There are two types of amnesia: retrograde amnesia (loss of memories that were formed shortly before the injury) and anterograde amnesia (problems with creating new memories after the injury has taken place). Both retrograde and anterograde forms may be referred to as PTA, or the term may be used to refer only to anterograde amnesia.
Frequently the last symptom to ameliorate after a loss of consciousness, anterograde amnesia may not develop until hours after the injury. A common example in sports concussion is the quarterback who was able to conduct the complicated mental tasks of leading a football team after a concussion, but has no recollection the next day of the part of the game that took place after the injury. Retrograde amnesia sufferers may partially regain memory later, but memories are not regained with anterograde amnesia because they were not encoded properly.
The term "posttraumatic amnesia" was first used in 1928 in a paper by Symonds to refer to the period between the injury and the return of full, continuous memory, including any time during which the patient was unconscious.
|Mild||13 to 15|| <1|
|Moderate||9 to 12|| 30 minutes|
to 24 hours
| 1 to 24|
|Severe||<8||>1 day|| >24 |
Classification systems for determining the severity of TBI may use duration of PTA alone or with other factors such as Glasgow Coma Scale (GCS) score and duration of loss of consciousness (LOC) to divide TBI into categories of mild, moderate, and severe. One common system is shown in the table at right.
PTA is considered a hallmark of concussion, and is used as a measure of predicting its severity, for example in concussion grading scales. It may be more reliable for determining severity of concussion than GCS because the latter may not be sensitive enough; concussion sufferers often quickly regain a GCS score of 15.
Longer periods of amnesia or loss of consciousness immediately after the injury may indicate longer recovery times from residual symptoms from concussion. Increased duration of PTA is associated with a heightened risk for TBI complications such as post-traumatic epilepsy.