FLH is a rare, slow-growing, and benign peripheral nerve tumor. The origin of FLH is still obscure, and it is more common in adolescents, which indicates that there may be a congenital cause, although there are a few cases reported in the elderly9. This disease was first reported by Mason in 1953 and was first described as having a painless and rare benign slow-growing lesion10. Subsequently, in 1964, Mikhail published 2 cases of diffuse fibrous fat overgrowth involving the median nerve fatty tissue11. In the same year, Yeoman reported 3 cases of patients with fatty infiltration of the median nerve and pointed out that the tight junction of the intraneural fibrous fatty tissues prevented the extraction of the swelling12. Johnson and Bonfiglio reviewed the literature in 1969, and presented their cases, and gave a detailed histological description of the lesion, which was officially named as FLH13.
In our research on limited literature reports, we found that the median nerve and its branches were the main involved parts, followed by the radial nerve, ulnar nerve, dorsal pedal nerve, brachial plexus, and cranial nerve; the involved parts in the median nerve were usually located in the wrist or hand, and rarely in the elbow6. This disease is more common in young people under the age of 30, including several cases of infants and young children reported by some scholars as a congenital disease12–14, the ratio of male to female is about 2:114, with more left hands than right hands. The main symptoms of clinical manifestations were gradually enlarged painless masses, and pain, numbness, paresthesia and carpal tunnel syndrome usually appeared in the later stage15,16. One-third of FLH patients had related macrodactyly symptoms in the innervation area of the affected area, which is called "seborrheic lipodystrophy", which was first described by Feriz in 192517. For patients with hand hypertrophy or macrodactyly, especially if the patient has signs or symptoms of nerve entrapment, FLH should be considered in the diagnosis18. In the case we reported, the patient was accompanied by painless masses, macrodactyly, and paresthesia and numbness along the median nerve of the hand, which mainly affected the ulnar side of the index finger and the radial side of the middle finger.
Neural fibrolipoma is an orange-yellow, fusiform, sausage like or ropelike enlargement of the nerve19,20. As seen in the case of this study, the appearance of the nerve was hypertrophic and variant, and it showed a fusiform change, the color of the outer membrane turned white, the color of the outer membrane from the distal end to the trunk along the nerve to the nerve thenar branch returned to light yellow, but still hypertrophic.
X-ray is not sensitive to FLH detection, and the X-ray film may show normal or thickening of soft tissue, which is not diagnosable21. Due to the special features of B-scan ultrasonography and magnetic resonance imaging of FLH, the B-scan ultrasonography shows that the hypoechoic sound beam is surrounded by the hyperechoic fibrous fat structure, showing a typical "snakeskin-like" change. MRI shows fat with high signal strength on T1-weighted and T2-weighted images, a characteristic coaxial cable appearance on axial images, and a "spaghetti-like" appearance on sagittal images21,22. Transverse sections may show the ‘lotus sign’ that occurs due to the thin hypointense septa within the fat tissue that separates the fat bundles. This is consistent with the characteristic pathological findings seen in FLH of the thickened perineurium due to perineural fibrosis with septation of nerve fascicles 5,23. Therefore, visualization of the coronal and axial characteristics obviates the need for a biopsy to make the diagnosis of FLH24–26. However, it should be differentiated from traumatic neuroma, neurofibroma, neuroma, schwannoma, ganglion cyst, lipoma, hereditary hypertrophic interstitial neuritis, vascular malformation and the like during diagnosis27,28.
In limited literature reports, patients with FLH of median nerve basically choose to undergo mass resection and pathological examination. Gross pathological specimens show that the involved nerve is thickened in spindle shape, with yellow fibrous fatty tissue infiltration, which is generally confined to the nerve sheath. Histology shows that there is a mixed infiltration of mature fatty tissue and fibrous tissue in and around the involved nerve sheath, which separates the nerve bundles, one of its remarkable features is that the fibrous tissue around the nerve is concentric29.Other changes in the involved nerve include the formation of the separated micro nerve bundles and the pseudo-onion skin-like structure, similar to the intraneural peripheral nerve tumor, as shown in this case; occasionally osseous metaplasia.
Jose A. Plaza29 once mentioned that there were 3 cases of FLH of median nerve whose pathological sections showed perineurioma-like changes, as seen in this case report. Because the pathological results cannot guide the clinic well, very few hospitals will continue to perform immunohistochemistry of pathological sections. Jenna-Lynn Senger4 once reported a case of bilateral acute carpal tunnel syndrome in a 3-year-old child, of which the pathological immunohistochemistry showed: CD34(+), S-100(+), vimentin(+), epithelial membrane antigen(-), desmin(-), glial fibrillary acidic protein antibodies(-). Jose A. Plaza29proposed S-100 (+), EMA (+ -), fibrous cells (-).
There is no clear and best treatment recommendation in the limited number of cases of FLH reported in the literature. Therefore, the current clinical treatment is mainly based on the presence or absence of symptoms and the severity of symptoms to determine the treatment plan. It is very important to understand the degree of nerve involvement of FLH for the preoperative plan. Prophylactic CTR is the main treatment for the disease. It has been clinically proven that CTR can relieve most of the symptoms of patients with carpal tunnel symptoms. Most scholars advocate early carpal tunnel decompression and fibrous lipid sheath decompression, anatomical and microsurgical resection is usually reserved for those patients who have undergone carpal tunnel decompression but still have progressive and disabling median nerve damage. However, this method may lead to permanent loss of motor and sensory functions, and the effect is not obvious3. Some scholars believe that the progressive worsening of neurological symptoms after decompression may be related to the direct compression of individual nerve bundles by massive fibrosis around the nerve rather than the compression of the median nerve in the carpal tunnel. Therefore, some scholars have achieved satisfactory functional results through large-scale resection of diseased tissue and sural nerve transplantation30–33.
In summary, the onset age of FLH is usually under the age of 30, and often at birth. while in this case, the patient was attacked by FLH at the age of 41-year-old, showing his rarity,the clinical symptoms of FLH of median nerve mainly include gradually enlarged painless mass, macrodactyly, pain, numbness, paresthesia and carpal tunnel syndrome, and the diagnosis mainly depends on B-scan ultrasonography and magnetic resonance inspection. The pathological sections of FLH sometimes show perineurioma-like changes, which need to be identified with clinical and MRI features. The treatment methods include CTR and nerve transplantation after the whole nerve resection. CTR has been proven to be helpful for many patients suffering from carpal tunnel symptoms and can relieve most of the symptoms. It is the main method of treatment of the disease, and the cause of the disease needs to be further explored.