The Ophthalmology Department,  Universiti Malaysia Sarawak (UNIMAS), Kuching, Sarawak.

The Ophthalmology Department, Sarawak General Hospital, Kuching, Sarawak, East Malaysia.

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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,

  • central retinal artery thrombosis and

  • cortical blindness.

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.