On the 2 nd day of illness a 27-year-old patient complains of the unbearable headache, repeated vomiting. Objectively: the patient is in a grave condition. He is conscious but adynamic. Lies in a forced position with his head thrown back. There is no skin rash. Occipital muscles are evidently rigid, there are Kernig's and Brudzinski's signs. to- 39,5°C, Ps-120/min., AP- 130/80 mm Hg. What is the reason for the leading syndrome of this disease?