Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your son, review his MRI, and obtain a full history, I can not comment on whether or not in his particular case he his arachnoid cyst is causing bipolar disorder, but I will try to provide you with useful information.
Arachnoid cysts are a collection of cerebrospinal fluid (CSF, the fluid that covers the brain and spinal cord) between two layers of tissue called the arachnoid. They are thought to be present from birth in the large majority of patients. They can occur in any location within the brain. They are often incidental findings with no symptoms and with no consequences. However, when they do cause symptoms, that could include headaches, seizures, and in some rare instances psychiatric problems. 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. There are several reported cases of arachnoid cyst causing psychosis and other psychiatric problems due to compression of surrounding brain tissue by the cyst, with resolution of the psychiatric symptoms following surgery.
In your son's case, review of his case by a qualified and well experienced neurologist and neurosurgeon is essential. The surgery is not without its risks, and only a neurologist and neurosurgeon together can determine whether the cyst is in fact causing his psychiatric symptoms. Further imaging may be necessary. The complexity of the surgery and recovery time depend largely on the location, size, and other details such as how the cyst is affecting the flow of CSF in general.
Thank you for the opportunity to answer your questions, I hope you find this information useful.
Did your son have any physical symptoms in addition to his BPD? Also, the brain cyst is related. Arachnoid cysts have been known to cause psychosis, depression, anxiety and other mental health maladies. You should look up the term on wikipedia, the Mayo Clinic database, and elsewhere.
He has complained of headaches. Have done a lot of research online --just wanted some validation that this could be the cause of his mental issues. Have not looked at the May Clinic database -- will do. Thanks for your response.