Descending inhibitory inputs from the medial prefrontal cortex (mPFC) regulate the transmission from the BLA to the CeA, which is hypothesized to play a role in the extinction of conditioned fear responses. While as a whole, amygdala hyperactivity is reported by meta analysis of functional neuroimaging in PTSD, there is a large degree of heterogeniety, more so than in social anxiety disorder or phobic disorder. Comparing dorsal (roughly the CeA) and ventral(roughly the BLA) clusters, hyperactivity is more robust in the ventral cluster, while hypoactivity is evident in the dorsal cluster. The distinction may explain the blunted emotions in PTSD (via desensitization in the CeA) as well as the fear related component.In a 2007 study Vietnam War combat veterans with PTSD showed a 20% reduction in the volume of their hippocampus compared with veterans who did not have such symptoms. This finding was not replicated in chronic PTSD patients traumatized at an air show plane crash in 1988 (Ramstein, Germany).Evidence suggests that endogenous cannabinoid levels are reduced in PTSD, particularly anandamide, and that cannabinoid receptors (CB1) are increased in order to compensate. There appears to be a link between increased CB1 receptor availability in the amygdala and abnormal threat processing and hyperarousal, but not dysphoria, in trauma survivors. A 2020 study found no evidence for conclusions from prior research that suggested low IQ is a risk factor for developing PTSD.
Cause
The most common causative organisms are (often intracellular living) bacteria:
Chlamydia pneumoniae
Mild form of pneumonia with relatively mild symptoms. Chlamydia psittaci
Causes psittacosis. Coxiella burnetii
Causes Q fever. Francisella tularensis
Causes tularemia. Legionella pneumophila
Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires disease. Mycoplasma pneumoniae
 Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.Atypical pneumonia can also have a fungal, protozoan or viral cause.In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans. Viral
Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), COVID-19
and measles. Diagnosis
Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all. This is occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by Streptococcus pneumoniae, as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.Infiltration commonly begins in the perihilar region (where the bronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe, but may affect any lobe or combination of lobes. Epidemiology
Mycoplasma is found more often in younger than in older people. Older people are more often infected by Legionella.