Visual acuity and the near triad:
In this demonstration we first tested visual acuity using a Snellen visual acuity chart. Remember that there are two numbers: the numerator gives the distance in feet from the chart to the subject, the denominator indicates the size of the test letter. In general, the denominator is the distance at which a “normal corrected eye” could read the letter in feet. (i.e. someone that can only read a 20/400 letter can see a letter at 20 feet that a “normal” eye could read from 400 feet away).
We then noted that when a person goes from reading at distance to a near target three things happen. The eyes converge (move towards the nose), accommodate, and the pupils constrict. This is called the near triad and is useful clinically to determine where a problem in pupil reflexes has occurred.
In this demonstration we viewed the retina in a dilated eye. We viewed the optic nerve and the central retinal artery. The artery enters at the nerve and then splits into various branches to supply the retinal surface with blood (the other major blood supply for the retina is the choroid). We also discussed that the nerve is given a clinical rating for the proportion of the central cup to the entire nerve. An increase in the amount of cup to overall nerve is a sign of progressing glaucoma. Remember that at the optic nerve is the reason we have a blind spot.
In this demonstration the layers of the cornea were visualized using a bio-microscope (slit lamp). The cornea (along with the tear film) is the main refractive surface of the eye. Of the approximate 60D of power that the average eye has, 40D are from the cornea and the other 20D are from the lens. The cornea is comprised of a highly organized stroma of collagen fibers between two cell layers. The outermost layer is the epithelium. This layer is fast healing and fast growing. The epithelium replaces itself entirely every ten days or so. The endothelium is the layer of cells that cover the inner layer of the cornea. This layer does not heal well or replace cells.
The cornea has no blood supply. This means that nutrients for the cornea have to be absorbed. Most oxygen is absorbed from the corneal surface. This is why contact lenses can cause neo-vascular growth. The lens can block oxygen to the cornea if not worn correctly, or worn too long. The oxygen starved tissue then grows new blood vessels to attempt to replace the oxygen it is not receiving. New lens materials attempt to avoid this problem by being more oxygen permeable. Nutrients also enter the cornea by being absorbed from the aqueous humor through the back surface of the cornea. The cells of the endothelium then pump fluid back out of the cornea to maintain a proper level of hydration. If the endothelium becomes damaged then the corneal stroma will swell and lose its organization and transparency.
In this demonstration we looked at the direct response of the pupil to light. We then viewed the consensual response by shining the light into one eye and watching the pupil in the non-illuminated eye constrict. We discussed the clinical importance of this reflex in determining the location of a problem.