Brain injuries can be broken down into two categories. Primary brain injuries occur when there is a direct injury or problem with the brain. Secondary brain injuries occur as a result of a primary injury. Hemorrhaging and swelling are examples of secondary injuries while blunt force head trauma is an example of a primary brain injury.
The most common type of primary brain injury that emergency veterinarians might see is a patient who has been hit by a car. The force from this kind of injury can cause concussions or even contusions on the brain. In some cases, blood vessels around the brain can start to bleed, and soft tissues can begin to swell. Bleeding and swelling will increase pressure inside of the skull and results in severe clinical signs such as seizures. If your dog has been hit by a car or has experienced any other type of head trauma, he needs to get to his emergency vet right away!
Too much or too little minerals and nutrients can also cause brain injuries. For example, very small young puppies run the risk of having unsafe drops in blood glucose when they don’t eat, and the brain uses up about half of your pup’s energy stores. Without enough glucose in the body, the brain cannot function, and this can result in severe depression, seizure activity, and death. Another important nutrient, sodium, can cause brain swelling and seizures if there is too much of it in your dog’s bloodstream. This kind of brain injury is commonly seen after a dog has been to the beach and ingested too much saltwater.
Anything that alters blood flow to the brain can result in a brain injury. Blood is comprised of red blood cells which are oxygen carriers. When the brain doesn’t receive enough oxygen, it can become irreversibly damaged. Blood clots to the brain can obstruct blood flow, and chronic diseases such as congestive heart failure can reduce blood flow to the brain. In the case of bleeding disorders like hemophilia, blood can still flow but the risk of bleeding into the brain becomes increased.
Like in humans, dogs have very tight junctions between cells that line the inside of the blood vessels in the brain. This is called the blood-brain barrier (BBB), and it is responsible for filtering things like toxins and bacteria away from the brain. In some cases, like with viral infections such as West Nile Virus, things can pass the BBB and cause infection and swelling. Prolonged fever (and prolonged hypothermia) can also cause brain injuries.
Dogs can also have problems with their blood-brain barrier directly. Collies and Australian Shepherds are at a higher risk of having a genetic mutation that affects their MDR1 gene. This is known as the multi-drug resistance gene, and when this gene is mutated, it deletes certain parts of the genetic code that are important to have a functioning BBB. This means that medications generally safe for other dogs like ivermectin, loperamide, and sedatives can cause significant neurologic problems in dogs with the MDR1 gene mutation.
Certain toxins are also known to cause brain injuries. Some can cause direct brain injuries while others can contribute to conditions that result in secondary brain injuries. For example, xylitol is a compound that is found in products like sugar-free gum. If a dog is exposed to xylitol, it can result in severe drops in blood glucose. Without supportive care, severe hypoglycemia can cause seizures and brain damage.