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Case 37a
by
Professor Chua Chung Nen, Dr. Ashok & Dr. Ngo Chek Tung
A 15 year-old boy developed a swollen and
tight right orbit after undergoing a 6-hour scoliosis operation in prone
position (Figure 1). His head was positioned in a horseshoe headrest during
the operation. The ophthalmologist on-call was consulted. The visual acuity in
the right eye was light perception and the eye movement was limited in all
directions (Figure 2) with a dense right afferent pupillary defect. The fundoscopy
showed swollen optic disc with flamed haemorrhages and retinal pallor with
"cherry red" spot in the macula. The intraocular pressure measured 60mmHg. A
lateral cantholysis was performed and the patient was given intravenous
acetazolamide. Although the intraocular pressure was lowered the vision
remained poor and failed to improve after 24 hours. He was started on oral
prednisolone which helped to reduce the orbital swelling and improve ocular
movement but the vision remained light perception one week post-operative.
The MRI scans (Figure 3, 4 & 5) showed swollen orbit with extraocular
muscles swellings and thickened sclera.
Figure 2. Swollen and tensed right orbit.
Figure 5.
Swollen extraocular muscles with thickened sclera.
a. What are the causes of
postoperative visual loss?
Postoperative visual loss after non-ocular
surgery has an estimated incidence varying from 0.01 to 1% depending on
the type of surgery. Visual loss has been
reported as high as 1 in 500 patients after spine procedures and 1 in 60
patients after cardiac procedures1,2 & 3.
The three recognized causes of
postoperative visual loss4,5 are:
Ischaemic optic neuropathy is the most
frequently cited cause of postoperative visual loss following general
anaesthesia. Recognized intraoperative risk factors for developing
ischaemic optic neuropathy include hypotension and anaemia. ION may be
anterior or posterior depending on the part of the optic nerve involved.
Most cases of post-operative ION involves the posterior optic nerve and
consequently, the optic nerve will appear normal initially in the
presence of relative afferent pupillary defect. In anterior ION, the
optic disc is swollen.
Retinal artery thrombosis is usually
secondary to intraoperative extrinsic pressure on the eye and
has a different opthalmological examination findings compared
with ION.
Cortical blindness is usually the result
of ischaemic stroke to the occipital cortex. The most common cause is
prolonged hypotension. The optic discs appear normal without relative
afferent pupillary defect.
b. What is the most likely cause of visual
loss in this patient?
Central retinal artery occlusion (CRAO) as a
result of prolonged orbital compression from the head rest is the most
likely diagnosis. The limited ocular movement and proptosis are the consequences of
compressive ischaemia to the extraocular muscles.
Improper head positioning in
horseshoe headrest has long been recognized as a case of CRAO because of its firmness and
close position to the eye with poor accessibility. However, other less
rigid headrests such as soft foam cushions and gel pads are also known
to cause CRAO if not positioned correctly.
Figure 7. Improper head positioning in the
prone position causes globe compression
and CRAO as is likely to be the case in our
patient. Pictures from Kumar et al 6.
c. How may the blindness in this
patient be prevented?
CRAO caused
by compression on the
globe can be prevented by careful and frequent attention with
documentation to the patient’s eyes when the patient is positioned prone especially
when using horseshoe headrest.
In cases where
constant ocular checking is difficult during the procedure, Mayfield pins are an alternative to the horseshoe
headrest in the prone position. The pins are designed for rigid skull
fixation and keep the face away from any pressure contacts. However, this
technique does not prevent visual loss from ischaemic optic neuropathy7.
Figure 8.
Mayfield clamp keeps the skull in rigid fixation and
prevents
globe compression.
References:
1. Roth S,
Thisted RA, Erickson JP, Black S, Schreider BD. Eye injuries after nonocular
surgery. A study of 60,965 anesthetics from 1988 to 1992. Anesthesiology.
1996;85:1020-1027.
2. Stevens WR, Glazer PA, Kelley SD, Lietman TM, Bradford DS. Ophthalmic
complications after spinal surgery. Spine. 1997;22:1319-1324.
3. Breuer AC, Furlan AJ, Hanson MR, et al. Central nervous system
complications of coronary artery bypass graft surgery: prospective analysis
of 421 patients. Stroke. 1983;14:682-687.
4. Roth S,
Gillesberg I. Injuries to the visual system and other sense organs. In:
Benumof J, Saidman L, eds. Anesthesia and perioperative complications. 2nd
ed. St. Louis, MO: Mosby; 1999:377-408.
5. Lee LA. ASA postoperative visual loss registry: preliminary analysis of
factors associated with spine operations. ASA Newsletter. 2003;67:7-8.
6. Kumar N,
Jivan S, Topping N, Morrell AJ. Blindness and rectus muscle damage following
spinal surgery. Am J Ophthalmol 2004; 138: 889–91.
7. Lee LA,
Roth S, Posner KL, Cheney FW, Caplan RA, Newman NJ, Domino KB. The American
Society of Anesthesiologists Postoperative Visual Loss Registry: analysis of
93 spine surgery cases with postoperative visual loss. Anesthesiology. 2006
Oct;105(4):652-9; quiz 867-8.
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