Here are the major reasons for a cat walking in circles:
Nasal cryptococcosis – a yeast-like fungus. Cryptococcosis is the most common systemic fungal infection affecting the domestic cat. Nasal cryptococcosis is one type and the most common. It is acquired when spores in the ground contaminated by bird droppings are inhaled. The infection can affect the brain causing neurological signs such as circling and seizures.
Inner ear disease – causes head tilt and circling. This is a disease of the vestibular system which orientates the cat and ensures balance. The semi-circular canals in the inner ear (the labyrinth) become inflamed (labyrinthitis). If things go wrong the cat stumbles (ataxia), loses balance and can circle. The cat might fall, roll over and wobble.
Portosystemic shunt (liver shunt) – an inherited anatomical defect. This is a defect concerning the portal vein that should take food nutrients to the liver but instead bypasses it and takes them to the heart. Ammonia products build up inside the cat causing non-standard behavior such as circling. There may be seizures and head pressing (cat presses head against hard surface). In addition to circling, there may be a range of other symptoms such as drooling, vomiting and diarrhea.
Diseases of the cerebrum – this is the largest part of the brain. It controls learning, memory and judgement. When it malfunctions the cat’s personality changes. Symptoms can include irritability, aggressiveness and forgetting what he has learned such as using the litter (inappropriate elimination). Also the cat might circle and pace. Seizures can also occur.
Stroke (cerebral hemorrhage – bleeding on the brain) – blood vessels feeding the brain rupture. This is rare in cats. High blood pressure can cause a stroke. Before the stroke there might have been a fever or illness. Residual symptoms are circling and seizures. Initial signs are vocalizations, paralysis, blindness and loss of coordination.
The cat in the picture is Pug the Cat. He was found on a front lawn. Initially diagnosed with a broken leg and then vestibular disease. It’s a still image from the video.
Source: Cat Owner’s Home Veterinary Handbook 3rd Ed. and myself.