Anal thrombosis (anal vein thrombosis | perianal thrombosis): a painful lump caused by a blood clot near the anus

Strictly speaking, anal thrombosis should be referred to as anal vein thrombosis or perianal thrombosis, but the common abbreviation among medical professionals is anal thrombosis. 

At the opening of the rectum (medically known as the anal canal) are blood vessels—the veins—through which deoxygenated blood is transported back to the heart.

A thrombus forms in one of these superficial veins in the anal region, obstructing blood flow and quickly leading to a painful lump. This lump usually develops directly at the anus or at the edge of the anus and can grow to the size of a plum. The blood clot causes local swelling that is visible from the outside and easily palpable.

The anal canal contains a fine network of blood vessels as well as the hemorrhoidal cushion. If areas of the vessels become enlarged or blood flow is impaired, a so-called thrombus can develop. Patients often confuse this condition with hemorrhoids, as the symptoms are somewhat similar. Unlike hemorrhoids, however, it is not a change in the mucous membrane or a protrusion of the hemorrhoidal cushion, but rather an acute blood clot in the vein.

Anal thrombosis presents as a swollen, usually bluish lump that can be painful, especially with movement or pressure. Since it tends to develop suddenly, many patients feel anxious. Nevertheless, anal thrombosis is considered a harmless condition that resolves on its own in many cases without extensive treatment.

Anal thrombosis and hemorrhoids have in common that both are conditions affecting the anal canal. Anal thrombosis is often confused with hemorrhoids, but the two are unrelated.

Hemorrhoids are protrusions of the mucous membrane in the anal canal, whereas anal thrombosis involves the formation of blood clots in the veins of the anal canal. If hemorrhoids cause an obstruction to blood flow in the veins of the anal canal, there is a risk of anal thrombosis developing. Conversely, if anal thrombosis leads to bulging and protrusion of the mucous membrane, this can result in the development of hemorrhoids.

Anal thrombosis is caused by a sudden blood clot in a superficial vein in the anal region. This condition often develops when blood flow is disrupted, leading to the formation of clots. Several factors can contribute to its development, including straining during bowel movements, prolonged sitting (especially on hard or cold surfaces), and physical exertion such as heavy lifting. Straining during bowel movements increases the risk, as it raises abdominal pressure and puts greater strain on the blood vessels around the anus.

Hormonal changes, such as those during pregnancy, are also considered contributing factors. Anal thrombosis often occurs after episodes of diarrhea or persistent constipation, as both can alter straining patterns. Spicy foods, high alcohol or coffee consumption, and a warm, humid climate can also be triggers.

If enlarged blood vessels, such as hemorrhoids, are already present in the area, a thrombus can develop more easily. 

A clear distinction is important, as anal thrombosis—unlike hemorrhoids—develops suddenly and resolves within a few days. 

Anal thrombosis (blue dots) © Armin Kübelbeck, CC-BY-SA, Wikimedia Commons

Anal thrombosis is primarily characterized by

• sudden pain at the anus (rectal opening), which makes sitting impossible,
• a severe sensation of pressure or a foreign body in the area of the anal opening,
• bluish lumps at the anus,
• swelling, and
• bleeding (especially when anal thromboses rupture).

In addition, those affected often suffer from

• itching,
• stinging, or
• a burning sensation

in the anus.

The diagnosis is usually made by visual examination of the anal region. The taut, tender, and externally visible lump usually allows for a clear diagnosis. An examination of the anal canal helps rule out other conditions, particularly external hemorrhoids. 

To rule out other causes, a proctological examination or colonoscopy may be advisable once the acute symptoms have subsided.

In contrast to serious thrombotic conditions, such as deep vein thrombosis in the leg, anal thrombosis is considered localized and harmless.

Treatment depends on the symptoms and their severity. In many cases, the condition usually resolves on its own within a few days. Local measures are typically used to relieve symptoms. Ointments or creams containing anti-inflammatory and pain-relieving ingredients, such as diclofenac or ibuprofen, can be applied. For more severe pain, pain relievers in tablet form may also be used. Cooling compresses and sitz baths can further reduce the feeling of tightness.

It is also important to avoid aggravating factors such as straining during bowel movements. Soft stools can help the symptoms subside. 

Minor surgery may be necessary only in cases of severe symptoms or intense pain. Under local anesthesia, the lump can be opened and relieved (incision made by a doctor). Such procedures are brief, low-risk, and quickly relieve symptoms.

Anal thrombosis usually heals without complications within a few days to weeks. In rare cases, however, blood clots can lead to the formation of residual skin, known as marisks, which are harmless. Recurrent episodes are possible if the triggering factors persist. 

Serious complications are rare. Infections, persistent symptoms, or delayed healing may occur in isolated cases. Overall, anal thrombosis is considered an uncomplicated condition with a good prognosis.

How long does anal thrombosis last?

In many cases, anal thrombosis heals within a few days; in cases with more pronounced changes, it usually heals within a few weeks.

Can anal thrombosis recur?

Yes, especially if triggering factors such as straining or prolonged sitting persist. Adjusting one’s habits can reduce the risk of recurrence.

Is anal thrombosis dangerous?

No, unlike other thrombotic conditions affecting internal veins, it is considered harmless and remains localized.

Should anal thrombosis be treated surgically?

Minor surgery is only recommended in cases of severe symptoms or very painful findings. It is usually performed on an outpatient basis and leads to rapid relief of symptoms.