Xanthomas are rare non-neoplastic lesions that predominantly occur in tendons and synovium. These lesions are characterized by the accumulation of lipid-laden macrophages, giving rise to nodular structures ^[1]. The tendoachilles (TA), extensor tendons of the hand and elbow, and patellar tendons are frequent sites for these nodules, although they can occur in other tendons as well ^[2]. Tendinous xanthomatosis is often associated with hyperlipidemia, particularly familial hypercholesterolemia (FH), a genetic disorder characterized by high cholesterol levels ^[3]. This association is due to the fact that xanthomas are essentially accumulations of cholesterol within macrophages, which are then deposited in tissues such as tendons [1, 3, 4]. Histologically, tendinous xanthomas are complex structures. They contain multinucleated giant cells, histiocytes, and hemosiderin deposits. Histiocytes are a type of immune cell that engulfs foreign substances, and hemosiderin is an iron-storage complex. The presence of these elements gives xanthomas their characteristic appearance and texture ^[1]. Xanthomas of the TA can lead to a variety of complications. They can cause pain, particularly during movement or physical activity. They can also lead to functional impairment, limiting the range of motion and flexibility of the affected tendon. In some cases, the size and location of the xanthomas can cause difficulties with footwear, as the enlarged tendon may not comfortably fit within standard shoes ^[5]. Despite their rarity, xanthomas represent a significant clinical entity due to their potential impact on patient quality of life and their association with underlying metabolic disorders. The exact incidence of Achilles tendon xanthomatosis is not definitively known, but studies suggest that it occurs in approximately 10.9% of patients with elevated cholesterol levels, most commonly associated with FH, where the prevalence is roughly 1 in 500 individuals; meaning a significant portion of FH patients may develop Achilles tendon xanthomas (TX) ^[6]. Therefore, understanding their pathophysiology, clinical presentation, and management strategies is crucial for clinicians dealing with musculoskeletal and metabolic disorders [1, 2, 4]. This case report aims to contribute to this understanding by presenting a unique case of bilateral xanthomatosis of the Achilles tendons.

Patient: A 42-year-old male.

Chief Complaint: Swelling of the distal aspect of both legs posteriorly for 6 years.

History: No trauma, surgery, or other significant medical history.

Physical examination

• Firm, non-tender, non-reducible, and non-mobile masses (Fig. 1)
• No signs of inflammation (skin discoloration)
• Full range of motion around the ankle joint with no tenderness.

Magnetic resonance imaging (MRI) of both ankle joints was done with the below findings: (Fig. 2)

The Achilles tendons appear diffusely bulky with mild PD fat sat hyperintense intrinsic signals within, however, there is no tear noted. It appears predominantly hypointense on T2 as well as T1 imaging. Their calcaneal attachments also show no abnormality. On the right side, the maximum width of the tendon is 2.4 cm and is 6 cm cranial from the calcaneal attachment site. There is no peritendinous fluid or fat stranding seen. On the left side, maximum width of the tendon is seen to be 2.5 cm and is 7 cm cranial from the calcaneal attachment site. The visualized bones show normal marrow signal. The ankle joints appear normal without any effusion. Rest of the visualized ligaments and tendons are normal in course and signal without any evidence of tenosynovitis (Fig. 2). As the MRI findings were suggestive of xanthomatosis of the tendon Achilles, the patient was advised to evaluate their lipid profile to look for underlying hypercholesterolemia or hyperlipidemia. Serum lipid profile showed hypercholesterolemia and hyperlipidemia (Fig. 3).

The patient had hence been diagnosed with hypercholesterolemia and hyperlipidemia. As the patient did not have any pain or restriction of movement at ankle joint, the patient was advised to start treatment for the above with a physician while monitoring for any changes in the swelling.

Xanthomas are harmless, rare, non-cancerous growths that develop due to the buildup of collagen, lipid-filled macrophages, giant cells, and inflammatory cells as a reaction to the deposition of low-density lipoprotein (LDL) in the tissue ^[1]. They can appear on the skin, particularly the eyelids, and less frequently in the tendons and synovium. Tendinous xanthomas typically affect the extensor tendons of the hand and elbow, the Achilles tendon, and the patellar tendon ^[2]. The most common condition associated with muscle xanthomatosis is FH. It has been reported that TX develops in homozygous FH patients starting from childhood, while TX develops in heterozygous FH patients at the age of 20. The proportion of patients with TX increases with age. 75% of people with FH develop the condition as they get older. Most serious diseases associated with tendinous xanthomas are due to defects such as LDL receptor defects ^[1]. Xanthomatosis of the Achilles tendon is uncommon, but early and precise diagnosis is crucial due to the strong correlation between xanthomatosis and primary hyperlipidemia (mainly of types IIa and III) [1,4]. It typically occurs in patients with FH from childhood and develops by the age of 20 in patients with heterozygous hypercholesterolemia ^[4]. It may be an early indicator of coronary artery disease. There have been instances of their development in patients on antiretroviral therapy due to drug-induced hyperlipidemia [1, 2, 4].

The clinical manifestations of Achilles TX primarily depend on the size of the lesions. Smaller lesions are unnoticeable, whereas larger clinically apparent lesions are mostly noticed with cosmetic disfiguration. Patients may also experience localized pain, irritation, and restriction of movements ^[5]. Plain radiography is often the first imaging modality in these cases, and lateral views are used for the evaluation of the anteroposterior diameter of the Achilles tendon. Xanthomatosis appears as an abnormal thickening with non-calcified soft tissue masses. A few studies showed that in patients with hyperlipidemia, the Achilles tendon was thickened with the thickness ranging from 7.5 to 21.5 cm in contrast to the normal thickness which is <0.7 cm in males and <0.6 cm in females [7,8]. Ultrasonography is widely performed for the detection of xanthomatosis in various tendons. The involved tendons show focal hypo-echoic areas or a diffuse heterogeneous echo pattern with loss of normal fibrillar pattern. It is also a practical modality to monitor the treatment response. Computed tomography is usually not considered due to its low contrast within soft tissue lesions [7,8]. MRI depicts morphologic and signal abnormalities in the form of loss of normal flat or concave ventral margin of the Achilles tendon as seen in our case. Previously reported cases showed uniform low signal intensity on MRIs, indicating collagen fibers and intervening high signal intensity areas that could be cholesterol-laden foam cells and/or an inflammatory response. However, more research is required to evaluate the genesis of various magnetic resonance appearances. MRI is also highly sensitive in ruling out early disease in other visualized tendons and in underlying bone and marrow. The involved tendons show higher signal intensity on T1- and T2-weighted spin echo images compared with the normal tendon. In some unusual cases, patients may have necrosis and hemorrhage [7, 8]. The differentials for bulky, enlarged, and disfigured Achilles tendon include xanthoma, tendinopathy, chronic degeneration, tendonitis, peritendinitis, bursitis, trauma, nodules of rheumatic arthritis, tophaceous gout, infection, and neoplasm. However, owing to the clinical history, bilateralism, and imaging findings, most of them were excluded [1-4,9]. TX warrants investigation for underlying lipid disorders, particularly FH due to its prevalence ^[3]. However, rarer conditions such as cerebrotendinous xanthomatosis and sitosterolemia cannot be excluded. A comprehensive workup with lipid profile evaluation, potential genetic testing, and consideration for cataracts, neurological symptoms, or intestinal issues is crucial for accurate diagnosis. Early intervention for the identified etiology, including lifestyle modifications and lipid-lowering medications, is essential to prevent further complications. MRI brain was done. It showed no abnormalities; hence, cerebrotendinous xanthomatosis was ruled out [10-14]. Treatment options could range from conservative treatment to total excision with reconstruction or subtotal/partial resection. Pharmacological treatment for symptomatic Achilles tendon xanthomatosis includes lipid-lowering therapies such as statins, fibrates, and LDL apheresis to reduce cholesterol accumulation, while surgical treatment involves subtotal or total resection of the affected tendon with reconstruction if necessary. Surgery is reserved only for patients having severe disfigurement, pain, or restriction of movements with only partial resection and removal of the lesions to preserve the functions. Our case did not present with restriction of movements [1,5].

To conclude, bilateral xanthomatosis of the Achilles tendon is a rare entity, characterized by a spear-like enlargement of tendons visible on imaging scans. These scans are crucial for diagnosing the condition, especially when accompanied by a pertinent medical history. Imaging becomes vital for a prompt and precise diagnosis. This aids in effective treatment and also serves as a warning to both the patient and doctor about its links to hyperlipidemia, hypercholesterolemia, and heart disease. MRI scans are employed to determine the degree of affliction, plan treatment, and detect early alterations in nearby tendons. Therapeutic approaches vary from non-invasive to surgical removal, with surgery being reserved for cases involving significant esthetic alteration, discomfort, and movement limitations. Our case did not have any restriction of joint movement or pain, hence advised to continue with conservative treatment in the form of observation and treatment for hypercholesterolemia.

Bilateral xanthomatosis of the Achilles tendon, although rare, should be considered in patients presenting with asymptomatic swellings in the distal legs, particularly in those with a history of hyperlipidemia. Clinicians should employ imaging techniques for accurate diagnosis, as early identification of this condition can serve as an important marker for underlying FH. Prompt recognition and management of lipid disorders are essential to prevent potential complications associated with untreated hyperlipidemia.