7 most common causes
1. Boil (furuncle)
- Pink or red bump on the arm
- May ooze pus from the center
A boil (furuncle) occurs when your hair follicle becomes infected. Bacteria (usually Staphylococcus aureus or "staph") gets in through a cut in your skin.
When on the arm, it might be from trauma or just from clothes rubbing against the skin. Once bacteria is under the skin, it can cause an infection at the root of the hair.
Small boils sometimes get better without any treatment or with warm compresses. Treatment at the doctor’s office may involve cutting and draining the infection. Your doctor may give you an antibiotic cream to apply to the area and/or oral antibiotics.
2. Skin abscess
- Red bump
- Oozing of pus from the skin
A skin abscess is a large pocket of pus that forms just beneath the skin. It is caused by bacteria, which usually gets in through a small cut or scratch.
The body fights the bacteria with white blood cells, killing some of the infected tissue. At the same time, pus forms within the area.
Skin abscesses are painful. A fever is rare but possible.
A small abscess may heal on its own. Most need to be drained or cut open by a doctor to clear out the pus. Usually, the doctor will prescribe antibiotics to help fight the infection.
3. Skin cyst
- Skin-colored, marble to golf ball-sized bump with a central opening (pore)
- Oozing of keratin (a white cheese-like material) from the center
- When inflamed or infected, it becomes red and painful
A cyst is a small sac or lump that’s filled with fluid, air, fat, or other material like keratin. A skin cyst can grow anywhere on the body, just beneath the skin. Most skin cysts on the arm are epidermal inclusion cysts, which contain keratin and have a pore in the center.
When cysts become infected or inflamed they become painful, red, and may ooze pus.
A small cyst does not require treatment. But if it bothers you or interferes with movement, a dermatologist can cut it out by doing a simple surgical procedure called an excision. Cysts need to be treated or they won’t go away.
If the cyst becomes infected, you should see your doctor for treatment. The treatment for an infected cyst is cutting open the cyst and draining the pus. You may also need to take antibiotics.
4. Joint cyst
- Skin-colored bump over the joint (usually the wrist)
- Cyst can be moved around under the skin
- Filled with a jelly-like material
Cysts that develop over joints or tendons, most often in the wrist, are ganglion cysts. They can also form over the joints of the fingers or elbows.
The cysts contain a jelly-like fluid and can be moved underneath the skin with a finger. They may become red and painful if they get infected or inflamed.
Ganglion cysts do not require treatment. But if they become painful or grow large enough that they limit movement of the joint, a hand surgeon can perform a procedure that removes the fluid from the cyst or can cut the cyst out.
- Painful red bump over the elbow or shoulder
- Stiffness in the joint
Bursitis is inflammation of the bursa, the fluid-filled sac that surrounds the joints. It is caused by repetitive movement of the joint or pressure on the joint, such as leaning on the elbows.
Bursitis can be treated with resting the arm, ice, and anti-inflammatory medications like ibuprofen. If it worsens or does not improve, the doctor may remove some of the fluid from the joint or inject a medication in the joint to reduce inflammation. Rarely, surgery is necessary.
- Golf ball sized bump
- Painful to touch
- Moves under the finger
Lipomas are fatty non-cancerous growths of the skin. When there are numerous blood vessels in the growth, it is known as an angiolipoma. The most common location for an angiolipoma is the forearm, but they can also occur on the upper arm.
Angiolipomas are beneath the skin and there is no overlying redness or skin changes that can be seen with the eye. The bump moves under your finger when you touch it. The cause of these fatty growths are unknown, but they often run in families.
Most of the time, treatment is not necessary unless the angiolipoma is causing a lot of pain, you don’t like how it looks or is growing rapidly. It can be removed through surgery by a dermatologist.
7. Squamous cell carcinoma
- Painful red or pink bump
- Bleeds easily
Squamous cell carcinoma is a form of non-melanoma skin cancer. It occurs in areas of sun-damaged skin. The top of the forearm is a common place for this cancer.
Squamous cell carcinoma looks like a pink or red bump that grows over time. It is usually firm and painful to touch. There may be some scale or crust on the bump. Sometimes, it can grow fast and have a central dip (called a keratoacanthoma). Squamous cell carcinomas can start to bleed without any trauma.
Most squamous cell carcinomas don’t spread to other parts of the body, but it can in rare cases. So you need to get them removed. First, a sample of the bump (a biopsy) will be taken by the doctor to confirm the diagnosis. Then they will remove it.
Other possible causes
A number of conditions may also cause painful arm bumps, though these are either rare or painful arm bumps are not usually the defining symptom. They include other cancers and inflammation.
When to call the doctor
If your painful arm bump continues to grow, becomes red and painful, limits motion of the arm, starts to ooze pus, or bleeds, you should make an appointment with your doctor.
Always check with your doctor if there are any movement limitations of the arm while the bump is healing. And here is a fun fact—ganglion cysts, which are cysts over the joints like the wrists, have the nickname “bible cysts.” Back in the day, one of the treatments was to smack the wrist with the bible to compress it. —Dr. Levy
Should I go to the ER for a painful arm bump?
You should go to urgent care or the ER if you have the following:
- Severe difficulty moving the arm
- Severe arm numbness, warmth, or color changes
- High fever
- Severe, sudden, or worsening pain and/or swelling of the area
- Nausea and/or vomiting
- Spreading redness up the arm
Once treated, most of these painful arm bumps do not come back. The one exception would be bursitis—if the same trigger (like repeated movement or leaning on the elbow) occurs again, bursitis could come back. —Dr. Levy
- Rest the arm.
- Apply ice to the area.
- Apply warm compresses if there is drainage from the bump.
- Take NSAIDs (nonsteroidal antiinflammatory drugs) like ibuprofen to help with pain.
Other treatment options
- Cutting open and draining the bump (incision and drainage) may be performed if the bump is infected.
- Oral antibiotics may be given if the bump is infected.
- Removing fluid from the bump, known as an aspiration.
- Inflammatory causes of painful arm bumps can be treated with medications that decrease inflammation.
- Surgery may be needed to remove both benign (non-cancerous) and malignant (cancerous) growths.
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