Why is seizure a sign of glioma?

It's not: Brain tumors can present with many problems, seizures being one. We are often taught that if a seizure is the first symptom a patient has that leads to the diagnosis of a brain tumor, that they will likely have a more favorable outcome. There are a multitude of "gliomas" - some are relatively benign and slow growing, and those are the ones in which seizure can be the first sign.

Brain irritation: A common presenting system of a glioma is a seizure. It is important to note there are other causes of seizures. The reason that a glioma is prone to a seizure is that it is an infiltrating tumor that frequently involves the cortex. It irritates the brain both by physical approximation and also by causing edema (extra fluid accumulation). The treatment is anti-seizure medications and surgery.