They appear grey-red with alternating light and dark lines (known as lines of Zahn) which represent bands of white blood cells and red blood cells (darker) entrapped in layers of fibrin. Cause
Virchows triad describes the pathogenesis of thrombus formation:
Endothelial injury: Injury to the endothelium (interior surface of blood vessel), causing platelet activation and aggregation;
Common causes include: trauma, smoking, hypertension, atheroma. Hemodynamic changes (stasis, turbulence):  Blood stasis promotes greater contact between platelets/coagulative factors with vascular endothelium. If rapid blood circulation (e.g., because of tachycardia) occurs within vessels that have endothelial injuries, that creates disordered flow (turbulence) that can lead to the formation of thrombosis;Common causes of stasis include anything that leads to prolonged immobility and reduced blood flow such as: trauma/broken bones and extended air travel. Hypercoagulability (also called thrombophilia; any disorder of the blood that predisposes to thrombosis);
Common causes include: cancer (leukaemia), factor V mutation (Leiden) - prevents Factor V inactivation leading to increased coagulability.Disseminated intravascular coagulation (DIC) involves widespread microthrombi formation throughout the majority of the blood vessels. This is due to excessive consumption of coagulation factors and subsequent activation of fibrinolysis using all of the bodys available platelets and clotting factors. The result is hemorrhaging and ischemic necrosis of tissue/organs. Causes are septicaemia, acute leukaemia, shock, snake bites, fat emboli from broken bones, or other severe traumas. DIC may also be seen in pregnant females.
To be recognized, a sufficient amount of consensus and clinical history needs to be present, and as a newly proposed condition, PDA had not met the standard of evidence required at the time of recent revisions. However, DSM-5 also moved from sub-type classification to the use of ‘Autistic Spectrum Disorder’, which allows for the behavioural traits of different profiles to be described. In 2011, the National Institute for Health and Care Excellence commented on the fact that PDA has been proposed as part of the autism spectrum but did not include further discussion within the guideline. NICE guidance also expects an ‘ASD’ diagnosis be accompanied by a diagnostic assessment, providing a profile of key strengths and difficulties. Demand Avoidance is listed as a ‘sign or symptom of ASD’ (Appendix 3).Christopher Gillberg wrote a commentary article in 2014 that reviewed recent research and stated, “Experienced clinicians throughout child psychiatry, child neurology and paediatrics testify to its existence and the very major problems encountered when it comes to intervention and treatment.”

Proposed diagnostic criteria
As of 2014 there are no recognized diagnostic criteria.