It is important to understand that the abused child’s reaction to stress is not abnormal

Stress
The experience of moderate and predictable stress in childhood has not been found to be particularly harmful – in fact this low level of stress can help children to develop ways of coping with life in general. However, chronic stress sensitises neural pathways and overdevelops the regions of the brain that deal with anxiety and fear responses.
Children who are constantly under threat of abuse will need to focus their brain’s resources on survival and responding to threats in their environment. The chronic stimulation of fear responses hinders the stimulation of parts of the brain involved in more complex thought and this effects learning.
Cortisol is a hormone that helps the body prepare to cope with stress, and studies have shown that children who have been subjected to abuse have abnormal secretions of cortisol (Hart, Gunner, Cicchetti, 1995).
It is important to understand that the abused child’s reaction to stress is not abnormal: their brain development is often an adaptive response to their negative environment. However, it is maladaptive to other environments. If the brain’s main focus is survival it will be more difficult to develop healthy cognitive and social skills. Abused children often have great difficulty when presented with kindness, nurturing and stimulation: their brains may have underdeveloped pathways to adapt to this new world.
Repeated abuse and resulting stress may cause an affect on the neurochemical systems which can cause changes in: attention, impulse control, sleep patterns, and fine motor control (Perry 2000).

http://www.teachingexpertise.com/articles/child-abuse-and-its-effect-on-brain-development-1448