Coup contrecoup injury

# Overview
In head injury, a coup injury occurs under the site of impact with an object, and a contrecoup injury occurs on the side opposite the area that was impacted. Coup and contrecoup injury is associated with cerebral contusions. Coup and contrecoup injuries can occur individually or together.  When a moving object impacts the stationary head, coup injuries are typical, 
while contrecoup injuries are usually produced when the moving head strikes a stationary object.
In a coup injury, the head stops abruptly and the brain collides with the inside of the skull.
Coup and contrecoup injuries are considered focal brain injuries, those that occur in a particular spot in the brain, as opposed to diffuse, which would occur over a more widespread area.
The mechanism for the injuries, especially contrecoup injuries, is a subject of much debate.  It is likely that inertia is involved in the injuries, e.g. when the brain keeps moving after the skull is stopped by a fixed object or when the brain remains still after the skull is accelerated by an impact with a moving object.  Alternately, movement of cerebrospinal fluid following a trauma may play a role in the injury.

# Mechanisms
The coup injury may be caused when, during an impact, the skull is temporarily bent inward, and impacts the brain. When the skull bends inward, it may set the brain into motion, causing it to collide with the skull opposite side and resulting in a contrecoup injury. The injuries can also be caused by acceleration or deceleration alone, in the absence of an impact. In injuries associated with acceleration or deceleration but with no impact, the brain is thought to bounce off the inside of the skull and hit the opposite side, potentially resulting in both coup and contrecoup injuries.  In addition to the skull, the brain may also impact the tentorium to cause a coup injury.
Contrecoup injury may be produced by tensile forces.
Cerebrospinal fluid (CSF) is also implicated in the mechanism of coup and contrecoup injuries.
One explanation for the contrecoup phenomenon is that CSF, which is more dense than the brain, rushes to the area of impact during the injury, forcing the brain back into the other side of the skull.  If this is the case, the contrecoup impact happens first.

# Features
Contrecoup, which may occur in shaken baby syndrome and vehicle accidents, can cause diffuse axonal injury. On very rare occasions, contrecoup injury can cause epidural hematoma.
Contrecoup contusions are particularly common in the lower part of the frontal lobes and the front part of the temporal lobes. A 1978 study found that the contrecoup mechanism was responsible for most of the brain lesions such as contusions and hematomas occurring in the temporal lobes of injured individuals. Injuries that occur in body parts other than the brain, such as the lens of the eye, the lung, and the skull and other bones, may also be labelled "contrecoup".  The contrecoup mechanism can play a role in pulmonary contusion.

# History
In the 17th century, Jean Louis Petit described contrecoup injury. In 1766, the French surgeon Antoine Louis coordinated a meeting of the Academie Royale de Chirurgie  on contrecoup injuries, at which papers were to be presented of which one would be chosen to receive the respected prize, the Prix de l'Academie Royale de Chirurgie. The presenter of the chosen paper was not awarded the prize because he failed to make recommended changes.  In 1768, the group met again on the topic, and Louis Sebastian Saucerotte won the prize for his paper describing contrecoup injuries in humans and experiments on animals and recommending tretaments such as bloodletting and application of herbs to patients' heads.