To the Editor:—
The thoughtful discussion of the case report “Unilateral Blindness after Prone Surgery” by Drs. Lee and Lam 1reasonably excluded systemic hypotension and anemia as independent etiologic factors. However, the discussion was deficient in three respects. First, although they claim that the eyes within the soft foam cushion (manufacturer and model unreported) were intermittently “checked” every 30 min, conventional eye checks in the prone position can only be performed by peeling or pressing the foam cushion away from the eyes to visualize them. However, peeling or pressing the foam cushion away from the eyes necessarily changes the relationship of the foam cushion to the eyes;i.e. , the test itself changes the results. The eyes can also be checked by palpation, but this is a blind procedure, which also requires the fingers to peel or press away the foam cushion from the eyes. Furthermore, since the patient's face at the end of procedure was “extremely edematous,” the physical relationship of the eyes to the foam cushion must have been continuously changing in the direction of increasing intraocular pressure. The point in stating these limitations of the traditional eye check is that the new Prone-View foam cushion system (Dupaco) allows the eyes to be continuously and directly visually monitored (by mirror image) without the need to manipulate the foam cushion. 2
Second, the authors failed to comment on the height of the nasal bridge of the patient. A low nasal bridge allows the medial aspect of the eyes to experience greater contact and pressure with the foam cushion, and as the periorbital area becomes relatively more edematous than the nasal bridge, the medial aspect of the eyes will press harder into the foam cushion.
Third, the 15° head down tilt and compression of the abdomen and thorax (not commented upon) of this 80 kg patient (height and body mass index were unreported) may have contributed to increased venous and intraocular pressures, facial edema, and decreased eye perfusion. As the authors point out, the selective effect of neosynephrine infusion on eye venous and arterial hemodynamics is unknown. It will be very important to elucidate the effect of neosynephrine infusion on the vascular supply of the eye since awareness of postoperative vision loss in the anesthesia community is rapidly increasing, and efforts to prevent hypotension by neosynephrine infusion will likely increase.
In summary, we agree that the cause of postoperative prone spine surgery vision loss is multifactorial. Drs. Lee and Lam have done a good job of ruling out systemic hypotension and anemia as independent causative factors in their particular case. We would be greatly interested in their thoughts about the multiple factors of technically inadequate eye checks, the relationship of their patient's eyes to the bridge of the nose, and the effect of 15° head down tilt and increased venous pressures on the etiology of vision loss in their patient.