Arachnoid Cyst

[AndreaLC]

After I was experiencing vertigo-like symptoms, my doctor ordered an MRI, which showed a 4cmx5cm arachnoid cyst on the left temporal lobe.  Through further testing (EEGs), it was also discovered that the vertigo-like symptoms were actually mild seizures in the area where the cyst is located.  My nuerologist and nuerosurgeon suggested that surgery should wait until it could be determined if the medication could control the seizures and/or the cyst did not change in size.  Six months past and I recently had a breakthrough seizure where I lost consciousness.  Blood work determined that my medication level was therapeutic at the time of the seizure.  Another 24-hour EEG was ordered and the surgeon wants to wait to see what those results are before ordering a PET scan.  My questions are these: When does a surgeon determine that seizures are intractable? What will a PET scan show and how can a surgeon determine if the cyst is putting pressure on the brain to cause the seizures?  Any information that you can provide would be greatly appreciated.

[CCF-Neuro-M.D.-SH]

First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes. Arachnoid cysts are congenital cystic spaces that are often incidental findings on neuroimaging.  However, sometimes these cysts can expand and compress brain tissue.  Other times, a problem can be attributed to the cyst (such as headaches, seizures, etc.) and there is really another explanation.  From the symptoms and history you provided, it sounds like you have focal epilepsy, that is possibly arising from your left temporal lobe (I do not know the results of your EEG, but if they suspect the cyst as the cause, I assume they localized the seizures to your left temporal lobe).  While it is possible that the cyst is pushing on your left temporal lobe causing the seizures, there are several other things that could be going on, and it is very important to know exactly what is going on before you conduct brain surgery.  Another possibility is that you have temporal lobe epilepsy, unrelated to the cyst, and thus removing the cyst would not cure your epilepsy.  Another thing to be ruled-out is that you could have a cystic tumor instead of an arachnoid cyst.  A PET scan will show the metabolic rate of your brain (how fast it uses glucose, which is the brain fuel).  If you have temporal lobe epilepsy then, the PET scan would be expected to show hypometabolism (less fuel being burned) in the area causing the seizures.  If you have a tumor, the PET may show increased metabolism in the area of the tumor.
   It sounds like you did not respond to the first medication that was tried.  In studies, this seems to indicate that you are likely to have medically refractive epilepsy (~60% chance), but there is still a chance that you may respond to the another drug.  We generally recommend that patients try at least 2-3 medications before having surgery.  If you do indeed have medically refractive focal epilepsy, then surgery may be the best option for you (and this should be discussed in depth with your epileptologist (neurologist who specializes in epilepsy) and neurosurgeon.
I hope this has been helpful.