Injury threshold (IT) = constitution divided by 2, round up.
A character that takes IT (or more) physical damage at once sustains an injury. This injury affects one of the physical ability scores: strength, dexterity, or constitution (determine which randomly). The afflicted score is decreased by 1d6 points.
Further, if the damage causing the injury reduced the injured PC to zero HP, immediately reduce the stat maximum by one permanently.
If the injury is not treated by a doctor or healer during or before the next downtime, the stat reduction is permanent. Put another way, it is dangerous to spend the downtime following an injury in an uncivilized or poorly equipped location.
Any permanent stat reduction from injury results in a visible scar or maiming of some sort. The player may decide how this manifests, or defer to the referee.
A mental injury threshold (MIT) could be handled in a similar manner.
I have been thinking about using a Call of Cthulhu rules base for a dark fantasy survival horror dungeon crawl game. The above injury rules were inspired by reading various Basic Roleplaying variants. Following is the original version I developed for use with BRP.
Damage from a single attack that equals or exceeds a target’s injury threshold (which is half maximum HP) causes an injury. An injury reduces one physical characteristic (appearance, constitution, dexterity, or strength) by 1d6 points. If not treated promptly, this decrease is permanent. Injuries may have additional consequences, such as shock or ongoing damage from blood loss, as determined fictionally.