Most traits we can measure in humans vary continuously. Like blood pressure. Blood glucose levels. Corneal thickness. Plot the measurements you obtain for any such trait across enough individuals and you obtain a bell curve. Individuals at the extremes of the bell curve are more rare... and if the measurement you're making is a physiological measurement, there may be a disease process associated with occupying the extremes of the bell curve.
Extremely high intraocular pressure may mean glaucoma.
Extremely thin corneas may predispose to ectasia.
The threshold concept: there is a certain physiological value or threshold, when crossed, results in disease symptoms.
LASIK patients, on average, have lost more than 40% of their corneal nerve density 3 years after LASIK. This may leave a young male, who is in a low risk group for dry eye asymptomatic, while a middle aged female who has lost a similar % of her pre-operative nerve density may experience debilitating dry eye.
Same situation with contrast sensitivity. All patients lose contrast sensitivity... depending on age and other factors, not all of them experience this loss as disabling/debilitating.
An important point is that all patients who have LASIK lose some visual and neural reserves. This means as they age they may cross the threshold to a symptomatic and/or disease state sooner.
People who never would have lost their ability to drive at night or enjoy comfortable vision may lose these abilities at middle age or even sooner because of laser eye surgery.
The point is that refractive surgery has robbed millions of their visual and corneal nerve reserves.
Millions of Americans have been pushed nearer the threshold for dry eye, loss of functional night vision, and corneal failure (ectasia).