Eye rubbing and keratoconus: the UV-A and sunburn analogy

My friend and I both rub our eyes; I have keratoconus but he does not.  How is that possible?”

“Why do I have keratoconus in only one eye?”

“Why is my keratoconus so much more severe than my sister’s? Is it related to how we rub our eyes?”

These are just some of the many questions asked by patients with keratoconus….

In our efforts to explain to our patients the intimate relationship that exists between eye rubbing and keratoconus, we often use the UV AND SUNBURN ANALOGY to shed some light on this interesting phenomenon. In fact, there are many remarkable parallels in the elements and mechanisms of these two totally unrelated entities, and this shall be the focus of this article. These analogies will also explain why eye rubbing leads to keratoconus in some patients, and not in others. 

Sunburn and UV exposure

Sunburn is a radiation burn that affects the skin.

It results from an overexposure to ultraviolet (UV) radiation, commonly from the sun.

Most of us have experienced the ill-effects of a sunburn, which include reddish skin which is warm to the touch, itchy and painful.

It is now universally accepted that the root cause of a sunburn is the UV-A radiation (and to a lesser extent UV-B) from sunlight, and that risk factors include sensitive, lighter skin, and young age.

Keratoconus and eye rubbing

As alluded to in this website and in many other articles (see: “the no rub, no cone conjecture”), the root cause of keratoconus is eye rubbing.

Our etiopathogenic model does not make eye rubbing the only ingredient in the keratoconus recipe, but still an indispensable one.

This point is crucial since the logical consequence of this conjecture is that the cessation of eye rubbing should prevent further evolution of the disease, and even more importantly, that the suppression of excessive eye rubbing would prevent keratoconus from occurring.

Root cause vs risk factor; Why not everyone who rubs gets KC

To better clarify the exact role of eye rubbing in the pathogenesis of keratoconus, and distinguish between direct causative vs risks factors, we will use several analogies between the mechanisms at play in the genesis of sunburn and keratoconus.

Analogy 1:
Root cause: The trigger for mechanical and inflammatory responses

– Sunburn is an inflammatory response triggered by direct DNA damage induced by UV radiation, which alters the mechanical barriers of the skin.

– Keratoconus is a mechanical response in the cornea triggered by direct trauma to the collagen fibers and matrix by the hands and fingers, which incites inflammatory responses in the cornea.

*Comment: In addition, some pre-existing corneal inflammation may also trigger eye rubbing (atopy, allergy, chronic ocular night compression etc) Yet, just as sunburn would not occur without the exposure of the skin to UV radiation, no spontaneous deformation of the corneal dome will occur without the force exerted by repeated eye rubbing.

Analogy 2:
Risk factors: why are some more susceptible than others

-Special populations including children and individuals with lighter skin tone and limited capacity to develop a tan after UV exposure have a greater risk of sunburn. There are also certain genetic conditions, eg xeroderma pigmentosum (XP), that increases a person’s susceptibility to sunburn.

Special eye populations including natively thin corneas and poorer collagen quality have a greater risk of keratoconus. The native corneal thickness is strongly genetically determined.

*Comment: A light and sensitive skin is a well-known risk factor for sunburn. However, having these traits will not be sufficient to develop a sunburn, so long as this type of skin is not exposed to excessive UV radiation. The same consideration can apply to keratoconus; a soft and thin cornea which is never excessively rubbed will never become thinner and warped. The best way to prevent keratoconus is to prevent excessive eye rubbing.

Analogy 3:
Intensity and severity

-The UV Index indicates the risk of getting a sunburn at a given time and location. Contributing factors include time of day, cloud cover, altitude and proximity of the reflective surface (sand, sea, etc.).

-The intensity, type, and duration of eye rubbing are correlated to the risk of developing keratoconus just as the intensity, type, and duration of the exposure to UV radiation is correlated to the risk of getting a sunburn. The use of knuckles and grinding forces during eye rubbing increases the risk and severity of keratoconus.

*Comment: Just as most of us are aware why we have gotten a sunburn (“I shouldn’t have stayed that long under that sun”) we have derived from our clinical experience and interaction with our keratoconus patients that the more frequent, prolonged and intense the eye rubbing, the higher the risk and severity of the keratoconus;

Analogy 4:
Focality

-A sunburn is focal, as it affects only the skin zones which have been exposed to the UV radiation and spares the areas which were covered or protected from it.

-Recent advances in the assessment of the biomechanical properties of the cornea (Brillouin microscopy) have shown that the biomechanical impairment observed in keratoconus corneas is focal, and affects the central or paracentral cornea, sparing its periphery.

*Comment: In keratoconus, the areas which are biomechanically weaker correspond to the zones directly exposed to the shearing forces inflicted by the fingers and knuckles…

Analogy 5:
Position / Laterality

-A person who lies exclusively on their back under the sun has no chance of getting a sunburn on his / her back, but may be burnt on the front and vice versa. In a sunbather who flips over regularly, a sunburn will first appear on the side that has been exposed to the highest level of UV radiation.

-A person who rubs exclusively one eye has no chances of getting a keratoconus on the contralateral eye. In patients who rubs both eyes to different extents, the keratoconus is more pronounced on the side that is rubbed more.

*Comment: The correlation between the severity of keratoconus and the side which is rubbed more frequently and vigorously is striking.

Analogy 6:
Inciting causes

-The application of certain tanning lotions,oils, ointments or fragrance on the skin prior to sun exposure may increase the risk of a sunburn.

-The exposure of allergens (dust mites, pollens etc) from the linen or pillows onto the ocular surface may increase the risk of developing keratoconus. Some sleeping patterns, that exacerbate this exposure, for example, side or front sleeping where one eye is in direct contact with the pillow or mattress during the night, are correlated with a higher risk of keratoconus and increased severity of the deformation.

*Comment: Chronic ocular compression may increase the local temperature and swelling of the cornea, making its tissue more vulnerable to the effects of eye rubbing.

Analogy 7:
Feel good factor

-Sunbathing is a pleasurable activity enjoyed by many people. Despite the risks of sunburn and premature aging of the skin, many people still continue to expose themselves excessively to the sun. Many are sometimes unconscious of their over-exposure, but the subsequent occurrence of a sunburn reveals to them the extent of their exposure to UV radiation and the long hours outdoors.

-Eye rubbing is a pleasurable activity for many individuals, who may not always realize the extent of their habit initially, but even when made aware, may find it difficult to cease rubbing due to the relief it gives from ocular irritation.

Conclusions:

The sunburn analogy is a useful tool to better pinpoint the role played by eye rubbing in the genesis of keratoconus. Just as not all sunbathers will get a sunburn despite spending the same number of hours at the beach, not all eye rubbers would develop keratoconus. For any one condition, risk factors predict the odds for developing that condition.

In the case of keratoconus, these risk factors include heterogeneous genetically inherited traits (related to the corneal thickness and resistance) and susceptibility of the immune system to develop allergies (which incite eye rubbing). However, it is important to reiterate that just as sunburn won’t occur in an individual even if they may have various risk factors predisposing them to a sunburn as long as their skin is adequately protected from UV exposure, no keratoconus would occur in an allergic patient with thin and soft corneas as long as they do not rub them.

The risks of excessive sun exposure to the human body have been promulgated ad nauseam (and rightly so) through various advertisements and campaigns. Recommendations include using skin lotions with high UV index protection (“sunscreen”), the use of proper clothing and hats, and the general avoidance of skin exposure to the scorching rays of the mid-day sun to prevent sunburn.  In the domain of eye medicine, the deleterious role of UVA has also been propagated, as it can increase the risk of cataract. Even despite the lack of true evidence, the idea that blue light (which is not filtered by the cornea) may be a risk factor for age-related macular degeneration (ARMD) has been embraced by the ophthalmic community.

In contrast, the deleterious effects of eye rubbing on the cornea and the eye have not been adequately propagated until recently. Many patients with keratoconus who consulted us for a second opinion had received little or no information from their previous eye care providers on the effects of eye rubbing in keratoconus genesis and progression.

This website is our contribution to compensate for this lack of information.

Ironically, the corneal crosslinking (CXL) procedure aimed at stiffening the corneal tissue in keratoconus relies on the energy brought by UV-A radiation to which the cornea is exposed,  albeit in a controlled manner by a specific delivery system.  The UVA interacts with electrons supplied by a molecule (riboflavin) and oxygen to create covalent bounds between the collagen fibrils of the cornea. The corneal stiffening (which interestingly has never actually been clearly demonstrated in clinical conditions) is probably not sufficient to make the corneal dome resistant to the trauma of vigorous eye rubbing, but the inflammation triggered by CXL followed by corneal desensitization may make the patient less prone to rub his operated eye, which may represent an indirect benefit of the procedure.