The immediate effects of a head injury can actually include dementia symptoms, according to a recent article in the Mayo Clinic newsletter.

Effects such as confusion, memory loss, and changes in speech, vision and personality can all develop. Depending on the severity of the trauma, these symptoms may clear up quickly, last a long time or never go away completely. However, such symptoms that begin soon after an injury generally don't get worse over time as happens with Alzheimer's Disease.

Certain types of head injuries, however, may increase your risk of developing Alzheimer's disease or other dementias later in life. The greatest increase in future dementia risk seems to occur after a severe head injury that knocks you out for more than 24 hours. (This explains why athletes playing sports like Boxing or Football are sometimes known to develop Parkenson’s Disease or similar ailments in later life. Muhammed Ali is an unfortunate example of this process.) However, a moderately serious head injury that causes unconsciousness for more than 30 minutes, but less than twenty-four hours, only seems to increase the risk to a smaller extent.

There's no evidence that a single mild head injury that doesn't knock you out, or that knocks you out for less than 30 minutes, increases your risk of developing dementia. However, repeated mild injuries may increase risk of future problems with thinking and reasoning.

You're likely at greatest risk of developing dementia or Alzheimer's later in life, after sustaining a head injury, if you also have other risk factors. For example, carrying one form of the apolipoprotein E (APOE) gene increases the risk of Alzheimer's in any individual. A head injury in such a person would increase his or her future risk of complications.

It's important to realize that many people who sustain a severe head injury never develop Alzheimer's disease or later dementia. Research is ongoing to better understand the link between head trauma and dementia.