The single most useful thing to understand about concussion is that the injury is chemical and metabolic before it is anything else. Once you know what is actually happening in the tissue, almost everything else about this injury stops being mysterious — including why the scan is normal.
What happens in the first seconds
Mechanical force stretches neuronal membranes and axons. That stretch opens ion channels indiscriminately, and the carefully maintained separation of ions across the cell membrane collapses. Potassium floods out of cells, calcium floods in, and the excitatory neurotransmitter glutamate is released in bulk, which drives further ionic flux in neighbouring cells.
None of this destroys tissue in a way a scanner can resolve. The cells are intact. What has failed is their electrochemical order.
The energy crisis
Restoring that order is expensive. The sodium-potassium pumps that push ions back where they belong run on ATP, so the brain’s demand for glucose spikes sharply, at exactly the moment when cerebral blood flow is typically reduced. The result is a mismatch between energy needed and energy deliverable — a period the literature calls a metabolic crisis.
Calcium accumulating inside cells also impairs mitochondria, which are the very structures that would otherwise produce the ATP required. The injury degrades the machinery needed to recover from the injury.
Giza CC, Hovda DA. The New Neurometabolic Cascade of Concussion. Neurosurgery. 2014;75(Suppl 4):S24–S33. doi:10.1227/NEU.0000000000000505
Why this explains the symptoms
The clinical picture follows directly from the biology, which is why it is so consistent across patients who have never spoken to each other:
- Headache and light sensitivity — glutamate release and cortical spreading depression share mechanisms with migraine, which is why post-concussive headache so often behaves like one
- Fatigue and cognitive fog — a brain in an energy deficit cannot fund sustained attention; thinking becomes expensive, so it becomes slow
- Symptoms provoked by cognitive or visual load — load raises energy demand that the tissue currently cannot meet, which is why reading a screen brings symptoms on
- Vulnerability to a second injury — while the metabolic crisis is unresolved, a further impact lands on tissue with no reserve, which is the physiological basis for return-to-play caution
And why the CT is normal
This is the whole point. A CT scan detects structural pathology — blood, fracture, displaced tissue. The neurometabolic cascade is an invisible injury at that resolution: no bleeding, no mass, no lesion. Cells that are electrochemically deranged and energetically starved look exactly like healthy cells on a CT.
So a normal scan is not evidence against a concussion. It is the expected finding, and any reasoning that treats it as exoneration has misunderstood what the scan measures. See diagnosis and testing.
What follows for treatment
If the problem is an energy supply-and-demand mismatch, then the treatment logic writes itself: do not add demand the tissue cannot fund, but do not withdraw all demand either, because prolonged inactivity has its own costs. That is precisely the graded, symptom-guided return to load that replaced dark-room rest.
It also explains why metabolic health is not a side issue here. A patient whose glucose handling, sleep and vascular function are already impaired is trying to resolve an energy crisis with a compromised energy system. See treatment.
Common questions
If my brain cells were not destroyed, why do I feel so bad?
Because the problem is functional rather than structural. Ionic balance collapses, glutamate floods out, and restoring order consumes more energy than the injured brain can supply. Intact cells that cannot meet their energy demands produce exactly the symptoms you are describing.
Why do my symptoms get worse when I concentrate or look at a screen?
Cognitive and visual work raises energy demand in tissue that is already in an energy deficit. The symptom is the tissue reporting that it cannot fund the task, which is why graded return to load works better than either total rest or pushing through.
Why is a second concussion more dangerous before I have recovered?
While the metabolic crisis is unresolved the tissue has no reserve. A second impact during that window lands on a brain that cannot absorb it, which is the physiological reasoning behind return-to-play restrictions.
Does this show up on any scan?
Not on routine CT or MRI, which resolve structure rather than metabolism. Advanced research imaging can observe aspects of it non-invasively, but in ordinary clinical practice the cascade is inferred from the examination and the symptom pattern, not imaged directly.