Individuals with meibomian gland dysfunction (MGD) have reduced enzymatic activity of sphingomyelinases (SMases) in tears, according to a study published in Ocular Surface. They may also have altered sphingolipid (SPL) composition in both meibum and tears.
Researchers collected data from 50 participants (mean age 57±9 years, 84% men) and stratified them according to meibum quality (n=25 good quality, n=25 poor quality). They analyzed meibum and tears samples to quantify the following SPL classes: ceramide (Cer), hexosyl-ceramide (Hex-Cer), sphingomyelin (SM), sphingosine (Sph), and sphingosine 1-phosphate (S1P). The team performed an ocular surface assessment consisting of tear breakup time (TBUT) measurement, corneal fluorescein staining, and Schirmer testing. They collected subjective symptomatology data using the Ocular Surface Disease Index (OSDI) and 5-Item Dry Eye Questionnaire (DEQ-5).
Investigators found that neutral and acid SMase (nSMase and aSMase, respectively) levels were lower in individuals with poor meibum quality compared with those who had good quality meibum. They determined that aSMase activity in tears negatively correlated with SM in meibum and tears and positively with Sph in meibum and S1P in tears.
Investigators highlight associations with age, noting that older individuals had lower SMase activity in tears compared with younger individuals (nSMase: ⍴=-0.41, P =.003; aSMase: ⍴=-0.25, P =.08). They noted no differences in SMase activity with respect to gender, race, and ethnicity.
Both subjective assessments revealed that nSMase was positively associated with dry eye symptoms (DEQ5: ρ=0.29, P =.045; OSDI: ρ=0.30, P =.03). Lower SMase activity was associated with MGD, particularly meibomian gland dropout.
“The findings of reduced SMase activity in the tears of participants with poor quality meibum with increased SM and decreased S1P suggest that SPL metabolism is altered in the MGD group,” according to the researchers. “Such alterations in the setting of MGD may point to a role in alterations in SPL metabolism playing a role in the development of the disease.”
Study limitations include the potential for geographic bias and the use of separate eyes to measure SMase activity and tear SPL.
Sanchez V, Galor A, Jensen K, Mondal K, Mandal N. Relationships between ocular surface sphingomyelinases, meibum and tear sphingolipids, and clinical parameters of meibomian gland dysfunction. Ocul Surf. 2022;25:101-107 doi:10.1016/j.jtos.2022.06.003