With Bangladesh forced to make not one but two concussion substitutes in the Kolkata Test against India, here's a quick recap on everything you need to know about this new introduction to top-flight cricket.
What is a concussion?
A concussion is a traumatic brain injury that results in temporary loss of normal functioning of the brain. It is usually caused by a blow to the head or a blow to some other part of the body that results in rapid movements of the head. It is not always accompanied with external signs of head trauma, nor is a normal brain scan a reliable test of concussion.
Why are there substitutes for concussion and not other injuries?
Pain caused by other injuries is conspicuous and more obviously debilitating. Concussions are not that straightforward, and if unattended, can leave players susceptible to a second impact because of reduced brain function. Such second impacts can lead to severe neurological complications, and can in some cases be fatal. It is, therefore, critical to take off the field a player who has a concussion or even suspected concussion to eliminate that risk. However, this is a competitive sport played by elite competitors. Cricket, in particular, tends to carry a problematic idea of "courage".
It can be difficult to convince a player to come off or a team to play with effectively 10 men when the symptoms are not that obvious. In a way, despite being more lethal than other injuries, the symptoms - or a possibility - of concussions are easier to brush off.
So, the ICC wants to "lessen the cricketing consequences of a decision to rule a player out", allowing players the chance to receive proper treatment for blows to their head.
What happens during a concussion test?
The symptoms of a concussion typically include nausea, dizziness, visual problems, lack of consciousness, lack of balance or co-ordination, disorientation, confusion, momentary loss of memory. Players are asked if they are feeling a headache or nausea. They are checked on balance. Their memory can be tested by asking the cricketing version of Maddocks questions, such as "what city are we in", "what session of the day is this", which two bowlers are in the attack at present". They might be asked to read signboards.
In rugby, doctors maintain baseline scores on these criteria and test concussions suspects on them. Cricket hasn't yet made baseline testing a matter of routine, but some teams already do that.
Even if a player is allowed to continue, a video review of the injury is carried out to look for immediate symptoms that might have been missed in the direct observation. If such symptoms are present in the video review and not being reported by the player, he still needs to go off the field.
How come Steven Smith and Hashim Amla continued playing then?
Amla, in the World Cup 2019 opener, and Smith, in the Lord's Ashes Test, did carry on batting and later developed concussion symptoms. It is not atypical even in other contact sports for a player to develop concussion symptoms belatedly, which actually makes it an even trickier injury to handle. A concussion can only be properly ruled out 48 hours after the first blow.
The absence of symptoms doesn't mean the absence of concussion, which is why rugby is extremely strict with how it deals with it. A rugby player involved in a collision that typically leads to concussion has no choice but to go off for testing. Although it advises regular observation, and a SCAT5 (Sport Concussion Assessment Tool) test at the first available moment when the player is off the field, cricket is not yet as strict as rugby. Smith, for example, decided by himself that he wanted to carry on. As did Mushfiqur Rahim when he waved off the physio during the Eden Gardens Test.
When asked about this, an ICC spokesperson told ESPNcricinfo: "The concussion protocol in cricket varies from rugby to some extent as cricketers have protective gear in the form of helmets, and hence a lesser proportion of head knocks result in concussions." While that might be true, examples of Smith, Amla, Liton Das and Nayeem Hasan - who all continued to bat for varying lengths of time before availing a concussion substitute - is evidence that might push ICC more towards rugby protocols.
Is a physio qualified to give a concussion test?
Ideally, you want a doctor to do it but, at the moment, not all teams travel with a team doctor. For example, Australia do and India don't. While the ICC has not made it mandatory for teams to travel with a doctor, a physio still remains pretty qualified to take medical fitness decisions on behalf of his team.
However, the ICC spokesperson told ESPNcricinfo of the work being done to regularise the process: "The ICC requires that a doctor who has knowledge of concussion management and the ICC Concussion Management Guidelines be available on match day to provide the necessary support to participating teams if required. The ICC has also been focussing on the professional development of all team healthcare personnel, in particular, the capability of team physiotherapists to oversee healthcare decisions such as concussion."
In time, perhaps the "if required" might be removed and an independent doctor or a team of doctors might become a mandatory presence at all ICC matches.
Can the system be gamed?
Perhaps yes, but you would always expect a team to name their best players in the starting XI; any further change will, at least in that team's estimation, weaken the side. The only conceivable advantage a team can gain by gaming the system is if it realises it has misread a pitch and picked a wrong combination. Not only will it be diabolical to go to such an extent, but the ICC match referee will also likely strike down any such request. The match referee is empowered to disallow a replacement that is not "like for like". Such assessment depends on the role the said replacement is likely to play for the rest of the match. If al allrounder replaces a batsman, he will not be allowed to bowl. A spinner might not be allowed to replace a fast bowler unless he just bats.
How long does it take to return for a concussed player?
Unlike rugby, cricket doesn't lay down a minimum requirement of time off training, but it tends to agree with the rugby's stipulation of a week away. Says the ICC protocol: "Generally, concussion symptoms will settle within two-three days and a player diagnosed with a concussion is ready to return in about a week but, in some individuals, this time might be shorter or longer. Medical oversight, as occurs in elite teams, is essential if a player is to return to play within a week or on a subsequent day of a multi-day match."
Sidharth Monga is an assistant editor at ESPNcricinfo