A mucoid cyst is a soft, fluid filled mass under the skin.  In the foot, it is found in the toes.  These cysts are a form of a ganglion cyst, which is a group of benign masses that have an outer shell that contains a gooey thick fluid very similar to joint fluid.  They are common throughout the foot, but on the toes the cyst is often referred to as a mucoid, myxoid, or mucous cyst.  The mucoid cyst appears usually in middle aged and elderly people, and more often than not they occur in women.  These cysts are typically much smaller than many traditional ganglion cysts as the space they occupy is often more limited,.  They form from the joint tissue in the toes, and appear externally as a small bump over one of the toe 'knuckles'.  The cyst is often compressible, but may also be firm to the touch, and the tissue over its top is often somewhat translucent, like a blister.  These are difficult to 'pop' because the tissue is deeper than a more superficial blister, and although they may sometimes drain these cysts often remain intact.
 
In many cases, mucoid cysts are not painful.  However, they can occasionally become painful when they are large or tense, or when tight shoes rub across the cyst surface.  The skin around the cyst can also become inflamed, furthering the pain.  After this process begins, direct pressure on the cyst becomes painful, and it can be difficult to wear tighter shoes.  If the cyst is located closer to the nail base, it can also disrupt the growth of the nail and lead to nail malformation or thickening.
 
Treatment of mucoid cysts often involves surgery to remove the cyst intact, and possibly even some of the joint whose tissue forms the cyst in the first place.  A oval shaped wedge of tissue is removed, containing the cyst and some normal tissue on its edges.  One can have the cyst drained, but unless the connection is severed between the joint fluid and the cyst lining, it will reform within a couple months.  By adding electrical burning, cold therapy, or less effectively steroid compound to the treatment after drainage, the chance that the cyst will return is lessened.  Mucoid cysts do not have to be removed as they are benign, but this treatment is often performed when they become painful.