What does it feel like?
Most patients complain of a lump that develops on the back of the finger, just shy of the fingernail cuticle that waxes and wanes in size, and occasionally pops, exuding a clear jelly-like fluid. Sometimes, they also notice that their nail just downstream from the lump has developed a scalloped appearance.
What is it?
It is a cyst that forms from the extrusion of fluid from the distal interphalangeal joint of the finger (or the interphalangeal joint of the thumb).
What causes it?
A deficiency in the lining of the joint results in a steady ‘leak’ of joint fluid that is unable to re-enter the joint to get absorbed, due to the presence of a one-way valve at the base of the cyst. This usually means that the cyst, when popped, is likely to recur, because the base or neck of the cyst still has the persistent one-way valve and fluid still continues to be formed in the joint, and is extruded out from the joint.
Usually, very early arthritis with a bony spur potentiates this condition. However, most patients do not feel pain in the joint that is typical of more advanced arthritis.
The nail changes that some patients see are a result of a pressure effect from the cyst, pressing on the germinal matrix, where the nail forms.
What are my treatment options?
Some people would consider mucous cysts to be annoying and cosmetically displeasing. If the skin overlying the cyst is translucent or transparent, it is a good idea to seek treatment promptly, because this is considered ‘threatened skin’, and has a very high likelihood of rupture.
Cyst rupture exposes the underlying joint to surface micro-organisms, and risks a deep-seated infection that may necessitate multiple surgeries, and long-term intravenous antibiotics. This is why it is generally recommended that you do not attempt to pop the cyst.
Cyst aspiration (inserting a needle and sucking the fluid out) has a high rate of cyst recurrence, because the presence of the one-way valve at its base has not been addressed. Surgical management has a significantly lower recurrence rate because the principle of surgical excision of the cyst is to excise the neck and the one-way valve as well as to debride the joint and bony spur. This can usually be performed as a day procedure with local anaesthesia delivered to the affected finger and light sedation.