A quick review of ocular physiology will help the anesthesia provider understand the pathology behind retinal damage in patients with glaucoma presenting for surgery requiring steep trendelenburg (ST) positioning particularly when presenting with glaucoma.
Two thirds of the aqueous humor is produced in the posterior chamber by the ciliary body. The remaining third is formed by passive filtration from the vessels on the anterior surface of the iris. The flow of the aqueous humor is from the posterior chamber to the anterior chamber. Production of aqueous humor depends on active secretion of Na+ into the aqueous humor giving it a significantly higher osmotic pressure than plasma. The osmotic gradient between these two compartments (plasma and aqueous humor) plays a large role in determing IOP. This relationship can be described by the following equation: IOP=K[OsmPress AqHumor-OsmPress Plsma]+Cap pressure. Therefore, hypertonic solutions such as mannitol are used to lower IOP by increasing Plasma osmotic pressure.
Another factor affecting IOP is the outflow rate of aqueous humor via the canal of schlemm. In order to arrive at Schlemm's canal, it passes through Fontana's spaces whose size plays a large role in overall IOP. The below equation demonstrates how important these spaces (Fontana's) are in determining the outflow rate of aqueous humor.
Volume of outward flow=r^4(IOP-venous pressure) / 8 n L
r=radius of fontana's spaces, n=viscosity and L=length of fontana's spaces.
The overall diameter of fontana's spaces are distinctly under the control of the anesthesiologist during surgery as they are exquisitely sensitive to miosis and mydriasis. Increased sympathetic discharge as may occur with anxiety or pain can result in mydriasis which causes a dramatic decrease in the overall diameter of Fontana's spaces. Since outward flow of aqueous humor is proportional to the fourth power of the radius of these spaces, this will have a disproportionate impact on overall outward flow.
From fontana's spaces the fluid enters Schlemm's Canal that carries it to scleral vessels that drain into the venous system which eventually drains into the Superior vena Cava. The choroidal venous plexus which drains into episcleral blood vessels ultimately leads to the superior vena cava. Therefore any obstruction along this pathway can increase IOP. Hypercarbia, hypoxemia, and increased metabolic rate have all been found to increase blood volume in the choroidal venous plexus and thus increase IOP. In robotic surgery, the Steep Trendelenburg (ST) position, creates massive venous pooling in the choroidal venous plexus, and as a result, remaining in this position for extended periods will increase IOP proportional to time in ST . Another study was able to demonstrate that this cause of increased IOP can be reversed with a 5 minute return to the supine position . In multivariate analysis in the previous study , the only other factor to result in IOP was increased etCO2. In fact, IOP increased 0.21 mmHg for every 1 mmHg that etCO2 was increased above normal .
Glaucoma is a pathologic increase in IOP (greater than 22 mmHg) to such a degree and for such a duration as to cause damage to the retina. There are two principal types that may concern anesthesiologists, open angle and acute closed angle glaucoma. Open angle simply means the areas of absorption of aqueous humor is not anatomically impaired but are impaired through chronic pathological conditions such as sclerosis of trabecular tissue. Closed angle glaucoma results from closure or narrowing of the anatomic space of the peripheral area of the anterior chamber. Mydriatics such as anticholinergics can cause an acute exacerbation in patient predisposed to this due to congenital anatomical defects of the anterior chamber.
Due to the danger of mydriasis, it was believed by clinicians that atropine was contraindicated in any patient with glaucoma. However, at clinical doses of 0.4mg to a 70 kg person, only a fraction is absorbed by the eye (0.0001mg) . Others have demonstrated that this may not be true for other anticholinergics like scopolamine . However, a study performed in patients with open angle glaucoma with scopolamine patches found no difference in IOP . Therefore, it seems reasonable to conclude at this time that in patients with chronic well treated open angle glaucoma, use of scopolamine for PONV is not an absolute contraindication when undergoing robotic hysterectomy.
The anesthesiologist's main role in the perioperative period is to render patient's unconscious of all surgical stimuli and provide adequate analgesia upon recovery from the anesthetized state. The expectation is that this is done while monitoring the patient carefully to avoid any end organ damage. In most cases this done by utilizing standard ASA monitors. However, in the ST position, retinal damage may occur with no detection of this event from standard ASA monitors. Currently IOP measurements are not standard and require special training. However, this information may be relevant to anesthesiologists caring for patients at high risk for retinal damage such as those with significant diabetes, glaucoma or significant vascular disease. There is evidence that when IOP is greater than 40 mmHg, autoregulation of blood flow to the optic nerve head was compromised . Bonnie Molloy found that in patients in the ST position, the presence of chemosis was predictive of patients with IOP of greater than 40 mmHg. She recommends maneuvers (placing patient in supine position) to reduce IOP if this physical exam finding is found during surgery. Unfortunately, this type of exam can prove nearly impossible during a robotic case when the face is covered with goggles and drapes. Nevertheless, it may be prudent to consider an exam of the eyes during prolonged robotic cases where the ST position is required (i.e. greater than three hours in the ST position).
In conclusion, new technology in the surgical arena creates new organs at risk. In this article the organ at risk given consideration is the retina secondary to malperfusion because of increased IOP. Fortunately, this has not yet proved to be a major cause of blindness in the perioperative period. Unfortunately, current anesthesia practice does not account potential retinal damage arising from IOP. It is suggested that in prolonged robotic hysterectomy cases (i.e. greater than three hours in the ST position) anesthesiologists should consider returning the patient to the supine position for five minutes or examining the eye for signs of chemosis; and if present, requesting a five minute break to place the patient supine. This is particularly important in patients with preexisting glaucoma.
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