The simple physics say a 4 month old cannot cause a concussion or even subconcussive impact.

But, the mind can cause problems by memorizing symptoms of a prior event and replaying those symptoms with the right trigger.

Sometimes, the trigger causes the person to 'look' for these symptoms.

I guarantee that anybody who looks for concussion symptoms will find them, whether they are real or not.

I don't deny the symptoms but the anxiety that often is triggered by those memorized symptoms needs to be addressed.

The symptoms are more of a PTSD like flashback. The continuing thoughts of, "Did I do brain damage?" and the attempts to justify those questions can lead to a spiral down into dysfunction.

Regarding Concussion Anxiety Syndrome. There has been extensive research into the impact depression and/or anxiety play in symptoms manifesting. The correlation is up to 80%.

Learning to recognize triggers and take appropriate immediate steps to mitigate any progressions is very important. I have rescue meds that I used after some triggers as I sensed things progressing.

So, as BW says, we need to take those steps... Migraine meds, thought modification, environment modification, and such can go a long way in minimizing the progression.

Often, the person is unwilling to seek the therapy to learn how to moderate these events. They may not have insurance that covers such therapy and are unwilling to pay out of pocket but end up losing wages for lost work days that exceeds the out of pocket costs for therapy.