This question is information regarding the effects of colliod cyst size and when to remove. My wife had a 5.5 mm colloid cyst in the third ventricle removed 5/22/09 at a hospital in Georgia "too small to possibly cause symptoms." The discussion of the size of the cyst alone required to cause problems by obstructing a duct is meaningless without the size of the passageway in which it resides included. The relevant issue is the clearance or lack thereof between the two. This missed fact almost caused her death four days prior to the surgery. A peanut in a fire hose has little effect in flow blockage compared to same in a straw.
What saved her life was the chief neurologist, who had had enough, recalling the neurosurgery team (having previously dismissed the case based on the same type of assumptions present in this forum), to require data supporting their assumptions. In the end analysis the symptoms in this case were strong enough to give confidence to the diagnosis that surgery was indeed needed.
The seating of the cyst in the ventricle duct normally occurs when the patient is vertical, acting like a small ball check valve positioned by gravity. The classic symptoms are severe headached relieved to some degree by lying down ("positional component"). The patient normally shows the back of the neck as the source, the focal point on either side (including swelling), painful to the touch. The cyst blockage of fluid causes fluid pressure in the cranium to drop, and the brain ----- normally floating like a sponge in a fishbowl ----- actually lowers and repositions in the cranium, causing the terrific pain, and in my wife's case, tremors and motor impairment.
MRI's are of little use beyond detection of cyst presense because they are performed in the horizontal position, and as a result does not show the cyst blocking the channel. The only person an MRI would show blockage of the channel on would therefore be a corpse: the patient dies if complete blockage occurs. The 5.5 mm cyst in my wife's case was confirmed to completely close off the ventricle by sitting her up during surgery after the a clear view was established before the cyst was removed.
There ARE are a handful of vertical MRI machines available in the US: a new tool first introduced in 1999.