Ainhum

Etiology

The exact etiology of ainhum is unknown. The prevalence of ainhum in individuals of African descent suggests a genetic predisposition to developing an excessive fibrous tissue response in reaction to mechanical or infectious injuries.[3] Pseudoainhum occurs when an underlying cutaneous condition or irritation leads to the development of a fibrotic band.[1][2][4][5] Pseudoainhum can be attributed to amniotic bands, the entanglement of hair or thread around a digit, and the mechanical effects of hyperkeratosis in individuals with palmoplantar keratoderma.[3]

Epidemiology

Ainhum predominantly affects males of African descent, usually between the ages of 30 and 50. The overall prevalence of ainhum in the population ranges from 0.015% to 2.2%, with a male-to-female ratio of 2:1, indicating a higher incidence in males.[5] Although ainhum has a worldwide prevalence, this condition is most commonly observed in African countries. Most case reports of ainhum in White patients originate from Brazil, which is another endemic region, and this occurrence may be associated with interethnic marriages.[1] 

Pathophysiology

True ainhum is considered as a rare idiopathic condition.[2] Patients affected by ainhum present with the development of a fibrotic band around the base of a toe or, less commonly, a finger. Although this condition primarily affects the fifth toe, in approximately 75% of cases, both feet are involved. Distal to the fibrotic band, the digit experiences swelling caused by lymphedema, and over time, there is bone resorption. Auto-amputation of the digit typically occurs after 4 to 6 years. The frequent association between walking barefoot in tropical regions and constricting bands of the feet in rural areas of Africa and South America has led to speculation about the potential impacts of ainhum.[2]

Histopathology

Although biopsies are usually unnecessary for diagnosis, they may reveal dermal fibrosis with longitudinally oriented connective tissue when performed. Electron micrographic findings often resemble those of keloidal tissue.[6]

History and Physical

Clinically, the primary feature of ainhum is the progressive development of a circumscribed fibrotic ring at the base of the toes or, less commonly, the fingers.[1] The most commonly affected location is typically above the fifth toe, and this condition often appears on both sides (bilateral).[2][7] In ainhum, a fibrotic band typically develops within a flexural groove, and the circumferential lesion gradually advances, causing alterations distal to the constriction.

The clinical criteria comprise the following 3 distinct findings in patients with ainhum:[2]

• Soft tissue constriction
• Bulbous distal enlargement
• Distal thinning or lysis of phalangeal bones

Cole identified and categorized ainhum into the following 4 distinct clinical stages:[2][8]

•  A small callus develops and eventually transforms into a circumferential groove or fissure.

• The portion distant to the band becomes bulbous or globular due to impaired lymph and venous drainage, as well as narrowing of arteries and bone.

• Pain develops as the bone undergoes separation, either intraosseously or intra-articularly, resulting in increased mobility of the affected digit.

• Bloodless and spontaneous auto-amputation of the digit occurs.

Evaluation

Ainhum is predominantly diagnosed through clinical evaluation. During the initial presentation, a fissure beneath the toe may appear nonspecific. The diagnosis of the condition is confirmed after the development of the fibrotic ring. Distinguishing ainhum from pseudoainhum is crucial.

Radiography is valuable for assessing the extent of the condition. The progressive features associated with ainhum can also be identified through plain films, ultrasound, computed tomography, or magnetic resonance imaging. Radiographs will initially show a radiolucent band at the base of the toe, followed by swelling. Eventually, osteolysis becomes apparent distal to the band. The bone continues to constrict until autoamputation occurs. 

Treatment / Management

Currently, no consistent treatment approach or targeted therapies have been established for ainhum, as the inciting event is often unidentified. Some attempts have been made using topical and injectable corticosteroids, retinoids, or salicylates to manage early lesions.[2][9] There are reports of a case in which psoriasis caused pseudoainhum, but it was successfully treated with a combination of topical pimecrolimus and low-dose narrowband UVB therapy.[10] However, extrapolating this to idiopathic ainhum appears unlikely, as this particular patient responded to treatment of the underlying causal condition. Early intervention aimed at addressing the constricting rings of fibrous tissue, akin to treating scars and contractures, may be beneficial.[1][2](B3)

Resection of the fibrous band followed by a Z-plasty surgical technique may help prevent progression if the procedure is conducted in the early stages.[1][11] The anticipated outcomes of the procedure involve either surgical amputation after bone separation or auto-amputation.

Differential Diagnosis

Clinically, the primary feature of ainhum is the progressive development of a circumscribed fibrotic ring at the base of the toes or, less commonly, the fingers.[1] The most commonly affected location is typically above the fifth toe, and this condition often appears on both sides (bilateral).[2][7] In ainhum, a fibrotic band typically develops within a flexural groove, and the circumferential lesion gradually advances, causing alterations distal to the constriction.

The clinical criteria comprise the following 3 distinct findings in patients with ainhum:[2]

• Soft tissue constriction
• Bulbous distal enlargement
• Distal thinning or lysis of phalangeal bones

Cole identified and categorized ainhum into the following 4 distinct clinical stages:[2][8]

• A small callus develops and eventually transforms into a circumferential groove or fissure.

• The portion distant to the band becomes bulbous or globular due to impaired lymph and venous drainage, as well as narrowing of arteries and bone.

• Pain develops as the bone undergoes separation, either intraosseously or intra-articularly, resulting in increased mobility of the affected digit.

• Bloodless and spontaneous auto-amputation of the digit occurs.

Pseudoainhum

Pseudoainhum occurs due to an underlying cutaneous disorder or an identifiable trigger.[1][2][4]

Congenital pseudoainhum

Amniotic band syndrome, also known as Streeter dysplasia, is believed to be a consequence of the rupture of amniotic membranes and subsequent constriction of developing fetal tissue. Variable clinical manifestations include constricting rings, digital amputations, or acrosyndactyly. More severe consequences may include anencephaly, lymphedema, and limb amputations.[12]

Acquired pseudoainhum

Acquired pseudoainhum develops either due to a hair or thread acting as a tourniquet around a digit or in association with palmoplantar keratoderma or, occasionally, other dermatoses. Keratodermas that may present with autoamputation include Vohwinkle syndrome, Olmsted syndrome, Mal de Meleda, loricrin keratoderma, keratosis linearis with ichthyosis congenita and sclerosing keratoderma syndrome (KLICK syndrome), and Papillon-Lefèvre syndrome.[13]

Acquired dermatoses and other conditions that can lead to the appearance of constricting bands include leprosy, tertiary syphilis, scleroderma, Raynaud syndrome, syringomyelia, diabetes, psoriasis, neuropathic plica, systemic sclerosis, yaws, spinal cord tumors, liver cirrhosis, and factitious pseudoainhum (from tourniquet application).[1][14]

Prognosis

Ainhum progresses gradually, and spontaneous amputation typically occurs 4 to 6 years after disease onset.[1]

Complications

Complications of ainhum are secondary infection, pain, imbalance, deformity, and psychological effects stemming from physical mutilation and ongoing pain.[2]