Types of Head Injuries in Dogs
A dog’s head can be injured in many ways, including a car accident, a fall, a blow to the head, or a gunshot wound. Since the brain is encased in bone and surrounded by a layer of fluid, it takes a major blow to the head to fracture the skull and injure the brain.
A skull fracture can be linear, star shaped, compound (a compound fracture opens to outside the body), or depressed (forming a depression). Skull fractures often extend into the middle ear, nasal cavity, or sinuses, creating pathways for bacteria to gain access to the brain and cause infection. In general, the larger the skull fracture, the greater the likelihood of brain injury. However, the brain can be injured even if the skull is not broken.
The skull is formed by three bone plates, and the area at the top of the skull where they come together is called the fontanel. Usually these plates fuse when a puppy is about 4 weeks old, but sometimes they never completely fuse, leaving a hole at the top of the skull called an open fontanel, or molera. The open area can range in size from a 50-cent piece to a penny.
Most of the time an open fontanel will close over by the time the dog is 1 year of age, but sometimes it will remain open throughout the dog’s lifetime. These areas can be susceptible to trauma but are generally not a problem. In some dogs this condition may be associated with hydrocephalus.
Congenital open fontanel is seen primarily in Chihuahuas, but the condition can be found in all the toy breeds. Since it’s likely a hereditary problem, dogs with an open fontanel should not be bred.
Injuries severe enough to fracture the skull are often associated with bleeding into and around the brain. Brain injuries are classified according to the severity of brain damage.
With a contusion, there is no loss of consciousness. After a blow to the head the dog remains dazed, wobbly, and disoriented. The condition clears gradually.
By definition, a concussion means the dog was knocked unconscious. With a mild concussion there is only a brief loss of consciousness, while with a severe concussion the dog may be unconscious for hours or even days. When she returns to consciousness, the dog exhibits the same signs as for a contusion.
A severe concussion causes the death of millions of neurons. Recent information indicates that brain cell death does not cease within a few hours of the injury, but can continue for weeks or months.
Seizures can occur at the time of injury or at any time thereafter. Seizures at the time of injury are particularly detrimental because they increase pressure in the skull and compromise blood flow. This worsens the effects of the injury. Seizures that occur weeks after the injury are caused by scars that form in areas where brain tissue has died.
Brain Swelling and Bleeding
Severe head injuries result in brain swelling and bleeding into and around the brain. Brain swelling, technically called cerebral edema, is always accompanied by a depressed level of consciousness and often coma. Since the brain is encased in a rigid skull, as the brain swells the cerebellum is slowly forced down through the large opening at the base of the skull. This squeezes and compresses the vital centers in the midbrain. Death occurs from cardiac and respiratory arrest.
Blood clots can form between the skull and the brain or within the brain itself. A blood clot produces localized pressure that does not, at least initially, compress the vital centers. Like cerebral edema, the first indication is a depressed level of consciousness. One pupil may be dilated and unresponsive to a light shined in the eye. Another sign is weakness or paralysis involving one or more limbs.