what's the difference?

[SusieD]

Can someone please explain:

What is the difference between BUT and tear evaporation? I always thought they were one and the same but when Im reading articles they are talking about them as two completely separate issues.

My understanding is that if you have a fast break up time (BUT) then your tears are evaporating too fast (hence, breaking up)?

can someone clarify for me?

A doc told me that I didnt have evaporative dry eye but I did have a fast BUT - but both are associated with MGD right? Im confused (again!)

Many thanks.

[kaypeeoh]

I can only comment on my own condition. My tear break up time ranges from one second to 5 seconds. Normal would be 10-20 seconds. I have adequate moisture. My eyes don't feel dry. What I've been told is I have a deficiency of lipid. Lipid is secreted by the meibomian glands. The lipid coats the tear film, slowing its evaporation. The small amount of lipid mixes with tears as well, giving the tear film some elasticity. So the tear film doesn't break up as quickly after a blink.

[Tony Barnes]

That was my understanding too Susie - the two would logically go hand in hand, and that had been my interpretation of some Clarymist/Tears Again materials I've read.

However, I just found this STUDY which states they are uncorrelated.... (it is an old one though!)

I'll see if I can get an answer from the manufacturers.

[kitty]

I believe it is the meibomian glands that secrete the lipid portion of tears, which is why MGD results in fast BUT.

[Tony Barnes]

I got this reply this morning Susie:

Hi Tony,

the TBUT indicates for how long the tear film is stable after a blink.

Of course there a several possible reasons for a unstable tear film, e.g. a deficiency of the mucus phase at the epithelial surface.

But the most common reason for an unstable tear film is a deficiency in the tear film lipid layer, which provides stability (e.g. it reduces the free energy of the tear film surface).

Thus, an abnormal lipid layer is linked to increased evaporation and decreased stability in general, although there might be other reasons in individual case.

Attached please find a newer publication of Tomlinson et al.:

“CONCLUSIONS: Where the human lipid layer is absent, or is not confluent, and the tear film is unstable, tear evaporation is increased …”

I hope this is helpful for you. If there are any questions, please feel free to contact me.

Hope it's some help

Tony

[SusieD]

Thanks for that explanation tony - so inevitably, the two are linked as I thought.