How do you detect invisible symptoms?

After a mild concussion, people generally start to feel like they are back to normal within 1-2 weeks. Symptoms start to resolve, and if being evaluated by a professional, patients will usually show a return to baseline scores on common tests such as SCAT and ImPACT within this window post-concussion. But is their brain really healed?

Unlike a broken bone where you can take an x-ray and see directly if the injury is healed, a head injury is assessed indirectly by measuring brain function. This is like assessing a broken arm by seeing if the person can move their arm — a significant break would be detectable, but a small fracture would be a lot harder to diagnose. Similarly with head injuries, a person with a minor concussion may have recovered enough after one or two weeks to the point that brain function is no longer affected, but that doesn’t mean the injury is completely healed.

An interesting way to assess the invisible symptoms that may persist is by placing the patient in more stressful conditions during a concussion test, specifically having them breathe air with a low oxygen concentration. Two examples of this we found in the literature are from Ewing et al. 1980 and Temme et al. 2013. Subjects in these studies had experienced a concussion 1-3 years (Ewing et al. 1980) or 0.6-9.7 years (Temme et al. 2013) prior. None of the subjects who had previously had a concussion showed any symptoms under normal oxygen ("normoxic") conditions, but when stressed with low oxygen ("hypoxic") conditions, the “recovered” concussion patients showed clear deficits relative to the healthy control group, specifically with regards to memory and judgment tasks. All subjects returned to normal functioning once they returned to breathing normoxic air. This suggests that the brain is able to compensate for deficits under normal, low-stress conditions, even when damage persists for years after the initial injury.

Temme et al. (2013) hypothesized that even though a person may be able to function normally after a concussion, their brain is using a lot more energy to complete the task. By adding the extra layer of difficulty (low oxygen), the brain is no longer able to hide the injury. Going back to the broken arm analogy, you might be able to lift a light object with your broken arm and think that it’s fine, but if you try to lift something heavy, you’ll be able to tell that something isn’t right.

This begs the question of, how can you really tell for sure that a head injury is healed? Regan et al. (2017) make the argument for preforming standard concussion tests under hypoxic conditions. While in theory this may make sense, in practical terms, it can be difficult and expensive to set up the necessary equipment, particularly in settings such as an athletic training room or smaller doctor’s office. We may be biased, but we see the Brain Gauge as an easier and cheaper alternative. We acknowledge that the side-by-side testing has not been performed to see how standard concussion tests under hypoxia stack up against Brain Gauge testing under normoxic conditions, but looking at biology and how the Brain Gauge works, we hypothesize that results should be similar. Brain Gauge tests are based on measuring how well pathways are working at a very basic level—it doesn’t matter how much mental energy you put into performing the task, your perception of what you feel shouldn’t change. Thus if you have even minor damage that may not show up as symptoms on other concussion tests, the Brain Gauge should still be able to detect the damage.

Regardless of what methods you use to assess brain damage, it is important to be aware that being symptom-free does not equate to a healed brain. Just like you would put a broken arm in a cast until it is healed, your brain needs to be protected from additional damage until it has completely healed. Using the right tools to assess and measure recovery is key to safely returning your brain to a fully functional state.


  1. Ewing R, McCarthy D, Gronwall D, Wrightson P. Persisting effects of minor head injury observable during hypoxic stress. Journal of Clinical and Experimental Neuropsychology. 1980 Oct 1;2(2):147-55.

  2. Regan PM, Bleiberg J, Onge PS, Temme L. Feasibility of using normobaric hypoxic stress in mTBI research. Concussion. 2017 Aug 22;2(3):CNC44.

  3. Temme L, Bleiberg J, Reeves D, Still DL, Levinson D, Browning R. Uncovering latent deficits due to mild traumatic brain injury by using normobaric hypoxia stress. Frontiers in neurology. 2013 Apr 30;4:41.

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