Cases & Commentaries

Blind Spot

Commentary By Lorri A. Lee, MD

The Case

A 36-year-old woman with no significant past
medical history underwent right nephrectomy in the left lateral
position. The surgery was uncomplicated—her blood pressures
intraoperatively were within 20% of her baseline, and she did not
have significant blood loss. Immediately after surgery, the patient
complained of “blurriness” in one eye. This was
attributed to eye ointment applied during the surgery and no
further work-up was pursued.

Two weeks later during a clinic visit, the
patient complained of blindness in the left eye. She was emergently
referred to an ophthalmologist and diagnosed with retinal ischemia
as a complication of surgery. Three months after surgery, the
patient still had partial visual loss.

The Commentary

This case illustrates the insidious and
surprising nature of many cases of postoperative visual loss
(POVL). Though not well appreciated by many clinicians, POVL may
occur in relatively healthy patients, even when the hemodynamic and
surgical course appeared to be uneventful.(1) Although the incidence may be as low as 1 in 61,000
non-ocular surgical procedures, for selected procedures it has been
reported to be as high as 1 in 500 patients (after spine
procedures) and 1 in 60 patients (after cardiac
procedures).(2-4)
Because of a perceived increase in POVL, the American Society of
Anesthesiologists (ASA) Committee on Professional Liability
established the ASA POVL Registry in 1999. This registry collects
detailed information on anonymous case submissions of non-ocular
POVL and currently contains 129 cases.

Major Postoperative Visual Loss
Lesions

The most common POVL defects are (i) anterior
ischemic optic neuropathy (AION), (ii) posterior ischemic optic
neuropathy (PION), (iii) central retinal artery occlusion (CRAO),
and (iv) cortical blindness. Specific ophthalmologic findings
determine the diagnosis.(5)
AION occurs at the optic nerve head as the optic nerve and retinal
vessels enter the globe. PION occurs anywhere posterior to the
optic nerve head to the optic nerve chiasm. Early funduscopic exam
will demonstrate disc edema in AION, whereas the fundus is entirely
normal in appearance with early PION. Many days to weeks later,
when disc edema has usually resolved, the funduscopic exam reveals
optic nerve pallor in both AION and PION, and the two entities are
indistinguishable at this point (Figure 1). Pupillary light reflexes in AION and PION
are either absent or delayed. Patients may complain of total
blindness or altitudinal field cuts. In just over half of the ION
cases in the ASA POVL Registry, both eyes were affected, consistent
with a systemic event.(6)
Complete recovery from either AION or PION is rare, and patients
are frequently left with a visual field defect, at a minimum.

AION has been associated primarily with
cardiac bypass procedures but can occur during prone spine
operations and other procedures. AION can also arise spontaneously
in the community with or without predisposing vascular
disease.(7,8)
PION is most commonly associated with spine procedures in
the prone position and bilateral radical neck operations. Although
numerous contributory factors for AION and PION have been proposed
(e.g., hypotension, anemia, edema, venous congestion, variations in
ocular anatomy and physiology), none has been causally linked in
randomized controlled trials or animal models. Data from the ASA
POVL Registry have disproved the erroneous theory that ION is
caused by pressure on the globe. Of the 43 spine patients with ION
from the ASA POVL Registry, eight patients had their heads placed
in Mayfield pins (Figure
2
) with their eyes free from external pressure.(6) These data clearly demonstrate that ION occurs in the
absence of pressure on the globe. Spine surgery cases with ION in
the ASA POVL Registry are associated with large operative blood
loss (median 2.3 liters) and long duration in the prone position
(median 8 hours). Since ION has been reported outside the operative
arena in critically ill patients, it seems unlikely that anesthetic
agents such as thiopental and volatile agents are significant
etiologic factors.(9)

CRAO is the sudden blockage of the blood supply
to the retina, often referred to as a “stroke” of the
eye. Patients typically present with acute, unilateral, painless
visual loss, and they rarely recover any vision. Findings on
physical exam consistent with CRAO include an absent pupillary
reflex or relative afferent pupillary defect. Funduscopic
examination reveals a pathognomonic cherry red spot at the macula,
narrowed retinal arterioles, and a pale and edematous retina. CRAO
can be caused by globe compression, emboli, venous congestion, and
vasculitis. CRAO caused by globe compression is usually associated
with periorbital trauma (e.g., bruising, proptosis, paresthesias,
or extraocular muscle paresis). The horseshoe headrest (Figure
3
) has long been associated with CRAO because of its firmness
and close position to the eye with poor accessibility. However,
soft foam cushions and gel pads can also cause CRAO if not
positioned correctly.

Cortical blindness is the loss of sight due to an
organic lesion in the visual cortex. Peripheral vision is
frequently affected, but total blindness may occur as well. Both
eyes are usually involved, and approximately two thirds of cases
may ultimately have some improvement in vision.(5) The physical examination of patients with cortical
blindness demonstrates normal pupillary light reflexes with a
normal funduscopic exam. Cortical blindness is usually associated
with embolic events or profound hypotension and most commonly
occurs in association with cardiac bypass operations.

Early Ophthalmologic Exam

The patient in this case was apparently not
examined early for visual loss, and we are not supplied with
follow-up information, so it is unclear if the patient’s
blurry vision in the recovery room was related to her subsequent
blindness in one eye 2 weeks later. Worsening or delayed visual
loss postoperatively implicates AION as the probable diagnosis.
Most cases of PION do not progress in the postoperative period if
the patient has stable hemodynamics, and CRAO usually presents with
complete loss of vision in the affected eye. Though we lack
detailed ophthalmologic exams on this patient, her clinical course
most closely resembles AION with progression or onset of visual
loss after discharge.

An ophthalmologic consultation and exam with
funduscopy should be performed as soon as possible on any patient
with POVL to rule out acute angle-closure glaucoma. Numerous drugs
given in the perioperative period (e.g., adrenergic agonists,
cholinergics, anticholinergics, histamine receptor antagonists) may
predispose to this condition. Though rare, it is one of the few
forms of POVL that is treatable if therapy is instituted promptly.
Moreover, early funduscopic exam allows easier distinction between
AION and PION, as the two lesions appear identical at later time
points.

Delayed Complaints of Postoperative
Visual Loss

Presumably, this patient either had significant
worsening, or onset, of her visual loss after discharge. Patients
may not alert their clinicians about early POVL, either because of
altered mental status or their mistaken beliefs that visual changes
are caused by residual anesthetic effects. Occasionally, visual
field defects in ION may be subtle, and patients may not recognize
or report them until several weeks after discharge. That being
said, the occurrence of complete blindness in one eye is
sufficiently dramatic and concerning that most patients will report
it promptly. Visual loss secondary to AION has been reported to
occur anytime from the immediate postoperative period to a few
weeks after surgery. Patients with significant gastrointestinal
bleeding have also been reported to develop AION between 1 and 3
weeks after their hemorrhage.(10)
Alternatively, an unremarkable hospital course, as in this case,
leaves open the possibility that the development of AION in some
patients may be coincidental timing and not necessarily related to
“perioperative stress.”

The finding of retinal ischemia in this patient 2
weeks postoperatively is most likely a misinterpretation of the
findings of optic nerve ischemia. Perioperative ION is best
diagnosed by either neuro-ophthalmologists or ophthalmologists who
specialize in ION. The actual funduscopic findings, which we are
not provided, are essential to making a diagnosis of the lesion.
Although retinal ischemia is a finding consistent with CRAO (and
not with AION), CRAO is not associated with progressive loss of
vision. However, we cannot entirely rule out the possibility that
the patient had complete blindness in one eye that went unreported
for 2 weeks, making CRAO a remote possibility in this case.

Diagnostic Studies for POVL

Screening for POVL after surgery is usually
reserved for high-risk procedures such as head and neck procedures,
cardiac bypass, and major spine operations, and generally involves
gross testing of vision in each eye. Complaints of even mild visual
changes after any procedure should be followed up with
ophthalmologic consultation if symptoms persist once the patient is
alert and able to provide a reliable exam. Ophthalmologic exam is
the only essential test for the workup of POVL and should include
testing for visual acuity, intraocular pressures, color
discrimination, gross visual fields, pupillary reflexes, and
funduscopy with pupillary dilation. The ability to read letters and
numbers or count fingers does not preclude the possibility of
scotoma or discrete visual field deficits as may occur in ION. If
possible, formal visual fields performed in the
ophthalmologist’s office prior to discharge may be helpful
for determining the extent of the visual defect and provide a
baseline comparison for future exams.

MRI may demonstrate occipital cortical strokes in
the event of cortical blindness, but it is not currently useful for
the diagnosis of ION. Electroretinograms are sensitive detectors of
retinal ischemia and will detect CRAO, but these are rarely used,
as funduscopic exam is sufficient for this diagnosis. Visual evoked
potentials (VEPs) detect abnormalities of the optic nerve and its
projections to the occipital cortex. VEPs are useful in the
diagnosis of ION before optic nerve pallor has emerged if visual
acuity cannot be assessed. Currently, VEPs are unreliable when used
in the presence of general anesthesia, whether volatile or total
intravenous-based, and therefore have not been utilized as an
intraoperative monitor of optic nerve function.(11)

Treatment

In the case presented here, an earlier exam and
follow-up might have been helpful in diagnosis, but it is unlikely
any meaningful treatment would have been possible. High-dose
steroids, hyperbaric oxygen treatment, mannitol, and furosemide
have been utilized for treatment of ION with inconsistent results.
Restoration of the mean arterial pressure to baseline and treatment
of significant anemia are frequently suggested interventions by
consultants, regardless of the diagnosis. However, no randomized
trials are available to document the efficacy of any
intervention.

Prevention

What can be done to prevent POVL? Data
delineating specific risk factors are lacking, making the
implementation of preventive measures difficult. However, certain
steps seem sensible. CRAO caused by pressure on the globe can be
prevented by careful and frequent attention (with documentation) to
the patient’s eyes when positioned prone or when objects are
close to the eyes. Mayfield pins are an alternative to the
horseshoe headrest in the prone position when the eyes cannot be
adequately protected or frequently checked during the procedure.
The usual safeguards to prevent air emboli (e.g., make operative
site lower than heart when possible to reduce venous pressure
gradient; de-air intravenous fluid bags and have air filters for
rapid infusion devices; consider an air-aspiration central venous
catheter for high-risk procedures) should be employed to prevent
cortical blindness. However, many of these cases are thought to be
caused by particulate emboli and thus may not be easily
preventable. Because the etiology of ION is unproven, any
interventions should be evaluated in terms of both their potential
efficacy and their potential to cause harm.

Take-Home Points

  • Postoperative visual loss is an
    uncommon, but not rare, complication of surgery, particularly for
    spine and cardiac surgery.
  • Patients with postoperative visual loss
    should be evaluated by an ophthalmologist as quickly as
    possible.
  • Isolated ION is not caused by pressure
    on the globe.
  • ION has many proposed risk factors, but
    none have been proven; thus, there are no evidence-based prevention
    strategies.
  • Postoperative visual loss caused by
    pressure on the globe results in CRAO and is usually associated
    with other signs of periorbital trauma. Frequent eye checks with
    documentation should be performed intraoperatively for all
    procedures performed with the patient in the prone position.
  • Mayfield pins will prevent CRAO from
    pressure on the globe, but not ION. The horseshoe headrest makes it
    difficult to perform adequate eye checks and provides little margin
    for head movement without compressing the ocular globes.
  • Except for acute angle-closure glaucoma,
    there are no proven beneficial treatments for postoperative visual
    loss.

Lorri A. Lee, MD
Director, ASA Postoperative Visual Loss Registry
Assistant Professor, Department of Anesthesiology
University of Washington

References

1. Lee LA, Lam AM. Unilateral blindness after
prone lumbar spine surgery. Anesthesiology. 2001;95:793-795.
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go to PubMed ]

2. 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.
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3. Stevens WR, Glazer PA, Kelley SD, Lietman TM,
Bradford DS. Ophthalmic complications after spinal surgery. Spine.
1997;22:1319-1324.
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4. 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.
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5. 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.

6. Lee LA. ASA postoperative visual loss
registry: preliminary analysis of factors associated with spine
operations. ASA Newsletter. 2003;67:7-8.
Available at:
http://depts.washington.edu/asaccp/ASA/Newsletters/asa67_6_7_8.pdf.

Accessed January 28, 2005.

7. Hayreh SS, Joos KM, Podhajsky PA, Long CR.
Systemic diseases associated with nonarteritic anterior ischemic
optic neuropathy. Am J Ophthalmol. 1994;118:766-780.
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go to PubMed ]

8. Characteristics of patients with nonarteritic
anterior ischemic optic neuropathy eligible for the Ischemic Optic
Neuropathy Decompression Trial. Arch Ophthalmol.
1996;114:1366-1374.
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9. Lee LA, Nathens AB, Sires BS, McMurray MK, Lam
AM. Blindness in the intensive care unit: possible role for
vasopressors? Anesth Analg. 2005;100:192-195.
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10. Hayreh SS. Anterior ischemic optic
neuropathy. VIII. Clinical features and pathogenesis of
post-hemorrhagic amaurosis. Ophthalmology. 1987;94:1488-1502.
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11. Wiedemayer H, Fauser B, Armbruster W, Gasser
T, Stolke D. Visual evoked potentials for intraoperative
neurophysiologic monitoring using total intravenous anesthesia. J
Neurosurg Anesthesiol. 2003;15:19-24.
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Figures

Figure 1. Funduscopic exam of (A) normal
fundus and early posterior ischemic optic neuropathy
(PION)—early PION has completely normal exam; (B) early
anterior ischemic optic neuropathy (AION) demonstrating narrowed
retinal arterioles and blurred optic disc margin/swelling; and (C)
late AION and PION demonstrating optic nerve pallor and loss of
disc edema. Source: Hayreh SS. Clinical features of non-arteritic
and arteritic AION. Available at: http://webeye.ophth.uiowa.edu/dept/AION/7-AION-features.htm.
Accessed June 6, 2005. Reprinted with permission from Dr.
Hayreh.


Figure 2. Patient undergoing major spine
surgery in the prone position with the head in Mayfield pins and
the eyes free from pressure. Note facial edema, which is present to
some degree in all major procedures in the prone position.


Figure 3. The horseshoe headrest’s
design puts the majority of the pressure of the face on the outer
rim without utilizing the chin or midface (i.e., cheeks/maxilla)
for pressure support. There is a very small margin between the
headrest and the eye. If the face is moved on the headrest during
surgery, significant pressure can be applied to one eye.