Lipomatosis of Nerve
Lipomatosis of nerve, also described in the literature as fibrolipomatous hamartoma of nerve (FLHN), lipofibromatous hamartoma, or neural fibrolipoma/intraneural lipoma, is a rare, benign congenital malformation. It is characterized by hamartomatous overgrowth of mature fat and fibrous tissue that infiltrates directly within the outer sheath (epineurium) and the sheath around each nerve bundle (perineurium) of a peripheral nerve trunk.
Unlike a solitary lipoma lying outside the nerve sheath (extraneural lipoma), which can be shelled out easily, lipomatosis of nerve is an inseparable interweaving of fat, fibrous collagen bands, and nerve fascicles. It overwhelmingly affects the median nerve (more than 80% of cases), causing slowly progressive carpal tunnel syndrome (CTS). Notably, 30–60% of cases are accompanied by macrodystrophia lipomatosa (macrodactyly), an overgrowth of the fingers supplied by the affected nerve.
At Haute-Derma, our diagnosis and management of lipomatosis of nerve put preserving nerve function above everything else. We combine 3.0T MRI to identify the "coaxial cable" sign, nerve conduction studies (NCV/EMG), and microsurgical decompression to relieve pressure without damaging the vital nerve fascicles.
Clinical & Pathological Features
Causes & Disease Mechanism
- Nature of the Lesion (Hamartomatous Proliferation):
- This is not a true neoplasm but a congenital hamartoma.
- Mature fat and fibrous connective tissue build up abnormally during embryonic development, producing a spindle-shaped (fusiform) enlargement along the length of the nerve.
- How Mechanical Compression Develops:
- As fat and fibrous tissue overgrow within the epineurium, the nerve trunk swells to 2–5 times its normal volume.
- Where the nerve passes through tight anatomical spaces (especially the carpal tunnel beneath the flexor retinaculum), pressure inside the sheath rises sharply and directly compresses the fascicles. This leads to microvascular ischemia, breakdown of the myelin sheath, and impaired nerve conduction.
Clinical Features
The condition usually appears in childhood or adolescence and progresses very slowly over many years:
- Spindle-Shaped Fatty Swelling Under the Skin:
- A soft or slightly firm swelling on the palm side of the wrist, in the palm, or on the forearm.
- The mass follows the course of the median nerve and moves slightly from side to side but not up and down.
- Nerve Compression (Carpal Tunnel Syndrome):
- Numbness, tingling, pins and needles, or aching in the thumb, index finger, middle finger, and the thumb-side half of the ring finger (the area supplied by the median nerve).
- Pain tends to worsen at night or when the wrist is bent for long periods.
- Late stage: Wasting of the thumb muscles (thenar atrophy), weaker thumb opposition, and loss of fine sensation in the fingertips.
- Associated Finger Overgrowth (Macrodystrophia Lipomatosa / Macrodactyly):
- Overgrowth of both soft tissue and bone in the thumb, index, and middle fingers makes them abnormally large from early childhood.
Diagnostic Imaging Signs (MRI & Ultrasound: the Gold Standard)
High-resolution MRI can confirm the diagnosis without an invasive biopsy:
- "Coaxial Cable" Sign on axial (cross-sectional) T1/T2 MRI: the enlarged nerve contains small round low-signal dots (the fascicles), each evenly surrounded by fat that is bright on T1-weighted images.
- "Spaghetti-Like" Sign on sagittal or coronal MRI: nerve fascicles run in parallel strands, alternating with bands of fat along the length of the nerve.
- High-Resolution Ultrasound: A fusiform, enlarged nerve containing hypoechoic bands (fascicles) set in bright, hyperechoic fat.
Microscopic Features (Histopathology)
- Intraneural Fibrofatty Infiltration: Mature fat interlaced with dense collagen bands diffusely infiltrates and widens the epineurium and perineurium.
- Preserved Fascicular Architecture: The fascicles are surrounded and separated by fibrofatty tissue, but in the early stages the axons and Schwann cells inside them keep their normal microscopic structure.
- Concentric Perineural Fibrosis ("Onion-Skin" Appearance): Layers of fibrous tissue build up in rings around each fascicle.
Differential Diagnosis & Immunohistochemistry (IHC)
| Condition | Distinguishing Microscopic / Imaging / IHC Features |
|---|---|
| Lipomatosis of nerve (FLHN) | Fusiform nerve enlargement; coaxial cable / spaghetti sign on MRI; fat infiltrating within the nerve sheath; S100 (+) (fat cells/Schwann cells); EMA (+) (perineurium). |
| Intraneural lipoma | An encapsulated fatty mass inside the epineurium that pushes the fascicles to one side (rather than weaving between them as in FLHN); shells out easily. |
| Schwannoma | Benign nerve sheath tumor; Antoni A/B areas and Verocay bodies; strong, diffuse S100 (+); can be dissected free of the nerve fascicles. |
| Neurofibroma | Infiltrative tumor that enlarges the nerve, but made of interwoven Schwann cells, fibroblasts, and mast cells (no mature fat); patchy S100 (+). |
| Well-differentiated liposarcoma (WDLPS) | Malignant fatty tumor; atypical lipoblasts; MDM2/CDK4 amplification (+) on FISH/IHC; extremely rare as a primary tumor within a nerve. |
Our Approach to Treating Lipomatosis of Nerve
At Haute-Derma, we manage lipomatosis of nerve on the principle of Maximizing Nerve Function & Nerve-Sparing Microsurgical Decompression:
- No Radical Excision: Removing the entire fatty mass would mean cutting through the median nerve, leaving permanent loss of hand movement (thenar muscle wasting) and sensation.
- Noninvasive Imaging to Confirm the Diagnosis: We rely on 3.0T MRI and electromyography (EMG) rather than needle or open biopsy, to avoid adding iatrogenic injury to fascicles that are already compressed.
- Early Decompression at the First Signs of Compression: Carpal tunnel release combined with opening of the outer nerve sheath (epineurotomy) protects as many surviving axons as possible.
How We Diagnose Lipomatosis of Nerve at Haute-Derma
- History & Hand Sensory/Motor Assessment: Semmes-Weinstein monofilament testing of fingertip sensory thresholds, thumb opposition strength testing, and Tinel's and Phalen's signs at the wrist.
- Dedicated 3.0T MRI of the Hand and Wrist: Pinpoints the location and length of the affected nerve segment, confirms the "coaxial cable" sign, and assesses the degree of compression in the carpal tunnel.
- Nerve Conduction Studies & Electromyography (NCV/EMG): Measures the compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) amplitudes of the median nerve to gauge axonal damage and conduction block.
- High-Resolution Vascular & Nerve Ultrasound: Measures the cross-sectional area (CSA) of the median nerve at the wrist and forearm to track progression over time.
Specialized Treatments at Haute-Derma
- Microsurgical Carpal Tunnel Release & Epineurotomy:
- Indication: The gold-standard procedure when a patient has carpal tunnel syndrome or reduced nerve conduction.
- Technique: Under a high-magnification operating microscope, the surgeon divides the transverse carpal ligament (flexor retinaculum) to fully release pressure in the carpal tunnel. The thickened, enlarged epineurium is then carefully opened lengthwise to decompress each fascicle directly without touching the axons.
- Debulking & Finger Reconstruction (for Associated Macrodactyly): Working with a hand plastic surgeon, we reduce overgrown fat under the skin, reshape bone, or stop growth plate activity (epiphysiodesis) in enlarged fingers to improve grip and appearance.
- Medical Treatment to Support Nerve Recovery: High-dose neurotrophic nutrients (methylcobalamin; vitamins B1, B6, and B12), alpha-lipoic acid, and neuropathic pain medicines (pregabalin or gabapentin) to ease numbness and tingling and support myelin repair.
- Resection & Nerve Grafting (Very Limited Indications): Reserved for nerves that have lost all function (complete sensory loss and severely wasted, paralyzed thumb muscles beyond recovery) where the fatty swelling is so large that it causes serious pain and deformity. The damaged nerve segment is removed and replaced with a sensory nerve graft (sural nerve graft), or thumb function is restored with a tendon transfer.
