A mental state in which a person has difficulties to think clearly or be fully present. Brain fogBrain fog A mental state in which a person has difficulties to think clearly or be fully present. Brain fog can be a symptom of complex post-traumatic stress. can be a symptom of complex post-traumatic stressComplex post-traumatic stress Complex post-traumatic stress is a possible diagnosis in people who have repeatedly experienced traumatic events, such as violence, neglect, or abuse..