IOP reduction after a shot.

Discussion in 'Optometry Archives' started by Don W, Mar 13, 2007.

  1. Don W

    Don W Guest

    When the eye receives a shot of Macugen, Avastin, or Lucentis, the
    IOP has a tendency to rise because of the volume of the injected
    drug. After awhile, the pressure reduces (hopefully) to some nominal
    value (i.e., the extra volume leaves). Question, what is the
    mechanism of this volume reduction? The aqueous humor has its own
    source and drain, and is blocked from the vitreous (wikipedia tells
    me, quote from them) "In health the aqueous humour does not mix with
    the firm, gel-like vitreous humour because of the lens and its Zonule
    of Zinn between the two."

    So if the shot is into the vitreous, how does that pressure reduce?
    Through the aqueous humor drain? Somehow?

    Don W.
     
    Don W, Mar 13, 2007
    #1
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  2. The momentary increase in IOP will be equilibrated immediately by
    draining off aqueous which is being compressed by the increase in
    vitreous volume transmitted through the crystalline lens (forward
    thrust) and this is mostly via Schlemm's canal. Then, the excess fluid
    in the vitreous (the shot) is gradually absorbed via the choroidal
    vasculature, and the normal homeostatic mechanisms (both routes of
    drainage) prevail.
     
    William Stacy, O.D., Mar 13, 2007
    #2
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  3. Don W

    Don W Guest

    Dr. Stacy, Thanks.

    In the event of cataract surgery, we are really talking about the lens
    capsule being intact to isolate these two humors?

    Probably not related, but it is a stunning coincidence that the
    aqueous humor and the vitreous humor have the same refractive index,
    1.336, to three decimals. They are completely different liquids, as I
    understand.

    Don W.
     
    Don W, Mar 13, 2007
    #3
  4. Right. they will cut a hole in the front of the capsule through which
    the nucleus/cortex are removed. The posterior capsule maintains the
    barrier, unless through complication, it is ruptured in which case
    vitreous can enter the anterior chamber, as was common with old style
    lens extraction.
    Well, there is some liquefaction that takes place as you age, so I think
    there's a fair amount of ordinary aqueous fluid back there in the
    vitreous body as well, and some of the glaucoma meds rely on the choroid
    to filter out excess aqueous, so it's not a completely closed system...
     
    William Stacy, Mar 13, 2007
    #4
  5. Don W

    Neil Brooks Guest

    Sounds both pastoral AND lovely.

    I'd buy a house alongside there in a minute.
     
    Neil Brooks, Mar 13, 2007
    #5
  6. Don W

    Don W Guest

    So what happens when the posterior capsule is ruptured?

    Don W.
     
    Don W, Mar 13, 2007
    #6
  7. Not much, as it would have to be a very large break for the vitreous to
    sqeeze through, but I could imagine a large hole allowing it to happen
    with some serious eye pressure. I haven't seen vitreous in the A/C
    since the bad old days of intracapsular extractions... If it reaches
    the endo, as someone said, bad news for corneal health.
     
    William Stacy, Mar 14, 2007
    #7
  8. Don W

    Don W Guest

    understand.
    Hard to believe that a gel would match the same RI of the aqueous
    fluid. But as you are pointing out, they are both 99% water. The
    water content of the vitreous is a big surprise to me.

    Don W.
     
    Don W, Mar 14, 2007
    #8
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