Mild traumatic brain injury (mTBI) can cause cognitive and psychological symptoms without any evidence of injury on a CT or MRI scan. Many scientists and clinicians now accept that this limitation is due more probably from the relatively low resolution of these scans, and not the absence of real damage.
A recent animal study has shown that, although an mTBI led to only small numbers of actual neuron deaths, this mild damage led to a more extensive degeneration of dendrites and synapses. Dendrites are the branches of a neuron, and are critical for the communication of one neuron to another. The reduction of dendrites led to a reduction in synapses, which are the spaces in between neurons that allow for electrical and chemical exchanges.
Dendritic and synaptic degeneration is too small to be detected by CT or MRI scans, and yet is considered to be physical proof of the damage caused by mTBI.
Gao X, & Chen J. Mild traumatic brain injury results in extensive neuronal degeneration in the cerebral cortex. Journal of Neuropathology and Experimental Neurology. (March 2011).