@Steel_Nation and @T3hPwnisher, both of you experienced corneal abrasions. The outer layer of the (5 or 6 layered, depending on who you ask) cornea is the epithelial layer. When the epithelium gets scraped or sheared off (by a stick, or an abnormal attachment to a contact lens, etc), it leaves underlying corneal pain fibers exposed. The cornea has one the densest distributions of pain fibers in the entire body, and the degree of pain is proportional to the size of the defect in the epithelium. The pain of a corneal abrasion can be utterly disabling.
As y’all note, topical anesthetics instantly resolve the pain, but only for 20 minutes or so. Unfortunately, topical anesthetics are quite toxic to the cornea, so we can’t simply give patients a bottle to use ad lib. However, this does not prevent the occasional pt from stealing a bottle and self-administering the drops at home. Such self-administration leads quickly to a vicious circle–the toxic anesthetic further damages the cornea, which leads to worsening of the pain, which leads to more frequent anesthetic administration, which leads to worsening of the corneal damage, etc. Every ophthalmologist has had a pt or two literally melt a hole in their cornea in this manner. On occasion, we have to admit such pts to the hospital and keep them under surveillance to prevent them from continuing to use drops. I had one such pt whose cornea continued to worsen in the hospital despite the fact that she had stopped (she swore) using the drops. Turned out she had a bottle of drops hidden under one of her breasts, which wasn’t found until we literally had her strip-searched by the nursing staff.
Fortunately, if not further abused, the cornea re-epithelializes very quickly, so even quite large defects will ‘fill in’ over the course of a few days. Unfortunately, in some individuals (I’m looking at you, Steel), the new epithelial cells fail to ‘put down roots’ properly, and thus are only loosely attached to the underlying basement membrane. When this occurs, trivial trauma will cause these loosely-attached cells to come off, thereby re-creating the original corneal abrasion. This situation is called recurrent epithelial erosion syndrome (REE). The classic REE presentation will sound very familiar to you. What happens is, while the pt is sleeping, a modest adhesion forms between the underside of the upper eyelid and the loose epithelium. Under normal circumstances, such an adhesion would not be strong enough to cause the corneal epithelium to shear away–the adhesion would simply break when the individual opened their eye. However, in an individual suffering with REE, the lid-epithelium adhesion is stronger than the epithelium-cornea attachment, and thus the act of opening their lids upon awakening literally tears away that section of epithelium. So it’s akin to waking up and having someone immediately rake their fingernail across your eye. Not a good way to start your day.
In some individuals (again, Steel) REE will eventually resolve with conservative treatment (moisturizing drops; etc), but others need more aggressive treatment to promote a firmer connection between the epithelium and the underlying tissue. Broadly, such treatment consists of mechanically stimulating the underlying tissue to promote better epithelial adhesion; this can be done by making multiple minute punctures through the epithelium into the main layer of the cornea (the anterior stromal micropuncture procedure), or by scraping off the epithelium and lasing the underlying tissue in a LASIK-like procedure called phototherapeutic keratectomy.