicular structures. They found Doppler US and MRI to have sensitivity of 74% and 92% respectively in identifying active reversal reactions, based on detection of endoneural colour flow signals, increased T2 signal and Gadolinium enhancement. Jebaraj et al. (5) described the role of ultrasound and MRI in a case of tender neuropathy due to leprosy. Ultrasound showed well defined cord-like hypoechoic lesion along the left common peroneal nerve. On MRI the peroneal nerve was enlarged and was isointense to muscle on T1W image and had high signal on STIR sequence. They concluded that MRI may exclude nerve abscess in cases of tender neuropathy. Recently Dawe et al. (6) have described MRI appearance of median nerve abscess in a patient of borderline tuberculoid leprosy. The patient while on treatment for leprosy developed a tender nodule over the right thenar eminence. MRI of wrist and hand showed abnormal thickening with heterogeneous increased signal intensity within the median nerve with an area of reduced T1W signal intensity at the centre of the nodule suggestive of fluid nidus consistent with nerve abscess.

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The differential diagnoses in the present case included ulnar nerve abscess, peripheral nerve tumor and reversal reaction. MRI appearance of well defined ovoid lesion with peripheral rim enhancement and central necrosis favours nerve abscess. Differentiation of ulnar nerve abscess from reversal reaction is important as reversal reaction can be managed conservatively with steroids whereas ulnar nerve abscess may need surgical decompression. (5)

In summary, the MRI can accurately diagnose nerve abscess. MRI features of ulnar nerve abscess are similar to abscess elsewhere in the body. Post gadolinium sequences are helpful in suggesting the diagnosis. In our case sagittal sequence was most useful in establishing the relationship of the lesion to the ulnar nerve.

Accepted for publication 8 September 2006

References

(1) Robert H Gelber. Leprosy. In: FauciAS, Braunwald KJ, and Iselbacher KJ (eds). Harrison's Principles of Internal Medicine, 15th edn. McGraw Hill, New York, 2001, pp. 1035-1040.

(2) Beneditti PF, Anderson MW, Masseli R, Rogero GW. Hypertrophic peripheral neuropathy due to leprosy. J Comput Assist Tomography, 1994; 18: 995-996.

(3) Robert JW, Leonard JZ, Michel AB et al. In musculoskeletal case of the day. AJR, 1999; 173: 795-803.

(4) Martinoli C, Derchi LE, Bertolotto M et al. US and MR imaging of peripheral nerves in leprosy. Skeletal Radiol, 2000; 29: 142-150.

(5) Jebaraj I, Rao A, Shyamkumar NK et al. Imaging of tender neuropathy in leprosy. Ind J Lepr, 2005; 77: 51-54.

(6) Dawe S, Lockwood DNJ, Creamer D. A case of post partum borderline tuberculoid leprosy complicated by a median nerve abscess, peptic ulceration and rifampicin induced hemolytic renal failure. Lepr Rev, 2004; 75: 181-187.

SMRITI HARI, SUBRAMANIAN SUBRAMANIAN &

RAJU SHARMA

Department of Radiodiagnosis, All India Institute of Medical

Sciences, Ansari Nagar, New Delhi 110029, India

Correspondence to: Raju Sharma, Additional Professor, Department of Radiodiagnosis, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India (e-mail: raju152@yahoo.com)