The principal blood supply to the anterior segment of the eye is from the anterior ciliary arteries, which travel in the four rectus muscles with anastomoses to the conjunctival and anterior Tenon’s vessels (Figure 53). These arteries are the major blood supply to the anterior segment. The rest of the blood to the anterior segment is supplied by the two long posterior ciliary arteries that come forward intrasclerally at the 3- and 9-o’clock positions. Both sets of arteries originate from the ophthalmic artery. Each of the rectus muscles has two anterior ciliary arteries, with the exception of the lateral rectus which has one.
Figure 53
There are seven anterior ciliary arteries, two each in the superior, inferior, and medial rectus muscles. The lateral rectus has one. The anatomy of these vessels is subject to marked variation.
These arteries lie within the muscle until near the tendon when they exit the muscle and travel in the muscle capsule. There they are visible until reaching the muscle’s insertion where they enter sclera to form the episcleral circle. From the episcleral circle, blood flows to the intramuscular circle and from there flows into multiple branches of the major arterial circle supplying blood to the ciliary muscle, ciliary process, and iris. This circle is discontinuous. The anterior ciliary arteries furnish 70 to 80% of the blood supply to the anterior segment. The long posterior ciliary artery bypasses the episcleral circle and joins the intramuscular circle. The recurrent ciliary artery serving the choroid is supplied by the intramuscular circle. The long posterior ciliary arteries provide less than 30% of the blood supply to the anterior segment. Of the two long posterior ciliary arteries, the medial provides the most blood (Figure 54).
Figure 54
Schematic of the blood supply of the anterior segment from Saunders, et. al.
ACA = anterior ciliary artery
IMC = intramuscular circle
LPCA = long posterior ciliary artery
RCA = recurrent choroidal artery
From Saunders RA, et al. Anterior segment ischemia after strabismus surgery. Survey of Ophthalmology, 1994, 38(5):456-466. Used with permission.
As a rule, at least one anterior ciliary artery should remain undisturbed by strabismus surgery to avoid anterior segment ischemia. However, anterior segment ischemic changes have been seen after as few as two muscles have been detached. Conversely, all four muscles have been detached at one procedure or in two, staged procedures separated by months or years without producing anterior segment ischemia, in some cases. Orge, et. al. have shown that detachment of rectus muscles can reduce blood flow in the ophthalmic artery 30%, presumably because of ‘downstream’ effects. Such a change in susceptible individuals could lead to acute anterior segment ischemia.
There is no evidence that severed anterior ciliary arteries ever recanalize to nourish the anterior segment, but time could be a factor in establishing collateral circulation. The conjunctival circulation and its contribution to the anterior segment have been discussed as factors that influence the type of conjunctival incision to be made. The cul-de-sac incision, which disrupts conjunctival circulation less than the limbal incision, has been suggested as being safer. In contrast, Awaya has detached all four rectus muscles at a single procedure to lower intraocular pressure as an alternative to cyclocryotherapy in advanced glaucoma. Patients achieved lowered intraocular pressure without experiencing the typical clinical signs of anterior segment ischemia.
The medial anterior segment circulation is the most protected because it is supplied by two anterior ciliary arteries and a long posterior ciliary artery (Figure 55). The superior and inferior quadrants are the least protected because they have no long posterior ciliary artery. In clinical practice, it is not known with certainty what factors ultimately influence the postsurgical dynamics of anterior segment circulation in a given case. Some useful guidelines follow:
-
When a muscle is detached and reattached, anterior ciliary vessels do not recannulate.
-
Because there are no long posterior ciliary arteries superiorly or inferiorly, detachment of the superior or inferior rectus muscles disrupts iris vessel filling more than detachment of the horizontal recti.
-
Older, vascularly-compromised patients may be more likely than young, healthy patients to develop anterior segment changes after eye muscle surgery.
-
Iris angiography is a valid way to assess anterior segment circulation at a given time, but it is not a valid or practical predictor to determine what might happen if eye muscles are detached.
-
Anterior segment ischemia is rare, fortunately.
-
If anterior segment ischemia occurs, it should be treated with atropine and frequent instillation of topical steroids.
A |
B |
Figure 55
A Normal iris filling after preoperative intravenous injection of fluorescein in a 30-year-old man.
B First postoperative day after detachment and transfer of the superior and inferior rectus muscles. Note superior and inferior sector filling delay.
From Olver J, Lee JP: Eye 3:318-326, 1989.
A variety of eye muscle transfer procedures, including the Jensen tendon-muscle splitting transfer, are designed to spare one anterior ciliary vessel in each of the split muscles. However, von Noorden has pointed out that the surgeon should look for anomalous anterior ciliary vessels that may crowd to the half of the muscle that is pulled over for the transfer and should then avoid ligating both vessels since this would leave no anterior ciliary vessel in the undisturbed portion of the muscle.
Both McKeown, et. al., and Roth, in separate papers published in 1989, described a technique for detaching a rectus muscle while sparing the anterior ciliary arteries (Figure 56). The technique requires meticulous dissection aided by magnification with the operation microscope. Because of the low incidence of clinically significant anterior segment ischemia and the difficulty of this procedure, its use is likely to be highly selective.
Figure 56
The anterior ciliary arteries are dissected from the superficial capsular muscle tissue allowing repositioning of the muscle while leaving blood flow undisturbed.
Print this Page