Rigorous eye/eyelid rubbing can change the corneal shape permanently, such as in Keratoconus. Rigorous or 15min-duration hot compresses can also change the corneal shape temporarily but it is not permanent from what I could find in the literature. If your vision is blurry after a long session of warm compresses, it is likely due to the release of meibum oil: the vision should recover in a few minutes with blinking. If more than a few minutes go by before the vision can be “blinked-clear” it may be possible the heat has changed the corneal shape and it should return to normal in less than an hour.
I have not seen a report where warm compresses, performed 15min x 2 per day without burning the skin, has caused permanent corneal shape changes.
Clearly, though, do not use hot compresses enough to burn the skin.
Gentle warm compresses with blinking and gently pushing on the glands (ie, not rubbing) is likely the safest option for those who have Meibomian gland loss and have to apply warm compresses daily.
Here are some articles that help explain the issue.
Warm Compresses and the Risks of Elevated Corneal Temperature With Massage
To quantify the changes in corneal temperature resulting from intensive warm compress (WC) application with minimal pressure and to review the significance of these changes within the context of the peer-reviewed literature.
WC were applied intensively and unilaterally at 45 ± 0.5°C for 30 minutes with the contralateral eye serving as a control. Outer upper eyelid and central corneal surface temperatures were measured using an Infrared pyrometer at baseline. The WC were removed for repeat measurements of the outer upper eyelid surface and central corneal temperatures every 2 minutes and replaced with a new WC heated to 45 ± 0.5°C. Lid and corneal temperatures were monitored for 10 minutes after the final WC application.
The mean age of the subjects was 37.1 ± 15.0 years (n = 12). The mean maximum outer upper lid temperature of 42.2 ± 1.3°C was reached after 6 minutes. The mean maximum corneal temperature of 39.4 ± 0.7°C was reached after 8 minutes of heating. The control eye showed no significant change in temperature from baseline throughout the experimental period.
These data show that WC use for lid warming, even when only minimal contact pressure is applied, also transfers significant heat to the cornea. Corneal temperatures reach peak temperature after about 8 minutes of WC application. Recent reports discussing the increased potential for transient and long-term corneal molding subsequent to the heat and pressure of WC application are briefly reviewed.
Warm compresses applied with gentle pressure are used therapeutically in many ophthalmic conditions. Results of recent studies have reported that topographic corneal irregularity and visual and/or refractive change may be induced with the application of warm compresses with accompanying ocular or lid massage. In addition, a literature review has suggested that, compared with the application of pressure alone, heat applied with pressure can induce greater changes in corneal surface asymmetry or regularity indices.
This study quantified the change in corneal temperature during warm compress application with minimal pressure to the closed right eyes of 12 healthy adults (mean age, 37 years). Compresses were heated to 45°C and applied for 2 minutes, after which time they were removed for temperature measurements of the eye and cornea with an infrared pyrometer, followed by replacement with a newly heated compress. The 2-minute cycles with temperature measurement and warm compress replacement were repeated over a 30-minute period. The left eye served as an untreated control.
Lid and corneal temperatures were significantly elevated during the application of warm compresses and minimal pressure. The mean maximum outer upper lid and central corneal temperatures were 42.2°C and 39.4°C, respectively, and were reached at 6 and 8 minutes; both temperatures were significantly higher than respective mean baseline temperatures of 35.4°C and 35.9°C (P < .0001). There were no significant lid or corneal surface temperature changes from baseline observed in control eyes, and baseline temperature values did not differ significantly between test eyes and control eyes.
This small study showed that there is significant heat transfer to the cornea with the application of warm compresses and minimal pressure. Should continuous heating methods be used to maintain the heat of the compress, there is potential for higher temperatures to be reached over a shorter time period.