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Medical Forum / General / Vision / March 2007IOP reduction after a shot.
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. > 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 [quoted text clipped - 10 lines] > > Don W. 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. > > 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 [quoted text clipped - 20 lines] > > - Show quoted text - 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. >In the event of cataract surgery, we are really talking about the lens >capsule being intact to isolate these two humors? > 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. >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. > > 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... So what happens when the posterior capsule is ruptured? Don W. 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. >So what happens when the posterior capsule is ruptured? > >Don W. > > > In the event of cataract surgery, we are really talking about the lens > capsule being intact to isolate these two humors? Not to maintain any sort of "seal" between them. Without the capsule, the vitreous tends to prolapse forward through the pupil and it only has to _touch_ the corneal endothelium to cause problems. > 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. But both are mainly water, and the RI is about the same as water. -MT > > understand. > > But both are mainly water, and the RI is about the same as water. 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. > this is mostly via Schlemm's canal. Sounds both pastoral AND lovely. I'd buy a house alongside there in a minute. |
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