For scleral contact lens wearers, mid-day fogging is a frequent nuisance. To better understand this conundrum, researchers from the University of Alabama at Birmingham looked into the relation between fogging, ocular surface leukocytes and scleral lens fitting characteristics.

“One of the most common modern complications we see in scleral lens wear is mid-day fogging,” says study investigator Jason J. Nichols, OD, MPH, PhD, associate vice president research and professor at the School of Optometry at The University of Alabama at Birmingham. “Different estimates suggest there is somewhere between 30% and 50% of scleral wearers who have mid-day fogging, and it’s really inconvenient.”

Adding to the inconvenience factor, these patients have to remove their lenses outside of the home in many cases, which means they have to have solutions on hand and the ability to clean the lenses and put them back in, he adds.

Fogging results from particulate matter trapped between the ocular surface and contact lens in the post-lens tear film. As such, there’s been a great deal of speculation about the post-lens tear film and scleral lenses, he says.

“When you have a scleral lens on the eye, you have a post-lens tear film behind it, and there’s not much tear exchange,” Dr. Nichols explains. “In fact, there is very little, if any, tear exchange from behind the scleral contact lens, so the tears are trapped. The question is, ‘What is it?’ People have speculated that perhaps it’s a lipid or protein or mucin.”

The study enrolled 39 eyes including 19 full-time scleral lens wearers, of which 46% had mid-day fogging issues. After at least four hours of wear, the lenses were rinsed with phosphate-buffered saline, and eyes were treated with 5mL of saline per eye. Leukocytes were counted and isolated from the wash solutions and assessed with flow cytometry. Investigators then stained the samples from the post-lens tear fluid with fluorescently labeled antibodies to detect leukocyte distribution.

Researchers found a great deal of leukocytes behind the post-lens tear film bowl of the scleral lens wearers with fogging. They also reported scleral lens corneal clearance was 246 ± 61μm for non-foggers compared with 308 ± 98μm for those who had fogging. On average, the number of leukocytes collected from the scleral lens bowl was greater than the number recovered from the eyewash. Researchers noted the scleral lens corneal clearance was associated with the presence of fogging.

 “If you think about it, leukocytes are white blood cells, and the fogging is a white film that happens in the post-lens tear film,” Dr. Nichols says,” so there seems to be a connection.”

As the scleral lens vaults higher and higher, researchers saw more neutrophils and more leukocytes. “Basically, it shows if you increase the vault, you get more hypoxia and more white blood cells, and this is possibly why fogging occurs,” he says.

While Dr. Nichols believes the study results show leukocytes could be a contributor to mid-day fogging in scleral lens wearers, he says there could be other factors as well.

It seems that hypoxia may be the driver for the release of the leukocytes into the tear film, investigators noted. If you keep the vault to a minimum—200μm or less—you’ll likely get fewer leukocytes and less fogging, Dr. Nichols says. Additional factors to consider would be ways to improve tear exchange and peripheral curves or by using more oxygen permeable material—factors that weren’t part of the study, but still relate to hypoxia, Dr. Nichols adds.

Dr. Nichols and his team may continue the investigation by using different materials and fits to see their impacts on white blood cells and mid-day fogging. 

Postnikoff CK, Pucker AD, Laurent J, et al. Identification of leukocytes associated with midday fogging in the post-lens tear film of scleral contact lens wearers. Invest Ophthalmol Vis Sci. 2019;60(1):226-33.