Fibrolipomatous Hamartoma of the Median Nerve.

We present the case of a 33-year-old woman who presented with a slowly enlarging mass over the volar aspect of the wrist that had been present since infancy which for the previous year had been causing progressive pain. The lesion was proven to be a fibrolipomatous hamartoma, a rare benign tumor that most commonly affects the median nerve. We discuss the characteristic radiologic appearance of this entity that is often pathognomonic and allows a confident diagnosis without the need for biopsy.

Introduction

Although rare, fibrolipomatous hamartomas are an important entity to be familiar with because their imaging findings are pathognomonic and allow for confident radiologic diagnosis.

Case Report

A 33-year-old woman presented with a slowly enlarging, previously painless mass over the volar aspect of the wrist that had been present since infancy. Over the course of the previous year she began complaining of increasing pain in the wrist and hand. Initial radiographs of the wrist were normal. Magnetic resonance imaging (MRI) demonstrated a mass at the level of the carpal tunnel where the palpable abnormality was felt (Figure 1, Figure 2, Figure 3, Figure 4). The MRI demonstrated fusiform enlargement of the median nerve at the level of the carpal tunnel. T1 images shower tubular low-intensity structures representing thickened nerve fascicles with interspersed high signal representing fat which homogeneously dropped signal on fat suppression images. On axial sequences the lesion demonstrated a “coaxial cable-like” appearance while on coronal images it had a “spaghetti string” appearance. Post-contrast images showed no enhancement. This constellation of findings is pathognomonic of fibrolipomatous hamartoma [1].

Figure 1

33-year-old woman with fibrolipomatous hamartoma of the median nerve. Axial T1 weighted pre (A) and post (C) gadolinium (TR 400/TE 13) and T1 fat-suppressed (C) (367/12) fast spin echo (FSE) MRI at the level of the carpal tunnel in location of palpable mass (vitamin E capsule denoted with green arrow). There is a fusiform mass (blue circle) in the expected location of the median nerve that is continuous with the nerve proximally and distally. On the non fat-suppressed images the high signal represents the fatty tissue infiltrating and enlarging the median nerve which drops completely with fat suppression. The fat is interspersed within thickened nerve fascicles which are low intensity. The mass displaces the flexor tendons.

Figure 2

33-year-old woman with fibrolipomatous hamartoma of the median nerve. Axial T2 weighted (3517/49) fat suppressed FSE MRI shows intermediate signal longitudinally oriented cylindrical structures representing the nerve fascicles. There is fusiform enlargement of nerve bundles caused by fatty infiltration.

Figure 3

33-year-old woman with fibrolipomatous hamartoma of the median nerve. Coronal T1 (500/11) FSE MRI shows thickened nerve fascicles with high signal fat interspersed (blue circle). The mass displaces the flexor tendons.

Figure 4

33-year-old woman with fibrolipomatous hamartoma of the median nerve. Sagittal T1 (600/11) FSE MRI shows mass (blue arrow) displacing the flexor retinaculum anteriorly.

Discussion

Fibrolipomatous hamartomas are benign tumors usually affecting infants and less commonly children and young adults. This uncommon lesion is also known as neural fibrolipoma, lipofibromatous hamartoma, perineural lipoma and intraneural lipoma [2, 3]. The median nerve is overwhelmingly the most commonly affected nerve (80% of cases), followed by the ulnar and radial nerves, dorsum of the foot and brachial plexus [4]. The most common presentation is that of a soft, slowly enlarging and often asymptomatic mass on the volar wrist or forearm often present since infancy. Occasionally nerve compression will lead to symptoms of pain, paresthesia or carpal tunnel syndrome. Frequently fibrolipomatous hamartomas are associated with macrodystrophia lipomatosa (20-66% of cases) [3, 4] which when present most commonly affects the second and third digits. There is equal prevalence among males and females. Although the pathogenesis has not been definitively elucidated it is postulated that a congenital abnormality of growth of fibrofatty tissues causes infiltration of the endoneurium, perineurium and epineurium resulting in fusiform nerve enlargement [4, 5].

Fibrolipomatous hamartoma may be imaged using ultrasound, computed tomography, and MRI; however MRI is the preferred modality for diagnosing fibrolipomatous hamartomas. Plain film is non-specific and may be normal or may show soft tissue prominence related to the fibrofatty tissue or osseous overgrowth. Sonographic findings show hyperechoic tissue (echogenic fatty tissue) surrounding smooth round hypo- or anechoic fascicles [5, 6]. Spectral Doppler will demonstrate no flow within the mass. CT and MRI imaging findings of fibrolipomatous hamartomas show fusiform nerve enlargement which is caused by thickening of nerve bundles and fatty and fibrous proliferation. Enlarged nerve bundles look like serpentine or tubular structures on all imaging modalities. The MRI characteristics of fibrolipomatous hamartomas are pathognomonic [1, 2, 3]. There will be fat signal (high intensity) on T1 and T2 weighted sequences which will drop out on fat suppressed images surrounding the cord or spaghetti shaped low T1/low T2 bands of enlarged nerve fascicles and fibrous tissues [7].

Differential diagnostic considerations include vascular malformations, ganglion cysts and nerve sheath tumors however the previously described distinctively characteristic imaging findings should allow confident non-invasive diagnosis.