A neurological disease is one that affects the brain or the network of nerves running throughout the body. The signs of illness can range from very mild (a weakness in one leg) to very severe (the inability to stand). In order for your vet to investigate the disease they need to know where the problem actually lies. If your pet has difficulty walking this may be because of a problem with the nerves in its leg, pressure on the nerves in its spine (like a slipped disc) or a problem in the brain. Only by careful examination can your vet identify where the problem is likely to be in order to perform the most appropriate tests.
Diseases affecting the brain can make cats very unwell. The brain is a very delicate organ and even minor changes within the brain can cause significant clinical signs. Animals with brain disease can show a variety of signs: depression and apparent headache (pressing their head against a wall), seizures (fits) and behavioural changes. If only a small area of the brain is affected then animals may show localised signs, eg lameness, head tilt, blindness, wobbliness.
Signs of brain disease may be caused by a disease within the brain or by a disease elsewhere in the body that results in toxins or other potentially harmful substances entering the blood and being transported to the brain. If your vet suspects that your pet has a brain disease they will need to do a variety of tests to work out where the primary problem is.