Date: 26th January 2012
Perioperative Visual Loss after Spinal Surgery - a rare complication
Perioperative visual loss (POVL) after spinal surgery is a rare and devastating complication.Although uncommon, reports are documented of post-operative blindness in spine and cardiopulmonary bypass surgeries with incidence varying between 0.05 % and 1 %.(Myers et al .A review of 37 cases.Spine.1997).Patil et al (Spine.2008) identified an overall incidence of visual disturbance after spinal surgery of 0.094 %.Spine surgery for scoliosis correction and posterior fusion surgery had the highest incidence of 0.28% and 0.14% respectively.Ischaemic Optic Neuropathy was present in 0.006% of cases.Hypotension,peripheral vascular disease and anaemia were the strongest risk factors for ION development.
The 3 major recognised causes of POVL after non-ocular and non-craniovascular surgery are:(i) Ischaemic Optic Neuropathy (ION), (ii)Retinal Vascular Occlusion (RVO) ,and (iii)Cortical Blindness.
ION following prone position spinal surgery is well reported,although the incidence and risk factors are not well appreciated by many anaesthetists and spinal surgeons(Kamming and Clarke.Br.J.Anaesth.2005.).Recognised pre-operative risk factors are hypertension,diabetes,polycythaemia,smoking,renal impairment,narrow-angle glaucoma,coagulopathy,arteriosclerosis and vasculitides.Recognised intra-operative risk factors for developing ION include hypotension ,anaemia,consequences of the prone position,raised central venous pressure and extrinsic pressure on the globe.With an increase in intra-ocular pressure in the prone position,even modest hypotension can lead to critical ischaemia and both ION and RVO.If the globe is compressed by virtue of lack of eye protection ,including failure to use skull pins,raised IOP can compound the other conequences of the prone spinal surgical position and lead to POVL.
The prone position used in spinal surgery has a number of physiological consequences, aside from the specific necessity to avoid direct globe pressure by using eye padding and skull tongs.It has been demonstrated that the prone position results in an increase in intaocular pressure(IOP).(Hunt et al.J Neurosurg Anaesthesiol.2004).This was confirmed in awake patients.It was also noted that 10 degrees of reverse Trendelenburg position actually normalised the IOP whilst prone.(Ozcan et al.Anaesth Analg.2004).The optic nerve has an interesting blood supply.The posterior part has its main supply from pial vessels derived from the ophthalmic artery.These vessels are unable to exhibit autoregulation and the posterior optic nerve is vulerable to a drop in perfusion pressure as a result of the factors discussed.Fundoscopic examination is often unremarkable.
In cases of extrinsic pressure to the eye ,the ophthalmological diagnosis is commonly RVO, with the fundoscopic appearance of a characteritic 'cherry spot'.Central retinal artery occlusion(CRAO) decreases blood supply to the entire retina ,whereas branch retinal artery occlusion (BRAO) affects a portion.These most commonly result from patient positioning and external compression of the eye complicating the complex prone physiology factors discussed.Kumar et al identified afferent pupil defect,periorbital oedema and bruising, ptosis, proptosis,cloudy cornea etc and use of horse shoe headrest prone (and not skull tongs) in cases of CRAO (RVO).
Roth (Br J Anaest.2009)advocates that in patients positioned prone for surgery, a foam headrest or skull pins should be used. For most procedures prone , Roth advises that any of the commercially available square foam headrests be used.For prone cervical spine surgery Roth however advises against the use of the horse-shoe headrest because of the greater risk of head movement caused by the surgeon and surgery, with compression of the eye.I agree- although the skull tongs are invasive and do cause scalp trauma, they are mandatory for prone cervical spine surgery to protect the eyes from extrinsic pressure, and allow accurate and stable positioning during surgery on the cord and posterior fossa..
Avoidance of raised CVP by careful chest padding to allow an uncompressed abdomen and no constricting ties around the neck are also well recognised adjuncts to reducing POVL in spinal surgery.The use of eye padding and skull tongs is superior over a padded horse-shoe in protecting against the devastating complication of POVL in prone cervical spinal surgery,evethough the skull pins do cause trauma to the scalp.
The mechanism of POVL due to RVO(either CRAO or BRAO) is usually due to venous congestion or arterial occlusion.The retina has a dual arterial blood supply-central retinal artery for the inner layer and choroidal plexus for the outer layer.
Kamming and Clarke report that the American Society of Anaestesiologists(ASA) cites that 81% of POVL is diagnosed as ION and the remainder as RVO and that 67% of all the reported cases of POVL in spinal surgery occurred in the prone position.
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