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Editors Selection IGR 9-4
Intraocular pressure: Patient positioning and vision loss
Comment by Paul Palmberg on:
19719 Changes in intraocular pressure due to surgical positioning: Studying potential risk for postoperative vision loss, Walick KS; Kragh JE; Ward JA et al., Spine, 2007; 32: 2591-2595
Walick et al. (1270) draw attention to an important problem, which may or may not hold clues to the damage that occurs in glaucoma. Spine surgery has replaced cardiac surgery as the leading cause of postoperative vision loss, with an estimated risk of 0.028 to 0.2% (Baig MN, et al., Neurosurg Focus 2007; 23:E15.) The authors performed their study in the Orthopedics and Rehabilitation unit at Brooke Army Medical Center. Since none of them is an ophthalmologist, it is to their credit that they considered the possible role of patient positioning for spine surgery as a risk factor for the rare loss of vision that occurs. However, it was already well known that the IOP is raised when one is positioned horizontally (supine) or with the head down, both of which are known to be associated with increased episcleral venous pressure, and they did not determine the effect of position upon ocular perfusion pressure. What else do we know about such cases of post-operative vision loss? According to the American Society of Anesthesiologists Postoperative Visual Loss Registry of 93 cases (Lee LA, et al., Anesthesiology 2006; 105: 652-659), 83 cases (55 bilateral) involved ischemic optic neuropathy, and occurred in lengthy cases (average ten hours) with considerable blood loss (average two liters). On the other hand, the ten cases with central retinal artery occlusion were unilateral and occurred in shorter cases with less blood loss. Therefore, the presentation of vision loss in such cases is not typical of glaucoma, but involves ischemic mechanisms that might be made worse by elevation of IOP.
What do we know about the effect of position upon retinal artery perfusion and optic nerve function? Friberg TR and Weinreb RN (JAMA 1985; 253: 1755-1757) found that both the IOP and central retinal artery pressure increased upon gravity inversion in normal volunteers. But while autoregulation maintains perfusion, Sandborn GE, Friberg TR and Allen R (Arch Ophthalmol 1987; 105: 774-776) documented reversible visual field defects in 11 of 19 eyes, suggesting a vision risk.
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