A mucoid cyst is a common, non-cancerous, fluid-filled sac that typically develops near the joints of fingers or toes. These growths are also known as digital mucous cysts or myxoid cysts. They are generally benign.
Characteristics of Mucoid Cysts
Mucoid cysts usually appear as small, dome-shaped lumps on the back of the fingers, most often near the fingernail at the last joint, known as the distal interphalangeal (DIP) joint. They can also affect the toes. These cysts range in size, typically from 5 millimeters to 1 centimeter across, and may feel firm or soft. Their surface often appears smooth and shiny.
The fluid inside a mucoid cyst is thick and gelatinous. Mucoid cysts are connected to the underlying joint capsule by a stalk, allowing synovial fluid from the joint to leak into the cyst. This connection explains why they form near joints and can fluctuate in size.
Causes and Associated Symptoms
Mucoid cyst formation is often linked to the degeneration of the joint capsule or tendon sheath, which allows synovial fluid to escape and accumulate. This process is frequently associated with osteoarthritis, a condition where cartilage cushioning the bones breaks down. As cartilage wears away, bone spurs (osteophytes) can develop, further irritating surrounding tissues and weakening the joint lining, leading to cyst formation. Studies indicate that between 64% and 93% of individuals with mucoid cysts also have underlying osteoarthritis.
While many mucoid cysts are painless, larger cysts or those under pressure can cause pain, especially with movement or direct contact. If a cyst forms near the nail matrix, it can exert pressure, leading to changes in the fingernail such as grooving, ridging, splitting, or even nail loss. The skin over the cyst may become thin and is susceptible to rupture, which can lead to fluid leakage and an increased risk of infection, potentially affecting the underlying joint.
Diagnosis and Management
Diagnosis of a mucoid cyst typically involves a physical examination, where a healthcare provider assesses the cyst’s appearance, location, and feel. Imaging techniques like X-rays or ultrasound may be used to evaluate the underlying joint for signs of osteoarthritis or bone spurs. These imaging studies help confirm the diagnosis and rule out other conditions.
For asymptomatic cysts, observation is a common non-surgical management approach, as some cysts may shrink or resolve on their own. If the cyst causes symptoms, aspiration involves draining the fluid with a needle, sometimes followed by a steroid injection to reduce recurrence. However, aspiration and steroid injections have a relatively high recurrence rate, ranging from 30% to 40%.
Surgical removal is often considered for symptomatic cysts, those that recur after non-surgical treatments, or those causing nail deformities. During surgery, the cyst, its stalk, and any associated bone spurs are typically removed to reduce the chance of recurrence. Surgical excision has a lower recurrence rate, approximately 2% to 25%, especially when the underlying joint pathology is addressed. Recovery usually involves wearing a bandage and splint for a period, with a full recovery taking several weeks. Consulting a healthcare professional is important for proper diagnosis and to determine the most appropriate treatment plan.