The reported incidence of ION after CABG is from 0.06% to 1.3%.
3,
4 Also the incidence of POVL after CABG has shown an increase over the years. This is possibly due to the heightened awareness of this complication.
5 ION can be anterior ischemic optic neuropathy (AION) or posterior ischemic optic neuropathy (PION). Johnson et al suggested three sites of ischaemia leading to POVL; occipital cortex, juxtalaminar optic nerve supplied by short posterior ciliary arteries (causing AION) and orbital optic nerve supplied by pial capillary plexus( causing PION).
6 AION typically presents with diminished vision, field defects and swollen optic disc which atrophies over a month. PION presents with a normal disc initially which atrophies later. In the cases being reported by us both had features suggestive of AION though the second patient had only an arcuate field defect without significant loss of vision in the right eye.
The risk factors for ION after CABG include prolonged CPB, lower haematocrit, high dose of inotropic drugs, hypotension and large volume of fluid infusions.
4 Nuttall et al showed preoperative coronary angiography 48 hours before CABG, postoperative Hb concentration of < 8.5 gm/dl and atherosclerotic vascular disease as significant risk factors but diabetes mellitus and smoking were not significantly associated with POVL.
3 Mansour et al showed that severe anaemia in diabetics undergoing CABG had a significant risk of developing ION and reversal of anaemia in the patients could salvage the vision.
7 CPB itself can induce shower of thromboemboli affecting the optic nerve perfusion and Kalyani et al showed an absence of any77 case of PION in their series where cardiac surgery was performed without CPB.
8,
9 But Holy et al found no significant haemodynamicfactors in the perioperative period in patients with PION following major surgical procedures.
10
In the cases reported by us the patients had risk factors for atherosclerotic vascular disease like hypertension and dyslipedimia (case 1 & 2), CPB (case 2), anaemia with haemoglobin concentration of < 8.5gm/dl (case 1) and carotid atherosclerotic plaque with stenosis (case 2).
The cause of ION after major surgeries including CABG is multifactorial and varies from patient to patient.
2 But the optic nerve is the most susceptible to vulnerable watershed zone as shown by Shapira et al since none of the patients in their series with ION (n=8) had cerebrovascular accident or myocardial infarction.
4 Altering any haemodynamic parameter to avoid this very rare complication of major surgery may be more harmful than beneficial. Also there is no proven effective treatment once the ION has set in.
2
These two cases are being reported because of the rarity of occurrence and the implications of severe visual loss in a patient undergoing non-ocular surgery. Lack of valuable perioperative data prevented us from evaluating for various other significant risk factors in these patients. Also a literature search regarding postoperative visual loss after CABG did not reveal any major series from India. Evaluation of POVL after CABG in centres with high volume CABG procedures can help to identify the risk factors as well as the magnitude in Indian scenario and help in understanding further about this rare manifestation.
