If someone has a headache and an arachnoid cyst, just given the statistical probabilities, they are more likely
to have a headache unrelated
to the cyst than
to have headaches caused by the cyst. Similarly, in a patient with bipolar disorder and an arachnoid cyst, the likelihood is that the cyst is NOT causing the psychiatric disorder. Having said this, it depends on several factors, including the location and size of the cyst, and more importantly its effect on surrounding brain structures.