Cataract is a common cause of visual impairment in the elderly population [1,2] and has a significant impact on patients' quality of life . Cataract surgery is the most commonly performed ophthalmic procedure, and it is estimated that 8 million cataract operations are performed worldwide every year . Due to advances in anaesthetic and surgical techniques, the majority of cataract operations are currently performed under ophthalmic local anaesthesia, during which the patient is awake and aware of his surroundings.
Previous research has indicated substantial national and international variations in anaesthesia management strategies for cataract surgery [5,6]. The most common forms of local anaesthesia include injection techniques (retrobulbar, peribulbar and sub-Tenon's blocks) and topical anaesthesia. Preferences of surgeons and anaesthetists, as well as the characteristics of patients influence the choice of anaesthetic management for cataract surgery .
The local anaesthetic agent is injected into the muscle cone during retrobulbar, outside the muscle cone during peribulbar and around the globe during sub-Tenon's block. These techniques can on rare occasions cause serious complications that require cardiopulmonary resuscitation [8-10]. Topical anaesthesia involves instillation of anaesthetic eye drops or gel into the conjunctival sac and is commonly used with sedation . Sedation may allow patients to tolerate unpleasant procedures while maintaining adequate cardiorespiratory functions and the ability to respond to verbal commands. However, it is recommended that intravenous (i.v.) sedation should only be administered by anaesthetists  since it is associated with a definite risk of adverse medical events . General anaesthesia is generally reserved for uncooperative or paediatric patients.
Unlike other types of surgery under local anaesthesia where the operative field is shielded from the patient's view, ophthalmic surgery is performed on the organ concerned with vision, and many patients fear that they might be able to see during surgery . Several recent studies have demonstrated that most patients retain at least some light perception in the operated eye during cataract surgery under regional or topical anaesthesia [13-21]. Many also experience a wide variety of other visual sensations, the details of which are summarized in Table 1.
Current standard anaesthesia or ophthalmology textbooks do not discuss the phenomenon of retained visual sensations in the anaesthetized eye. Anaesthetists and ophthalmologists often give adequate technical information about cataract surgery, its anaesthesia and perioperative management to patients but usually very little or no information regarding potential intraoperative visual experiences. Since many patients are concerned that they might be able to see during surgery, it is important that healthcare professionals should be aware of the potential and inform the patient what to expect.
Visual sensations and different forms of local anaesthesia
The majority of patients experience a variety of visual sensations in their operated eye during cataract surgery under retrobulbar [15,16,18,22], peribulbar , sub-Tenon's [19-21] or topical anaesthesia [15,17,19]. These include perception of light, movements, flashes, colours, surgical instruments, the surgeon's hand or fingers, the surgeon and changes in brightness of the operating lights (Table 1).
Several generalizations can be derived from these studies. Up to 20% of patients have no light perception at all [13-18,20,21], but most experience at least light perception at some time during surgery [13-17, 19-21]. A higher proportion of patients receiving topical anaesthesia retain light perception (89.7-100%) [15,17,19] than those with sub-Tenon's (81.0-87.8%) [19-21], peribulbar (80.9%)  or retrobulbar anaesthesia (80-84.3%) [14,16]. A possible explanation for this observation is that topical anaesthesia has no direct effect on the optic nerve , therefore the function of the optic nerve is not disrupted in any way.
The published studies generally show that most patients simply experience perception of light, colours and movement whereas, with the exception of patients seeing surgical instruments reported by Levin and colleagues  fewer than 25% perceive more detailed images such as surgical instruments, the surgeon or the surgeon's hands and fingers (Table 1).
The images patients perceive during cataract surgery vary considerably between patients . One senior ophthalmologist has provided a detailed description of his personal visual experiences during phacoemulsification . Several professional artist [26,27] and non-artist patients [13,28] have documented their visual experience in the form of paintings. Some patients sketched a single image seen at a specific stage of the surgery, while others produced several drawings representing their experiences at different stages of the operation. The interpretations of patients varied immensely with some describing ovals, circles, ‘simmering blue stars’, ‘balls of yellow’ and even a ‘pair of cat's eyes’ . Thus, while these pictures do not necessarily depict the specific visual sensation that the individual patient will experience, some patients who were shown these drawings preoperatively found them useful in terms of knowing what to expect during the surgery [24,26,28].
Regional ophthalmic block and optic nerve function
Anaesthetic fluid has been shown to surround the optic nerve, giving a distinctive T-sign after retrobulbar [29,30], peribulbar  or sub-Tenon's block [29,31]. Numerous studies have shown that retrobulbar block significantly reduces [32-38] but does not seem to completely abolish optic nerve function. Temporary reduction in visual acuity may occur during cataract surgery using regional blocks [22,39-41]. Transient pupillary defects  and marked reductions in the amplitude of visual evoked potentials [33,37] following orbital regional block also confirm temporary impairment of optic nerve function. The injected local anaesthetic agent does not affect retinal function . Sub-Tenon's block is also known to decrease optic nerve function . The effect of peribulbar anaesthesia is variable and debatable [35,37,44,45]. The use of topical anaesthesia, with or without intracameral lidocaine, does not affect the optic nerve function .
While it is clear that there is a definite reduction in the optic nerve function after ophthalmic regional anaesthesia, the effect is short-lived and often incomplete. Therefore, many patients retain their vision after ophthalmic blocks.
Causes of visual sensations during cataract surgery
Persisting function of the retinal photoreceptors and optic nerve could account for perception of simple visual sensations during cataract surgery in most patients. Light passing through the pupil and the remaining lens material stimulates the retinal photoreceptors, resulting in visual phenomena, although a focused image may not be obtained due to the disruption of the lens and impairment of its refractive function. As the operation progresses, an increasing amount of light stimulates the retina as lens opacity is reduced and implantation of an intraocular lens towards the end of the surgery restores the eye to a more emmetropic state. These may explain why some patients perceive an increase in brightness of the operating lights during the course of the surgery [13-18,20,21].
Au Eong and colleagues believed that the visual images seen by patients are a combination of images of objects outside the eye (fingers, instruments) and entoptic phenomena produced by objects and structures on the corneal surface and in the eye. Dynamic factors such as moving fluids and bubbles on the corneal surface and in the eye, moving instruments in the eye and the continually changing shape and opacity of the lens as it is being phacoemulsified and aspirated produce the changing kaleidoscope of colours and shapes .
Verma has suggested that stimulation of the colour photoreceptors by ultrasonic energy from the phacoemulsification probe is a possible explanation for the ‘wavelike colour haloes’ experienced by some patients  and cited evidence in the Russian literature that ultrasound energy may affect the photoreceptors . However, this mechanism does not explain the perception of similar sensations by patients undergoing extracapsular cataract extraction [14,18], in which ultrasound is not used.
Khan has proposed a possible optical mechanism by which objects in the anterior chamber could be seen by the patient . He noted that the posterior surface of the cornea acts as a concave mirror of reflecting power +298.5D and that if an instrument such as a phacoemulsification probe is placed in the anterior chamber 3.9 mm from the posterior cornea surface, an image of it would be formed 24 mm behind the posterior cornea surface, bringing it to a focus on the retina . However, this explanation does not account for the perception of complex images such as surgeon's hands and fingers, which are obviously not situated behind the posterior corneal surface.
Clinical significance of retained visual sensations
During cataract surgery 3-16.2% of patients are reported to be frightened by their visual experience (Table 2) [13-16,19-21]. Fear and anxiety may cause some patients to become uncooperative during surgery, increasing the risk of perioperative complications and making continuation of surgery difficult or impossible without sedation or conversion to general anaesthesia. Furthermore, the stress response may trigger a sympathetic surge leading to hypertension, tachycardia, myocardial ischaemia, hyperventilation and acute panic attack. These responses are especially undesirable since the majority of patients undergoing cataract surgery are elderly and often have concurrent medical problems such as hypertension, diabetes mellitus and ischaemic heart disease, which may increase the risk of morbidity .
The impact of the fear experienced by patients is clearly illustrated in one study in which 28.8% of patients who had undergone phacoemulsification under topical anaesthesia stated that they would have preferred a retrobulbar or peribulbar block if the technique specifically reduced intraoperative visual perception . Although these patients were counselled in detail about the small risk of retrobulbar haemorrhage and globe perforation inherent to the injection technique, 7.7% of the patients were still willing to accept the additional risks. These findings illustrate the significance of a frightening visual experience and suggest that the intraoperative visual experiences can have significant effect on patient satisfaction.
Except for colours [15,20], flashes  and the volume of anaesthetic used  most authors have found no significant association between a frightening visual experience with factors such as age or sex of the patient or the duration of surgery [13-21]. While the difference was not statistically significant, patients below 65 yr of age were more likely to be frightened by their intraoperative visual sensations [19,20]. Other studies also suggest that the average age of patients who were frightened was lower than that of patients who were not frightened [14,15]. It is possible that the unexpected nature of the visual experience, rather than a specific visual sensation, causes alarm.
Measures to reduce retained visual sensations
Since a frightening intraoperative visual experience may adversely affect a patient's clinical condition as well as satisfaction, appropriate steps should be taken to reduce its impact. The most practical measures include appropriate preoperative counselling but there are other possible interventions.
If patients are not properly counselled, they are uncertain if the visual sensations that they experience during cataract surgery are normal. For example, patients who have no light perception during surgery may become alarmed because they assume that some complication has occurred and fear that their eyesight may be permanently lost. If patients are counselled, they will accept these visual sensations (including no light perception) as normal.
One study found that patients who had prior cataract surgery in the fellow eye were less likely to be frightened (6.0%) than those who were undergoing cataract surgery for the first time (15.8%) . Although this difference was not statistically significant, it suggests that patients with previous intraoperative visual experiences may have received an indirect, practical form of ‘counselling’ and are therefore, less likely to be frightened by their subsequent experiences.
Preoperative counselling should be comprehensive, accurate and should cover the full range of possible intraoperative visual experiences [13,28,49]. Incomplete information such as that contained in the patient information leaflet in ‘Cataract Surgery Guidelines’ produced by the Royal College of Ophthalmologists, London, in contrast to other published literature, states that the patients operated under local anaesthesia ‘will not be able to see what is happening, but will be aware of a bright light’ . This is inaccurate since up to 20% of patients do not experience light perception throughout the surgery. In addition, most patients will perceive other visual sensations besides light during the surgery (Table 1). It is therefore possible that patients who receive this inaccurate information may become unduly anxious and frightened when they experience either more or less than what they were expecting [49,51].
Preoperative counselling may be aided by the use of drawings obtained from artists or other patients [26,28]. Sumich and colleagues showed a series of drawings produced by an artist patient who had previously undergone cataract surgery to 30 consecutive patients who were about to undergo phacoemulsification in their first eye. Twenty percent of the patients subsequently graded the value of having seen the drawings as ‘very useful’ while another 30% found it ‘quite useful’. The authors now routinely provide the drawings to all their patients preoperatively as an adjunct to their informed consent .
Only one of 30 patients in Sumich's series reported an experience ‘similar’ to that of the artist's depiction . This could be because each patients' impression is unique, and in fact, the drawings of this artist differ from those reported by Murdoch and Sze  and Verma . It may therefore be more helpful to show a variety of sketches from different patients who had undergone surgery so that new patients will know that these retained images are highly variable.
A recent nationwide survey of ophthalmologists in Singapore on their knowledge and practices regarding visual sensations during cataract surgery under topical anaesthesia showed that 51.1% of respondents believed that their patients might be frightened by their intraoperative visual experience and an equal number felt that appropriate preoperative counselling would be helpful in alleviating patients' anxiety. However, only 11.1% of respondents routinely offered preoperative counselling on possible intraoperative visual sensations to their patients and 17.8% counselled their patients occasionally .
Other measures to reduce the ability of the patients to see during surgery may include general anaesthesia, sedation or deep needle block techniques. General anaesthesia will completely eliminate any visual experience, but its routine use for this purpose in cataract surgery is unwarranted. Although sedation may reduce the alertness of patients to their visual environment (and their recall of retained images), the optimal level of sedation may be difficult to achieve as the titration of the dose of sedative in elderly patients is problematic. Sedation may also result in respiratory, cardiovascular and neurological complications as well as uncontrolled and unexpected movements of the head . In a very large study, the use of i.v. sedatives was associated with a significant increase in adverse medical events for topical (1.20%) and injection anaesthesia (1.18%) compared to topical anaesthesia without i.v. sedation . The injection of an adequate amount and volume of local anaesthetic agent nearer the optic nerve may also be helpful. It has been shown in a recent randomized controlled trial that more patients operated under topical anaesthesia perceived light, colours and change in light brightness compared to those under retrobulbar anaesthesia  although retrobulbar anaesthesia did not completely abolish the visual experiences. However, one has to be aware of potentially serious complications associated with a retrobulbar block.
It is well established that many patients experience a variety of visual sensations during cataract surgery under ophthalmic regional and topical anaesthesia, and that these frighten many patients. A frightening visual experience may cause undesirable complications and reduce patient satisfaction. Preoperative counselling appears to be an appropriate measure to reduce the fear from this phenomenon. Anaesthesia providers and other healthcare professionals who care for these predominantly elderly cataract patients should be aware of this phenomenon.
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