When I'm examining the front surface of the eye in the slit lamp microscope, I'm carefully evaluating your eyelids, lashes, conjunctiva, and cornea.  The cornea is the clear tissue that acts as the window for light entering your eye -- because it is clear light penetrates unobstructed through your pupil and onto the retina for visual processing.  But when the cornea is not clear, this can greatly impact vision and ocular health long term.  Scars, corneal degenerative conditions, inflammatory events, or oxygen deprivation can all permanently cloud the cornea.  These areas of cloudiness generally aren't reversible, but  if it has happened already to you, it's your doctor's job to make sure that you know everything you need to do to prevent further corneal opacification (cloudiness).  A real common finding I see in the exam chair looks like this:

The above picture shows pannus caused by contact lens overwear/abuse.

Do you see that whitened spot at the bottom of the cornea?  There are little blood vessels inside which you can faintly see within the whitened portion.  This is a mild case of corneal pannus, about 1 mm in size which is what I will often see in my patients.  Pannus is growth of vascularized tissue within the otherwise avascular cornea -- to make that simpler, you have blood vessels growing into your cornea which causes a cloudy/whitened area within a tissue that is ideally perfectly clear for maximum light penetration.

What Causes Pannus?

1. Oxygen Deprivation From Contact Lens Wear: When you wear contact lenses, the material each lens is made from has a certain number of oxygen particles that can pass through the material in a given time.  This breathability score is called Dk/t and a quick google search can tell you what the oxygen transmission is for your current contact lens brand.  Most of today's most popular multiple wear lenses have really high Dk/t scores -- many over 100 units which is very highly breathable.   But if you don't throw your lenses away at the appropriate time, or you sleep in your contact lenses when you don't have the proper lens to do so, you are risking starving your cornea of that vital oxygen it needs. If the body let's your cornea starve for oxygen underneath your lenses, your cornea will die -- that means blindness!  Instead, it tries to save you by shooting blood vessels from the conjunctiva or the white of the eye into the cornea to help get by, but the result is pannus -- a permanent cloudy place that you will never be able to see through again.   When your doctor sees pannus they know that your contact lens habits aren't working for you -- that means changing to a higher oxygen lens material, changing your wear schedule, or if severe enough you may have to permanently change to glasses wear only.

   Much more severe pannus in a patient with limbal stem cell deficiency via

2. Chronic Inflammation: Not all of my patients with pannus are contact lens abusers.  Many of them suffer from chronic inflammatory conditions like dry eye, previous severe viral infections of the cornea, or injuries like burns.  Pannus is like scar tissue, showing areas that have been damaged by acute events or chronic irritation.  What's going on in dry eye that could cause this kind of damage?  Your tear film is the cornea's only protection from the harsh outer world -- ceiling fans, vents, bright sun, staring at the computer or your cell phone for hours at a time.  It's a hard world out there.  When you don't have an adequate tear film, your cornea is exposed to these harsh environments, and additionally the tissue begins to become inflamed.  Dry eyes are red and irritated because of the inflammation they are associated with -- the lower tear volume on your eye's surface means an excessive amount of inflammatory mediators hanging out on the eye.  Their job is to trigger your eyes to start watering to put moisture onto the eye when it is needed, but with that watering comes more inflammatory mediators and you enter a feedback circle of continuous eye irritation.  When I see pannus in a patient with ocular rosacea or dry eye, it means that we need to significantly increase lubrication with eye drops or gels, and it's probably a great time to consider a topical anti-inflammatory like Restasis or pulses with steroids like Lotemax or Alrex.