​PRONE POSITION: PERI-OP VISUAL LOSS (POLV), ION, CRAO, CRVO AND INDUCED HYPOTENSION


Visual loss after spinal fusion surgery is a rare but devastating complication with a reported incidence of 0.017 to 0.1 percent.

The major causes of visual loss in this patient population include
1. Ischemic Optic Neuropathy (ION)
2. Central Retinal Artery Occlusion (CRAO) and
3. Central Retinal Vein Occlusion (CRVO).


POSITIONING AND CRAO and CRVO
CRAO and CRVO are attributed to embolic load and/or direct globe compression, emphasizing the need to protect the eyes from direct pressure while in the prone position.



ISCHEMIC OPTIC NEUROPATHY (ION)*****
ION almost universally results in PERMANENT VISUAL LOSS. In 2012, the Postoperative Visual Loss (POVL) Study Group reported the findings of their multicenter case control study that used data from known ION cases registered in the American Society of Anesthesiologists POVL Registry and control cases from 17 United States academic medical centers.

INDEPENDENT RISK FACTORS FOR THE DEVELOPMENT OF ION
1. Male sex
2. Obesity
3. Use of the Wilson frame (head lower than the heart)
4. Longer anaesthetic time
5. Greater estimated blood loss and
6. Lower percent of colloid in the nonblood fluid replacement

Using these risk factors and the reported incidence of POVL from ION in the literature, the POVL Study Group created a risk prediction model that may help surgeons and anesthesia providers modify their care plans and define the risk of ION to patients .

RISK MODIFICATION STRATEGIES FOR ION AFTER PRONE SPINE SURGERY
1. Positioning with the head level with or above the heart (Tilt The Whole Table)
2. Minimizing anaesthetic duration and blood loss ( Staged Procedure if necessary)
3. Use of both colloids and crystalloids for intravascular volume replacement
4. Monitoring hemoglobin periodically during cases with significant blood loss and
5. Optimization of hemodynamics with maintenance of blood pressure within 20 percent of baseline.

Patients scheduled for historically lengthy (longer than four hours) spine surgery in the prone position should be informed of the small but increased risk of ION. When possible, staging of long procedures should be considered.

CONTROLLED/ INDUCED HYPOTENSION


The use of controlled hypotension is NOT recommended in patients undergoing spine surgery. Induced hypotension has historically been advocated as a mechanism to reduce blood loss during a variety of surgical procedures.


WHY LOW BP DOES NOT REDUCE BLOOD LOSS
Reduced wound blood flow as a result of lower arterial blood pressure has been the mechanism cited for reduction in blood loss. However, epidural venous plexus pressure and intraosseous pressure, both important determinants of blood loss in spine surgery, are independent of arterial blood pressure.



INDUCED HYPOTENSION and SPINAL CORD ISCHEMIA
The most important reason to avoid the use of induced hypotension is the potential for end-organ ischemia. In particular, patients with severe spinal stenosis are at risk for SPINAL CORD ISCHEMIA and should be maintained during anaesthesia at or near their usual blood pressure.

In addition, spinal instrumentation and distraction can reduce SPINAL CORD PERFUSION AND RESULT IN ISCHEMIA. Therefore, adequate arterial blood pressure should be maintained during instrumented spinal surgery as one measure to avoid neurologic damage.

INDUCED HYPOTENSION AND POVL


As explained above, visual loss is a rare but potentially devastating postoperative complication of prone spinal surgery. Ischemic optic neuropathy (ION) is the most common cause of postoperative visual loss. Although the causes of ischemic optic neuropathy have not been fully elucidated, tissue edema with reduced perfusion of the optic nerve is a suggested etiology. 
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“Most of the successful people I've known are the ones who do more listening than talking.” —Bernard Baruch  

One of the traits that makes one an excellent clinician is that they are good listeners. The doctor’s distinctive listening tool—the stethoscope. Many children have played game of doctor-doctor as a child and the most important toy in that game is the stethoscope.

This year, 2016, marks the stethoscope’s 200th birthday. Invented by a French physician, René Laennec, in 1816 it is perhaps the true symbol of medicine.

First used in a Paris hospital, the story goes that Dr. Laennec invented the stethoscope because he wasn’t comfortable putting his ear on the chest of a woman to hear her heart.  Born in 1781 and raised by one uncle who was a priest and another uncle who was a medical school dean, Laennec was, like most doctors, a superior student. Although his lawyer-father tried to discourage him from a medical career, he would go on to become the father of clinical auscultation.

According to a very good profile of this pioneering physician by the National Center for Biotechnology Information, Dr. Laennec “wrote the first descriptions of bronchiectasis and cirrhosis and also classified pulmonary conditions such as pneumonia, bronchiectasis, pleurisy, emphysema, pneumothorax, phthisis, and other lung diseases from the sounds he heard with his invention.

He perfected the art of physical examination of the chest and introduced many clinical terms still used today.”   In writing the classic 1819 treatise on his application of clinical auscultation, he offered: “I happened to recollect a simple and well-known fact in acoustics ... the great distinctness with which we hear the scratch of a pin at one end of a piece of wood on applying our ear to the other. Immediately, on this suggestion, I rolled a quire of paper into a kind of cylinder and applied one end of it to the region of the heart and the other to my ear, and was not a little surprised and pleased to find that I could thereby perceive the action of the heart in a manner much more clear and distinct than I had ever been able to do by the immediate application of my ear.”  

Dr. Laennec, who was also trained by Napoleon’s personal physician, died of tuberculosis in 1826 at age 45. According to the NCBI story, “during those last months, he asked his nephew to auscultate his chest and to describe what was heard. The findings were as alarming as they were familiar to this great chest physician who had heard the same sounds a thousand times before. By his own invention, he could no longer escape the ironic truth that he was dying from tuberculosis—the disease that he helped to elucidate and understand with his stethoscope would soon take his life.”  

With today’s amazing technological advances in medical care, one might think the stethoscope will become a relic of the past—an antique. For many doctors maybe that’s the case even today, but Stethoscope still hangs around the necks of all good physicians. It may have advanced with some modifications but the principles remain the same.

Modified from a blog