When I was a kid and someone got a concussion, I seem to remember people telling me that it was when your brain impacts the side of your skull, accelerating fast enough to do so despite the cerebrospinal fluid buffer.

But wikipedia doesn't mention this explicitly. Is this just one possible cause for concussions? Is it possible to generalize about them? How much don't we know?


Concussion generally refers to the set of clinical symptoms that are secondary to the head injury and are characteristic by altered state of consciousness like instant onset of transient neurologic dysfunction, lack of consciousness or temporary respiratory arrest and loss of reflexes.

Since it's connected to the head trauma by definition, there is in principium no other cause of it.

The patophysiologic mechanism of concussion is not well understood yet, it's thought it results from two main sources:

  • the acceleration of brain causes increase of pressure
  • the impact of brain on the hard shell causes deformation and sheer stress in brain tissue (probably stronger cause of cytologic changes discussed later on)

These two mechanisms might lead to changes in the tissue structure and also trigger cytologic changes in the astrocytes and neurons such as:

Astrocytes respond to a focal mechanical stimulation by propagating intercellular waves through their network. Mechanically stimulated astrocyte networks show changes in the cytoskeleton, organelle function, and biochemical cascades over time.

The changes in neurons detected in vitro include microstructural changes, direct membrane permeability change and change in the receptor abundance, structure of subunits and

Early evidence showed that these physical insults can affect the properties of important synaptic glutamate receptors that can regulate neurotransmission and plasticity in networks. Moreover, inhibitory synaptic receptor functions can be altered with a physical force, showing that the balance of excitation/inhibition coupling is important to consider when assessing the effects of these physical forces. Perhaps equally important is the alteration in the receptor composition and intracellular signaling that occurs after a microinjury.

Microinjury is also linked to triggering of cell death pathways in brain cells.

The importance of these mechanisms are not yet clearly understood though and there probably are many more playing their role.


  1. Overview study
  2. Robbins and Cotran pathologic basis of disease
  • $\begingroup$ Does "concussion" imply "brain impacted the side of the skull"? $\endgroup$
    – Eli Rose
    May 21 '17 at 13:00
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    $\begingroup$ The brain doesn't have to impact the skull. Acceleration itself and the rise of pressure it causes if big enough can according to the article cause enough stress for some of these changes to occur. Morover the impact is always hindered by the cerebro-spinal fluid, but the thing that matters is how much of the energy is absorbed by the fluid and how much by brain tissue. If impact is small, much of the energy is absorbed by the fluid and little damage is done to the neural tissue. If it's big, proportionally less is absorbed by the fluid and the rest can cause the damage to the brain. $\endgroup$
    – mpribis
    May 21 '17 at 14:32

The CDC has a nice webpage about concussions and traumatic brain injury, as does the Mayo Clinic. The brain and head move rapidly back and forth due to impact or sudden acceleration or deceleration, in spite of the cerebral spinal fluid bathing the brain cavity. This causes chemical changes in the brain and/or damage to brain cells, affecting brain function.

Symptoms are not always instantaneous; sometimes it can take hours or days for symptoms to arise. There are multiple diagnostic tools that exist, including graded symptom checklists, the Standardized Assessment of Concussion, neuropsychological assessments, and the Balance Error Scoring System. These are summarized in this American Academy of Neurology (AAN) article from 2013.

  • 1
    $\begingroup$ "...sometimes it can take hours or days for symptoms to arise." I think that is true of some symptoms perhaps, but all people diagnosed with a concussion show one of a number of symptoms immediately after impact. I've never, ever heard of a concussion diagnosed without any symptomatology - even relatively minor, such as dizziness or cognitive impairment - post impact, and as an ED doc, I've assessed many, many head injuries. Diagnosis may be delayed because people think the symptoms are minor. $\endgroup$ May 21 '17 at 15:11
  • $\begingroup$ Also, just checked your references, which do not support your statement. The closest it comes is in the Neurology article, which states "...many of whom do not obtain immediate medical attention." $\endgroup$ May 21 '17 at 15:13
  • $\begingroup$ The note about symptoms taking time to arise might be referring to symptoms caused by bleeding in the brain, which could be caused by trauma but is separate from concussion. $\endgroup$
    – Bryan Krause
    May 22 '17 at 20:07

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