Podoconiosis is a disease of the lymph vessels of the lower extremities that is caused by chronic exposure to irritant soils. It is the second most common cause of tropical lymphedema after filariasis, and is characterized by prominent swelling of the lower extremities, which leads to disfigurement and disability.
Signs and symptoms
The primary symptom of podoconiosis is swelling and disfigurement of the lower extremities. The swelling can either be soft and fluid or hard and fibrotic. Multiple firm nodules may develop over time, as well as hyperkeratotic papillomata that resemble moss, which has led to the disease's alternate name of Mossy Foot. The edema of podoconiosis is usually bilateral and asymmetric. Prior to development of lymphatic failure and frank lymphedema, a prodrome consisting of itching, burning, hyperkeratosis, plantar edema, and rigid digits may occur. As with other forms of tropical lymphedema, chronic disease can lead to fusion of the toes, ulceration, and bacterial superinfection. The disease has an acute component, and sufferers may experience recurrent episodes of lower extremity warmth, firmness, and pain.
After parasitic filariae were discovered to be a cause of tropical lymphedema in the 19th century, early investigators assumed that filariae were the sole cause of the disease. It was later discovered that the distribution of tropical lymphedema and filaria did not perfectly overlap, and researchers began to recognize that some forms of tropical lymphedema were not associated with filariasis. Ernest W. Price, a British surgeon living in Ethiopia, discovered the true etiology of podoconiosis in the 1970s and 1980s by studying the lymph nodes and vessels of those afflicted with the disease. Using light microscopy, Price discovered macrophage cells laden with micro-particles in lymph nodes of the affected extremity. Then, examining the same tissue using electron microscopy, he was able to identify the presence of silicon, aluminum, and other soil metals both in the phagosomes of macrophages and adhered to the surface of lymphocytes. Price demonstrated that the lymphatic vessels of these patients experienced subendothelial edema and eventual collagenization of the lumen leading to complete blockage.
Podoconiosis is most frequently seen in the highland areas of Africa, India, and Central America. The highest prevalence is seen in Uganda, Tanzania, Kenya, Rwanda, Burundi, Sudan, and Ethiopia. In some areas of Ethiopia, the prevalence is as high as 9%. The incidence of podoconiosis increases with age, likely due to cumulative exposure to irritant soil. It is very rare to see podoconiosis in the 0–5 year old age group, and the incidence rapidly rises from 6 to 20 years of age, with the highest prevalence after 45 years of age. Podoconiosis is most commonly seen in higher altitude areas with volcanic soil, and it is estimated to affect 4 million people worldwide.
The pathophysiology of podoconiosis is a combination of an uncharacterized genetic susceptibility and a cumulative exposure to irritant soil. In susceptible individuals, irritant soil particles are absorbed through the feet and collect in lymphatic vessels and nodes. Over time, subendothelial edema occurs within the lymphatic vessels and collagenization of the lumen leads to complete blockage. The genetic susceptibility to podoconiosis has not been elucidated, and it has alternatively been suggested to be autosomal recessive or autosomal co-dominant.
The differential diagnosis for podoconiosis includes other causes of tropical lymphedema, such as filariasis or leprosy, and mycetoma pedis. Podoconiosis begins almost exclusively in the foot, as opposed to filariasis, where the initial edema can appear anywhere in the lower extremities. Podoconiosis is usually asymmetrically bilateral, whereas filariasis and mycetoma are usually unilateral. Additionally, groin involvement with podoconiosis is extremely rare and is usually indicative of filariasis.
If a clinical distinction between podoconiosis and filariasis can not be made based on history and examination alone, blood smears and ELISA antigen testing can be useful to screen for filariasis.
The disfigurement associated with podoconiosis can include soft or firm edema, and in later stages firm nodules and a mossy appearance, whereas mycetoma is characterized by firm nodules and edema, usually without the mossy appearance of podoconiosis. Additionally, the edema of podoconiosis is typically more striking and extends more proximally than the edema of mycetoma. Radiology can help distinguish between podoconiosis and mycetoma if the diagnosis is questionable.
Local epidemiology can also be a clue to diagnosis, as podoconiosis is typically found in higher altitude areas with volcanic soild, whereas mycetoma is found along the "mycetoma belt" between latitudes 15 south and 30 north, and filariasis is uncommon at higher altitudes and other environments in which the mosquito vector is less prevalent.
Podoconiosis can be distinguished from leprosy by the preservation of sensation in the affected limb and the isolation of disease to the lower extremities.
Prevention and treatment
The cornerstone of prevention and treatment of podoconiosis is avoidance of exposure to irritant soils. Wearing shoes in the presence of irritant soils is the primary method of exposure reduction. In Rwanda, a country of high disease prevalence, the government has banned walking barefoot in public, in order to curtail podoconiosis and other soil-borne diseases.
Once the disease has developed, rigorous foot hygiene including daily washing with soap and water, application of an emollient, and nightly elevation of the affected extremity has been shown to reduce swelling and disability. Compression wrapping and decongestive physiotherapy of the affected extremity has been shown to be effective in other forms of lymphedema, but the benefits of these therapies have not been rigorously studied in podoconiosis. Nodules will not resolve with these conservative measures, although surgical removal of the nodules can be performed.
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