You can have a concussion without losing consciousness. Blacking out is only one of four ways to meet the medical definition of mild traumatic brain injury. More than 90% of concussions involve no loss of consciousness at all. Losing consciousness also does not predict who develops lasting symptoms.
You never lost consciousness. You were not taken off the field on a board. You may not even have gone to an emergency room. Weeks later, you still cannot read a screen without a headache. You lose the thread of a conversation. And you have been told the imaging was normal, so nothing is wrong. That combination is common. It is not evidence that you are fine.
Can you have a concussion without losing consciousness?
The American Congress of Rehabilitation Medicine set the criteria, and the World Health Organization revised them. Under those criteria, a mild traumatic brain injury is defined by a Glasgow Coma Scale score of 13 to 15, measured 30 minutes after injury. That score comes together with any one of the following: loss of consciousness lasting under 30 minutes, post-traumatic amnesia (memory loss) lasting under 24 hours, an altered mental state at the time of the accident such as confusion or disorientation, or a transient (brief) neurological deficit.
Read that list again. Loss of consciousness is one item on a menu of four. The other three are far easier to miss. Feeling dazed at the roadside, losing a few seconds of memory, or a moment of double vision all meet the definition on their own. A concussion diagnosis has never required blacking out.
Definition per Lefevre-Dognin C, et al. Definition and epidemiology of mild traumatic brain injury. Neurochirurgie. 2020;67(3):218–221. doi:10.1016/j.neuchi.2020.02.002
Can concussion symptoms show up days later?
They can, and after an injury with no blackout they often do. The chemical disturbance described in what a concussion does to the brain keeps unfolding for hours to days, while adrenaline at the scene and attention on the car, the neck or the police report cover up what is already wrong. The headache, the fog and the light sensitivity tend to surface when ordinary load returns: the first full day at a screen, a loud office, a night that does not deliver sleep.
A late start does not make it a lesser injury or a different one. What matters is that the symptoms followed the blow and that someone examines the systems that explain them, which is what concussion diagnosis and testing covers. One exception is not a clinic question: a headache that keeps building, repeated vomiting or deepening drowsiness in those first days goes to an emergency department the same day.
How many concussions happen without losing consciousness?
This is not a clinical impression. It has been measured again and again in large surveillance studies, and the numbers are consistent.
- In a study of 17,549 high school and college football players, 888 had at least one concussion. Only 8.9% involved any loss of consciousness — while 86% involved a headache.
- A separate national high-school surveillance year covered 544 concussions. In it, 4.6% lost consciousness, while 93.4% reported headache.
Depending on the population, that puts the share of concussions with no loss of consciousness at roughly 91% to 95%. Blacking out is the exception, not the rule. Treating it as the threshold for evaluation means missing the large majority of these injuries.
Guskiewicz KM, et al. Epidemiology of concussion in collegiate and high school football players. Am J Sports Med. 2000;28(5):643–650. doi:10.1177/03635465000280050401 · Meehan WP, d’Hemecourt P, Comstock RD. High school concussions in the 2008-2009 academic year. Am J Sports Med. 2010;38(12):2405–2409. doi:10.1177/0363546510376737
Does losing consciousness mean a worse concussion?
This is the finding that matters most. It is also the one least often shared with patients. If blacking out marked the serious injuries, it should predict who is still symptomatic months later. It does not.
A prospective study of 1,507 NCAA athletes tracked who went on to develop post-concussion syndrome. That was defined as symptoms persisting four weeks or longer. Recurrent concussion roughly doubled the odds. Retrograde amnesia (memory loss for events before the injury), difficulty concentrating, light sensitivity and insomnia each raised them. Loss of consciousness did not. It showed no association with developing persistent symptoms at all.
So consider the single feature most often used at the roadside, on the sideline and in the claims file. It is used to sort “real” concussions from imagined ones. Yet it carries no predictive weight for the outcome that actually disrupts a person’s life. Meanwhile, the symptoms that do predict it — trouble concentrating, sensitivity to light, disrupted sleep — are exactly the ones dismissed as subjective.
Zuckerman SL, et al. Predictors of postconcussion syndrome in collegiate student-athletes. Neurosurg Focus. 2016;40(4):E13. doi:10.3171/2016.1.FOCUS15593
Why do so many concussions go undiagnosed?
When the threshold for taking a head injury seriously is whether someone was knocked out, the injuries that do not meet it simply go unrecorded. Across two concussion clinics, 486 patients were asked a question. Had they ever taken a blow to the head that was followed by concussion symptoms but never diagnosed as a concussion? Nearly one third — 30.5% — said yes.
Those patients were not describing trivial events. When they were later concussed again, they had higher symptom scores than patients without a prior undiagnosed injury. The first injury did not stop counting because nobody wrote it down.
Meehan WP, Mannix RC, O’Brien MJ, Collins MW. The prevalence of undiagnosed concussions in athletes. Clin J Sport Med. 2013;23(5):339–342. doi:10.1097/JSM.0b013e318291d3b3
Can you have a concussion with a normal CT scan?
A CT scan in the emergency department looks for bleeding, a skull fracture, or a mass effect that would require a neurosurgeon that night. It rules those out, and that is a real and important thing to know. But it is not designed to detect the diffuse, microstructural and physiological disturbance that produces concussion symptoms. A normal result does not exclude that disturbance.
A normal CT means you are not in immediate surgical danger. It is the beginning of the evaluation, not the end of it.
What we look for instead
Concussion is a clinical diagnosis supported by targeted testing. It is not a radiology finding. One of the most useful areas to examine is the vestibulo-ocular system — how the eyes and the balance system work together.
Consider children and adolescents assessed at a multidisciplinary concussion program. Vestibulo-ocular dysfunction was present in 28.6% of those with acute sports-related concussion. It was present in 62.5% of those already in post-concussion syndrome. Its presence was associated with roughly four times the odds of going on to develop post-concussion syndrome. It was also associated with a median symptom duration of 40 days rather than 21.
This is findable on examination. It does not appear on a routine CT. And it will not be found by anyone who stops looking because the patient never lost consciousness.
Ellis MJ, et al. Vestibulo-ocular dysfunction in pediatric sports-related concussion. J Neurosurg Pediatr. 2015;16(3):248–255. doi:10.3171/2015.1.PEDS14524
Why this matters beyond the clinic
Say your injury is tied to a motor vehicle collision, a fall, or an incident at work. Then the absence of a documented loss of consciousness will be used to argue that no significant injury occurred. The evidence above does not support that argument. But it can only be answered with a record — a documented examination, objective findings, and a clear account of function before and after.
The time to build that record is now. Not after the symptoms have been blamed on stress and the file has closed.
Frequently asked questions
Can you have a severe concussion without losing consciousness?
Yes. Blacking out does not mark the serious injuries. In a study of 1,507 college athletes, loss of consciousness showed no link to who developed post-concussion syndrome. Here, that means symptoms lasting four weeks or longer. A repeat concussion, memory loss for events before the injury, trouble concentrating, light sensitivity and insomnia did raise the odds. An injury with no blackout can still leave symptoms that last.
What are the symptoms of a silent concussion?
The early signs are easy to miss: feeling dazed or confused at the scene, losing a few seconds of memory, or a moment of double vision. In the weeks that follow, people describe headaches, trouble reading a screen, losing the thread of a conversation, sensitivity to light and poor sleep. In the studies on this page, 86% to 93% of concussions involved a headache.
If you lose consciousness, is it a concussion?
A brief blackout counts. Losing consciousness for under 30 minutes is one of four ways to meet the medical definition of a mild traumatic brain injury. That goes alongside a Glasgow Coma Scale score of 13 to 15. The other three are memory loss lasting under 24 hours, confusion or disorientation at the time of the accident, and a brief neurological deficit. Any one of the four is enough.
What will an ER do for a concussion?
The emergency department usually orders a CT scan. It looks for bleeding, a skull fracture or a mass effect that would need a neurosurgeon that night. Ruling those out matters. But the scan is not designed to detect the diffuse, microstructural injury behind concussion symptoms. A normal CT means you are not in immediate surgical danger. It is the start of the evaluation, not the end.
If you took a blow to the head and something still is not right — whether or not you blacked out, and whether or not a scan was called normal — we will evaluate it properly. Call (314) 887-5866.