Injury to the eyes is a well-known problem during non-ophthalmic surgery performed under general anaesthesia : corneal injury is the commonest . Although most of the factors responsible for such injuries remain elusive [2,3], unprotected eyes and pressure on the eyeball have been mostly blamed [4,5]. Closure of the eyelids with adhesive tape and the application of ocular lubricants has reduced such injuries in patients operated upon in the supine position [5,6]. As it seemed that little is documented about keratoconjunctival injury, and the value of ophthalmic ointment on its incidence, in surgical procedures performed in the face-down prone position, we decided to conduct a study using Rose-Bengal dye which stains  dead and dying epithelial cells of both cornea and conjunctiva.
After obtaining approval from our Institute's Ethics Committee and informed consent from patients or their parents, we conducted this prospective randomized investigation in 90 ASA I-II patients scheduled for elective cranial or spinal cord surgery in the prone position. Preoperatively, patients who had painful and/or red eye(s), visible corneal opacity, previous corneal pathology, past eye surgery, contact lenses, documented/suspected allergy to eye ointment and adhesive tape, proptosis, history of any other ocular disease and those unable to communicate properly with the investigator, were excluded. After induction of anaesthesia, endotracheal intubation and establishment of stable general anaesthesia, the patients were divided randomly into two groups. Patients of Group 1 did not receive any eye ointment before eye closure, but Group 2 patients received antibiotic eye ointment (Neosporin®, Burroughs Wellcome Ltd.) in the lower cul-de-sac of each eye, as a lubricant.
Before turning the patients into the prone position, the eyes were closed by applying adhesive tape to the lids. A small 4 cm2 of sterile cotton-filled sponge was placed over each eyeball and secured with a long strip of adhesive tape. The whole of the patient's face, including both the eyeballs and the forehead, was covered with two appropriately sized, cotton-filled sterile sponges approximately 4-cm thick. These sponges were partially split in the middle, to accommodate the endotracheal tube, and were secured with adhesive tape at the level of both forehead and mandible. The horse-shoe of the operating table for receiving the patient's head was cushioned with extra cotton pads . In the prone position, the patient's forehead rested on the margin of the head-rest and every effort was made to avoid pressure on the eyeballs. Due to the close association between hypotension and external pressure over the eyes as a recognized complication of general anaesthesia , any patients who became hypotensive (mean arterial pressure <60 mmHg) for any reason were excluded from the study.
When surgery had been completed, the patients were turned supine and the eyepads gently removed. Before antagonizing the residual effect of the muscle relaxant, we tested both eyes of each patient with sterile Rose-Bengal dye 4% strips to detect any devitalized (dead or about to die) tissue of the cornea and conjunctiva. Gentle irrigation of the eyes with physiological saline was performed in Group 2 to remove the ointment before proceeding with the dye test. In positive cases the area of staining was noted.
The total duration in the prone position was recorded. All patients were followed up for 24 h in the intensive care unit for any fresh ocular complaint. Statistical analysis was performed with software Sx version 4.0 for Windows. The t-tests and χ2-test with Yate's correction factor were used as applicable; P < 0.05 was considered significant.
Six patients (one from Group 1 and five from Group 2), were deleted from the study (see above). Thus, 44 patients in Group 1 and 40 patients in Group 2 were analysed. The two groups were comparable in respect of age, body weight and duration in the prone position (Table 1).
No patient of either group had staining of the cornea, but 32 (70.4%) patients in Group 1 and 22 (55%) patients in Group 2 showed conjunctival staining with the Rose-Bengal dye (P = 0.14). Among these, irrespective of the stained areas, only one eye was involved in 25 patients of Group 1 and 12 patients of Group 2 (P = 0.12). The remaining patients had bilateral conjunctival staining (P = 0.12) (Table 2). A significantly greater number of patients in Group 1 had single-eye conjunctival staining in the six o'clock position (19 and 6 patients in Groups 1 and 2, respectively, P = 0.04). Five patients in Group 1 and seven patients in Group 2 (P = 0.35) had staining in the six o'clock position of both eyes; the remaining patients had staining in areas other than the six o'clock position (8 and 9 patients in Groups 1 and 2, respectively, P = 0.34). Neither the operative area (cranium or spine), duration of prone position nor patient's age had any bearing on the incidence of conjunctival injury.
We found a decreasing trend for the development of conjunctival injury in patients who had ointment instilled; nevertheless, use of the ointment did not reduce the overall incidence of conjunctival abrasions significantly. The majority of patients who developed conjunctival injury did so at the six o'clock position of the eyeball - the incidence of unilateral conjunctival injury at this site was significantly less in the ointment group.
We have no explanation for damage to the conjunctiva only with sparing of the cornea. The reasons for the conjunctival damage - mostly in the six o'clock position - are also unclear. In a closed eye, the upper lid margin meets the lower one at the inferior region of the globe. The eyeball is globular in shape and the convex cornea in its centre is surrounded by sclera covered by conjunctiva. The slope of the globe decreases from the centre of the cornea towards the periphery. Thus, the force of contact between eyelid margins and globe is maximum at the most convex part of globe, i.e. the cornea and inferior pericorneal region. In the head-flexed position, the eyeballs roll upwards but the compensatory vertical eye movements - because of active vestibulo-ocular reflexes - restores the position of the eyes in space. Patients with paralyzed extraocular muscles from neuromuscular blocking agents are unlikely to have an effective vestibulo-ocular reflex; therefore, their eyeballs remain persistently deviated upwards when the patient's head is flexed. The prone position itself may produce some degree of head flexion resulting in upward rolling of the eyeball. The eyelid margins then come into contact with the inferior sclera with its covering of loose conjunctival epithelium. Thus, instead of cornea, the inferior sclera now becomes the most anteriorly placed portion of the globe and so bears the brunt of any pressure injury. This might have been the reason for highest incidence of injury to the conjunctiva in the six o'clock position.
Craniospinal surgery often requires heavy surgical instruments for craniotomy and cutting the vertebrae; this may involve application of vigorous force to the skull and vertebrae. These manoeuvres also produce movement that culminates in a sheering force between the body and its supporting structures. The apposed and often inwardly pushed (because of pressure) lid margins stay over the inferior sclera when patients are in the prone position; sheering movements of the relatively tough lid margins against the lower conjunctiva might have produced more injury in the six o'clock position than in other areas. The eye ointment, by decreasing the intensity of such sheering forces, might have reduced the incidence of abrasion in the lower conjunctival areas where pressure and sheering effect were presumably at a maximum.
No patient developed corneal or eyelid injury and none complained of visual impairment in the post-operative period. Application of cotton pads over eyes, face and head-rest probably protected the eyes from such complications. In conclusion, despite all preventive measures asymptomatic conjunctival trauma occurs frequently during neurosurgery performed in the face-down or prone position. Prior instillation of ophthalmic ointment does not decrease the overall incidence of conjunctival injury, but it reduces the incidence in a subgroup of patients who sustain conjunctival trauma at the six o'clock position.
B. K. Biswas
P. K. Bithal
N. S. Lamba
Department of Neuroanaesthesia; All India Institute of Medical Sciences; New Delhi, India
Department of Ocular Pharmacology; All India Institute of Medical Sciences; New Delhi, India
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