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The arterial blood supply to the anterior segment of the eye comes primarily from the ophthalmic artery and is carried to the eye by the anterior ciliary arteries and the long posterior ciliary arteries. The usual seven anterior ciliary arteries follow a course along the four rectus muscles; however, the two long posterior ciliary arteries take an intrascleral course and are located deep to the medial and lateral rectus muscles. The anterior ciliary arteries and long posterior ciliary arteries contribute to several collateral circulatory systems including the episcleral limbal plexus, the intramuscular circulation within the ciliary body, and the major arterial circle of the iris root.1-3 Because of this extensive collateral supplying the anterior segment, ischaemia following strabismus surgery or manipulation of the ciliary arteries is relatively rare.
Anterior segment ischaemia was first documented in experimental animals after ocular surgery.45 Investigators described irregular dilated pupils and iritis after rectus muscle surgery in primates. The first clinical reports of anterior segment ischaemia resulting from ocular surgery date back approximately 50 years.6 Anterior segment ischaemia has now been reported to be associated with retinal detachment surgery, cyclocryotherapy, and laser photocoagulation. Anterior segment ischaemia associated with strabismus surgery has usually been reported when the surgery involves more than two rectus muscles simultaneously. Although scattered reports have documented the unusual occurrence of anterior segment ischaemia in patients having surgery on only two rectus muscles,7anterior segment ischaemia is primarily a complication in adults and is often associated with systemic disease including hypertension, leukaemia, and thyroid disease.8
Because the majority of anterior segment blood flow normally derives from the anterior ciliary vessels, anterior segment ischaemia is rarely associated with long posterior ciliary artery disruption. Under normal circumstances the long posterior ciliary arteries are thought to be responsible for less than one third of the blood flow of the anterior segment of the eye.9 Because the blood supply to the iris is usually sectorial in nature iris blood flow has been felt to serve as an important indicator of the status of the anterior ciliary blood flow in each quadrant. Assessment of the quality of blood flow in the iris has been found to be a useful indicator for the potential risk of anterior segment ischaemia related to strabismus surgery. In the lightly pigmented iris this can be accomplished with standard fluorescein angiography.1011 Iris angiography can demonstrate a delay or absence of iris vessel filling in the quadrant that corresponds to a recently operated on rectus muscle. Iris angiography studies have documented that in normal patients disruption of the anterior ciliary vessels on the horizontal rectus muscles usually does not produce an alteration in iris blood flow that is detectable.12 In contrast, in both experimental animals and in human patients abnormalities of iris perfusion are frequently seen after disinsertion of one or both of the vertical rectus muscles.13 Regrettably, standard fluorescein angiography can only be successfully performed on lightly pigmented irides.
In this issue of the BJO (p 214) Chan and co-workers have successfully demonstrated that indocyanine green angiography can document iris perfusion changes following strabismus surgery even in the dark iris. Their findings demonstrate that delayed iris perfusion persists for 3–22 weeks following strabismus surgery. For that reason the authors have suggested that an interval of 2–3 months should be allowed to pass before additional strabismus surgery be performed. This is an important observation and provides the strabismus surgeon with another tool to evaluate the patient with strabismus in whom anterior segment ischaemia is thought to be a significant risk. However, one should emphasise that re-establishment of iris perfusion on angiography does not guarantee that further surgery on the rectus muscles will not produce anterior segment ischaemia. The two major risk factors for anterior segment ischaemia are the patient's own susceptibility (in small part age related) and the extent of the strabismus surgery itself.
In patients who are felt to be at high risk for the development of anterior segment ischaemia alternative forms of therapy for the strabismus may be entertained including botulism toxin injection, anterior ciliary vessels sparing surgery, and Wright's modified rectus tuck procedure.14 One should also note that there is evidence to suggest that anterior segment ischaemia occurs less commonly following a fornix conjunctival incision than with a limbal one.15
We should recall that there are data that suggest that anterior ciliary vessels generally do not recanalise after primary rectus muscle surgery. Blood flow from the anterior ciliary arteries disrupted by the surgical procedure is thought to be compensated for by collateral flow.13 Thus, iris perfusion studies are not always a direct reflection of the anterior ciliary vessel perfusion in that quadrant. Nevertheless, for the patient who appears to be at high risk for developing anterior segment ischaemia evaluation of iris perfusion would seem to be a prudent part of the preoperative evaluation. Now thanks to the work of Chan and co-workers it appears that such flow studies can be done no matter how pigmented the involved irides.
Contributors please note:
Communications fromall countries exceptthe UK and Republic of Ireland should be sent to Professor C Hoyt, Editor, British Journal of Ophthalmology, University of California, Department of Ophthalmology, 10 Kirkham Street, K 301, San Francisco, CA 94143-0730, USA (tel: 001 415 502-6871; fax: 001 415 514-1521). Manuscripts from the UK and the Republic of Ireland should be sent to Professor Andrew Dick, UK Editor, British Journal of Ophthalmology, Division of Ophthalmology, University of Bristol, Lower Maudlin Street, Bristol BS1 2LX (tel: +44 (0) 0117 929-4496; fax: +44 (0)117 929-4607).