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Fibrolipomatous hamartoma of the proximal ulnar nerve associated with macrodactyly and macrodystrophia lipomatosa as an unusual cause of cubital tunnel syndrome
  1. B-U MEYER,
  1. Department of Neurology, Charité, Virchow Klinikum, Humboldt-Universität zu Berlin, Berlin, Germany
  1. Dr Bernd-Ulrich Meyer, Department of Neurology, Charité, Virchow Klinikum, Augustenburger Platz 1, 13353 Berlin, Germany. Telephone 030-450-60012; fax 030-450-60901.

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Fibrolipomatous nerve tumours are rare benign tumours that are usually slowly progressive and that predominantly affect the median nerve at the level of the wrist and hand, usually causing carpal tunnel syndrome.1 The tumour is sometimes associated with macrodactyly and lipomatous macrodystrophy of muscles and subcutaneous fat in the region supplied by the affected nerve.1 2 In this report we focus attention on fibrolipomatous hamartoma at a previously undescribed location as an unusual cause of cubital tunnel syndrome. Furthermore, we point out that on the basis of the characteristic features of fibrolipomatous hamartoma on MRI, a non-invasive diagnosis can be made.

The 35 year old female patient was born with a giant fourth finger (about twice as thick as the neighbouring fingers) of the right hand which grew proportionally with the other fingers during early childhood. The family history was unremarkable. For cosmetic reasons, the finger was amputated at the age of three. When the patient was 27 years old, she experienced hypaesthesiae and paraesthesiae in the cutaneous area of the right ulnar nerve and a stabbing pain in the ulnar arm. She also noticed a slight weakness in the flexion of the fourth and fifth fingers, of wrist flexion, and flexion of the hand muscles innervated by the ulnar nerve. At the same time she noticed a thickening of the ulnar nerve at and above the elbow, with electrical sensations elicited by nerve percussion. During the next eight years, the symptoms were progressive. Recently, a clinical examination disclosed the sensory deficits mentioned above and weakness of muscles supplied by the ulnar nerve (MRC grade 3). The ulnar side of the forearm and the hypothenar and the fifth finger showed hyperplasia. The flexion of the elbow was restricted by a palpable sausage-like elastic tumour in the sulcus nervi ulnaris and the distal upper arm. There were no naevi, angioma, or neurofibroma.

In nerve conduction studies, the maximal motor conduction velocity over the elbow nerve segment was 44 m/s on the affected and 57 m/s on the non-affected side and the amplitudes of the elicited muscle compound potentials in the abductor digiti minimi muscle were reduced to 0.6 mV. Antidromic sensory nerve action potentials could not be elicited in the right ulnar nerve with stimulation at the wrist or proximal to the elbow. Electromyography disclosed signs of chronic neurogenic changes in all muscles supplied by the right ulnar nerve. Clinically and electrophysiologically the diagnosis of cubital tunnel syndrome was made.

T1 weighted MRI of the right upper arm and elbow (figure A-D) showed a fusiform enlargement of the ulnar nerve, with an extension from 6.5 cm proximal to 2 cm distal of the olecranon. The largest axial diameter of the nerve tumour was 2.3 cm. The coronal section showed serpentiform fibrous components within the nerve (figure A). On the axial section through the upper arm 6 cm proximal to the olecranon, circular structures of fatty and fibrous tissue could be seen within the nerve (figure B). The nerve had its largest diameter within the sulcus nervi ulnaris and mainly consisted of fibrous tissue, with a bulb-like configuration on the axial slice (figure C). At the entrance to the cubital tunnel, the nerve was compressed by the transverse fibres of the arcuate ligament (figure A and D). Here, the nerve seemed pathologically hypointense, indicating a fibrous degeneration of the nerve induced by compression. In T1 weighted MRI of the forearm, the ulnar nerve was surrounded by fatty tissue and had a normal diameter in its course along the forearm. The flexor digitorum profundus and flexor carpi ulnaris were found to be increased in volume and had a high content of intramuscular fat (figure E). In the distal forearm, fatty tissue infiltrated the spaces between the tendons and the muscle bellies. The blood vessels were of normal diameter.

(A) T1 weighted (TR 660 ms/TE 18 ms) coronal MRI through the right elbow which shows the serpentiform course of the fibrous component of the fibrolipomatous hamartoma of the ulnar nerve (white star). (B-D) Axial T1 weighted (TR 660 ms /TE 18 ms, contrast enhanced) sections through the upper arm and elbow. Within the sheaths of the fusiform nerve, circular structures of fatty and fibrous tissue (white arrow in B, 6 cm proximal to the olecranon) and fibrous tissue of bulb-like shape (white arrow in C, within the sulcus nervi ulnaris) can be seen. At the entrance to the cubital tunnel the nerve appears very hypointense, indicating a fibrous degeneration of the nerve induced by compression under the arcuate ligament (white arrow in A and D). (E) T1 weighted (SE, TR 360 ms, TE 15 ms) coronal MRI through the forearm, with enlarged, fat infiltrated flexor digitorum profundus and hypothenar muscle and fat infiltration into the spaces between the tendons and the muscle bellies in the distal forearm.

The patient has a non-hereditary congenital malformation, with a combination of a slowly progressive fibrolipomatous hamartomatous tumour of the ulnar nerve at an unusal location in the elbow region, unusual macrodactyly of a single finger, and lipomatous dystrophy with enlargement of ulnar forearm flexors and hypothenar. The causal relation between fibrolipomatous hamartoma, macrodactyly, and lipomatous macrodystrophy of soft tissues remains obscure. Besides others, a neurogenic cause has been discussed on the basis of findings in neurofibromatosis.3

Fibrolipomatous hamartoma is usually located in the distal median nerve and causes carpal tunnel syndrome.1 2 4 Only one case of fibrolipomatous hamartoma proximal to the elbow, in the brachial plexus, has been described before.5 In our patient, MRI detected an enlarged flexor carpi ulnaris muscle and a thick ulnar nerve as a previously unmentioned combined cause of a clinically relevant ulnar nerve entrapment in the proximal and distal part of the cubital tunnel. The proximal fibro-osseous tunnel is formed by the medial collateral ligament and the distal sulcus nervi ulnaris; the distal part of the tunnel is formed by the humeral and ulnar insertions of the flexor carpi ulnaris muscle and the arcuate ligament. Furthermore, the massively thickened nerve can also be mechanically lesioned within the sulcus nervi ulnaris by flexion-extension movements in the elbow.

The differential diagnosis of a palpable, unilocal fusiform nerve enlargement comprises fibrolipomatous hamartoma, lipomas within the nerve sheath, and segmental or plexiform neurofibromatosis. In our patient, MRI disclosed serpentiform nerve fascicles surrounded and separated by fibrous and fatty tissue within the expanded nerve sheath as typical features of fibrolipomatous hamartoma.2 4Fibrolipomatous hamartoma can clearly be distinguished from lipomas within the nerve sheath, which are characteristic focal masses that dislocate and compress the normal nerve bundles,6 and from segmental and plexiform neurofibromatosis, in which the neurofibroma has MRI signal characteristics of soft tissue and not of fat. Furthermore, in plexiform neurofibromatosis, the tortuous nerve is studded by small tumours.6 7 The unique features of fibrolipomatous hamartoma as identified by MRI , allow the identification of this benign nerve tumour preoperatively. This facilitates the decision to decompress affected nerves at the preferential sites of nerve entrapment and helps to avoid diagnostic nerve biopsy or even resection of an ambiguous nerve tumour.