Post-traumatic stress disorder is unique in the diagnostic and statistical manual of mental disorders (DSM-5) in that you need to have been exposed to a trauma to be diagnosed with it. For this reason it is often, rightfully, referred to as a "trauma-related anxiety disorder" — but alterations in memory represent such a central part of its symptomatic picture that some researchers see PTSD as a disorder of basic memory function.
How exactly can PTSD impact a sufferer's memory, and why?
PTSD, intrusive memories, and the brain
Traumatic memories, research has suggested, often don't get processed and "filed away" like other memories. Instead, they stick around in the subcortical and primary perceptual areas of the brain, where their raw material kind of floats about aimlessly, making it hard for us to access them at will (though chances are we won't want to). When faced with a trigger that reminds us of what happened, the memory is suddenly brought to the forefront. The result is an intrusive memory, or a flashback.
Because the memory wasn't processed like others and integrated into the person's chronologically-semi-accurate "brain photo and video album", if you like, these memories are often experienced as if they're playing out in real time. These memories then seem like an immediate threat. It's no surprise that they trigger a fear response, and they're often very primal, involving senses like smell, touch, sounds, and images rather than abstract cognitive ideas.
Nightmares, dissociation, and hyperarousal symptoms — all of which are key symptoms of post-traumatic stress disorder — are all related to this "faulty" memory filing. All this goes a long way towards explaining why processing the memories of trauma in therapy, and thereby integrating them, can ultimately offer PTSD sufferers (some) relief from debilitating symptoms, even though therapy may initially increase symptoms.
PTSD can also lead to memory gaps and short-term memory problems
As a person who was diagnosed with PTSD, I personally, as an example, have detailed "snapshots". I remember exactly what the lighting was like, what place we were at, what the room smelled like during traumatic certain events (relating to long-term childhood sexual assault). But I don't remember related events from life beyond trauma — what grade I was in, what books I read, or what movies came out that year. Perhaps that's true because that part of my life was on autopilot, and didn't consciously register at all.
The example that shocks me the most is that I wasn't able, years after, to physically recognize a person who came into my life when the trauma was ongoing. I remembered the person's name, but their face was completely gone, along with most of our shared experiences. On the whole, everything from that period of time is more or less a blur, and this is not an unusual experience.
For me, this has manifested as short-term memory issues like forgetting something I had done the day before or committing to doing something that then slipped my mind — long after I ceased to be exposed to trauma and had definitely reached the "P" in PTSD. People in your life asking whether you "remember the time that we...", and you drawing a complete blank may be a good indicator that you are dealing with similar issues.
Can therapy help you get rid of PTSD-related memory issues?
In my experience, talk therapy can absolutely help you reduce reexperiencing symptoms like intrusive memories, flashbacks, and nightmares in which your brain involuntarily processes traumatic events. It can help you integrate past memories into your conscious brain by making sense of them in a safe environment. Even now that I no longer experience debilitating PTSD symptoms, however, I do have some short-term memory issues. Writing everything down helps me remember to complete important things on my to-do list.