CONDITION
Hydrocephalus
Hydrocephalus describes a buildup of cerebrospinal fluid within or around the brain. This fluid normally cushions and nourishes the central nervous system, but when drainage is impaired or production exceeds absorption, pressure can rise inside the skull. In dogs and cats, this may be present from birth or develop later in life, and the signs depend on how quickly the pressure changes and which parts of the brain are affected. Owners often notice changes in behaviour, learning difficulty, altered gait, or an unusually shaped head in young animals. In toy and small-breed dogs, a persistent soft spot on the skull may be palpable. Some animals live with mild hydrocephalus and show minimal signs, while others develop more pronounced neurological changes over weeks to months. This page explores the signals that may prompt concern, the mechanisms that lead to fluid accumulation, the imaging and clinical tests used to assess the condition, and the medical and surgical approaches that exist. The goal is to help you understand what hydrocephalus can look like and how veterinary teams think about it, without implying what may be happening in your own animal.
Why this matters now
Hydrocephalus most often appears in young animals, particularly toy and small-breed dogs such as Chihuahuas, Yorkshire Terriers, and Maltese, where congenital malformations affecting fluid drainage tend to be recognised in the first few months of life. Cats can also be affected, though less commonly, and both species may develop the condition later in life following inflammation, infection, bleeding, or masses that obstruct normal cerebrospinal fluid flow. The age at which signs emerge often reflects whether the problem is structural from birth or acquired after an injury or disease process.
When hydrocephalus is present from birth, signs may become apparent as the brain grows and fluid pressure rises during the first weeks to months of life. In some animals, the skull bones remain flexible enough to accommodate the extra volume, and signs may plateau or remain subtle for years. Acquired hydrocephalus, developing after trauma, infection, or bleeding, can progress more rapidly, with neurological changes appearing over days to weeks as pressure builds and the adult skull offers no room for expansion. The pace and severity of progression vary widely between individuals, influenced by the underlying cause, the rate of fluid accumulation, and the capacity of surrounding tissues to adapt.
Signals & patterns
Early signals
Domed or enlarged skull
In young animals, the head may appear unusually rounded or swollen, particularly across the top. This occurs when the skull bones have not yet fused and can separate slightly to accommodate rising fluid volume inside the cranium.
Persistent open fontanelle
A soft spot on the top of the head, where skull bones normally close after birth, may remain palpable for longer than expected. This gap can sometimes be felt gently through the skin and tends to be more noticeable in toy-breed dogs.
Slow or incomplete house-training
Puppies or kittens may struggle to learn predictable toileting patterns or seem less responsive to routine training cues. This reflects altered brain function affecting memory, learning, and awareness of environmental signals.
Quieter or less engaged behaviour
Some young animals appear less curious, slower to explore, or less interactive with littermates and people than might be expected. These changes can reflect altered brain activity or reduced sensory processing.
Unusual eye position or movement
The eyes may appear to point downward and outward, a pattern sometimes called a 'setting sun' appearance, or show involuntary flickering movements. This occurs when pressure affects the nerve pathways controlling eye muscle coordination.
Later signals
Circling or compulsive pacing
Repetitive walking in tight circles or along walls can emerge as pressure affects deeper brain structures involved in motor planning and spatial orientation. The pattern may be more pronounced after rest or during periods of activity.
Seizures or collapse episodes
Sudden, uncontrolled muscle activity or loss of consciousness may occur when rising intracranial pressure disrupts normal electrical signalling in the brain. These episodes can vary in intensity and duration between animals.
Loss of vision or coordination
Animals may bump into objects, misjudge distances, or show unsteady gait as pressure affects the optic nerves or regions of the brain responsible for balance and spatial awareness. These changes often develop gradually but can accelerate if pressure rises quickly.
Reduced responsiveness or altered consciousness
Some animals become less aware of their surroundings, slower to react to sounds or touch, or appear drowsy even when awake. This reflects widespread disruption of brain function under sustained or increasing pressure.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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