Ординатура / Офтальмология / Английские материалы / Modern Cataract Surgery_Kohnen_2002

Bases of Topical/Intraocular Anaesthesia in Small Incision

Cataract Surgery

Small incision cataract surgery has anaesthesia requirements that are greatly different from the requirements of extracapsular cataract surgery. Instruments do not move within large wounds, but are moved as levers through small incisions thus preventing fluid leakage and hypotension. Usually two incisions are made, allowing the surgeon to stabilize and to direct the eye with two instruments. As a result, akinesia is no longer needed, yet the retained ocular motility can help the surgeon in some passages of surgery by simple instruction of the patient. In addition, phacoemulsification can be performed without painful manoeuvres typical of extracapsular surgery like muscle sutures, conjunctival incisions, iris manipulations, and also postoperative pain is very limited. Although this reduction in the need for analgesia promotes lighter anaesthesia techniques, still there are surgery procedures that usually cause pain: nucleus rotation stretches the zonula that elicits pain in the ciliary body, the increase in intraocular pressure with irrigation is also appreciated, iris touch can be painful especially at the end of the case. The patient’s safety lies in that cataract surgery with self-sealing small incisions can be interrupted at any time in case of pain sensations, for further anaesthesia delivery with no harm for the eyes under operation. Apart from these considerations about the feasibility of topical anaesthesia, there are many advantages over retroor periocular injections. The risks of needle penetration within the eye or the optic nerve are obviously avoided, as orbital haemorrhages. During surgery, the level of analgesia obtained is more consistent and the intraocular pressure is lower than following periocular infiltrations. The limited amount of drug employed inhibits the general side effects commonly observed with local anaesthesia. After surgery, the prompt return of sensitivity allows ambulatory patients to realize immediately an unexpected ocular pain that could require treatment. Patients are delighted with topical anaesthesia, that suppresses all they remembered painful about surgery – periocular needle injections. With injections, sedation was needed more for anaesthesia delivery than for surgery itself.

Topical anaesthesia brings a different concept than nerve block: receptor block. Nerve block involves the anaesthetic agent to come in touch with the sensory nerve, exerting its activity on non-myelinated fibres or on Ranvier nodes of myelinated fibres. Three to five nodes of Ranvier must be blocked to suppress impulse propagation, for a length of 3–7 mm. This relatively long portion could explain the variability we find in the level of anaesthesia after peribulbar injections. Receptor block involves the inhibition of sodium channels at nerve endings or receptors by the anaesthetic agents, thus blocking the production of nervous impulse. The exposed receptors could be reached more completely, being the drug in solution. As for drug concentration, A-delta and C fibres, carrying pain,

Table 1. Anaesthetic agents most used for topical applications in ophthalmic surgery

The low pH of the solution is associated with subjective burning on application. If the pKa is high, the molecule is more dissociated at physiologic pH (low % base at pH 7.4) with higher surface activity but poorer corneal penetration.

temperature and touch, are blocked by lower concentrations of drugs than motor fibres. Usually thermal and tactile sensations are thought to be more resistant to anaesthetic agents, because of slight differences in conductivity among the various types of nerves.

Anaesthetic Agents for Topical Intraocular Anaesthesia

Topical anaesthetic agents are tertiary amines composed by an aromatic hydrophobic ring and an amidic hydrophilic group, with an ester (proparacaine, tetracaine, benoxinate) or an amidic (lidocaine, etidocaine) intermediate chain. The ester compounds are rapidly hydrolyzed by plasmatic esterasis, while the amide compounds are degraded more slowly. All the anaesthetic agents are stable in solution at relatively acid pH. They have to gain the non-dissociated form to cross the tear film and the cornea, and to return to the dissociated form at nerve endings or axons to exert their activity. The chemistry of body fluids favours these passages. The low pH of commercially available solutions is the main cause of the burning sensations perceived on the first eyedrop applications.

The most used ester anaesthetic agent is tetracaine. The application of 0.5% solutions opened the way to topical anaesthesia for phacoemulsification [5], but at present it is less used because of the short duration of action and of the esterase deficiency that can be found in some patients.

Currently the most employed drug is the amide lidocaine, available in many countries because it is widely used for cardiac diseases. It is available in concentrations from 1 to 4%, with or without preservatives. The unpreserved drug is selected for topical applications, because of the toxicity for the corneal epithelium exerted by chemical agents used for preservation. The unpreserved form is mandatory in case of intraocular irrigations. A comparison of tetracaine and lidocaine can be read in table 1.

Table 2. Aqueous humor concentration of topical lidocaine

Intracameral irrigation with anaesthetics was first proposed by Gills et al. [6], and then widely adopted to suppress pain coming from intraocular structures [7, 8]. The drug employed was lidocaine, at 1% concentration probably because of simplicity in preparation. We are not aware of pharmacological studies with different concentrations. Lidocaine 1% was mainly prepared from 4% solutions by diluting in BSS or BSS plus. Obtained solutions have a pH of 6.39 and 7.11 respectively [9]. Lidocaine is taken up quickly by the iris/ciliary body and cornea and rapidly removed from these tissues after BSS washout. Irrigation during phacoemulsification seems to limit lidocaine exposure to the ocular tissues, resulting in a short duration of anaesthesia. Lidocaine is not metabolized or broken down by the iris or cornea during this short period [10].

Intraocular and Systemic Levels of Topical and Intracameral

Lidocaine

Studies about intraocular penetration of anaesthetic drugs have been carried out for lidocaine at 4% concentration (table 2). Zehetmayer et al. [11] found a huge dependence on the pH of the solution, as expected from chemical properties. Behndig and Linden [12] measured the lowest aqueous humor levels among published investigations. Higher levels were found in our study following instillations at 10-min intervals, probably because the damage of corneal surface favoured penetration: 8.7 2.4 g/ml after 3 instillations, and 23.2 8.9 g/ml after 6 instillations [13]. In this study, pain sensations during surgery were higher when the intraocular level of lidocaine was 12 g/ml.

With the commonly used irrigation of 1% unpreserved lidocaine, intraocular levels of the drug are a hundred times more elevated than after eyedrop applications: Behndig and Linden [12] found 341.8 151.6 g/ml in their study. The systemic absorption of topically applied lidocaine was studied by us [13]. Blood

levels found after 1 h from the last instillation were 0.009 0.001 g/ml following 3 instillations, and 0.12 0.02 g/ml following 6 instillations. A study from Wirbelauer et al. [14] after intraocular irrigation gave the same results. These amounts are too low to cause systemic problems even in diseased patients, and are much lower than the 2.13 g/ml found by Salomon et al. [15] after periocular injections. In our studies no differences were found in pulse rate, blood pressure and oxygen saturation with peribulbar and with topical anaesthesia [9], with some variations within the peribulbar group that were confirmed in larger studies [16].

Local Side Effects and Toxicity

Although lacking in the potential risks of needle injections, topical/intraocular anaesthesia can have side effects that are mainly local. The application of an anaesthetic agent to the cornea impairs the tear film because of dilution and because of the pH of applied solutions. The inhibition of cellular sodium channels causes some swelling of the corneal epithelium, with the possibility of superficial punctate keratitis [17] that could be more important in older patients with low tear secretion. Epithelial toxicity is more pronounced when preservatives are added to the solution [18], and pushes to have patient’s eyes closed after instillation. A part of this toxicity can last a few days after surgery, slightly affecting vision, as we learn from the fellow non-operated eyes receiving some anaesthetic drops to prevent blinking during surgery. These epithelial side effects can impair visibility during surgery, and are an argument favouring the reduction of eyedrop instillation and the adjunct of intracameral anaesthetic irrigation.

Intraocular lidocaine has been extensively tested for tolerance, starting from the amaurosis encountered in some patients after posterior capsule rupture [19]. Experimental studies on rabbits showed the lack of toxicity of common preparations both for corneal endothelium [20–23] and for the retina [24]. Reversible cellular swelling could be observed when the concentration is at least 1% [23], with permanent damage only at 2% [22]. Clinical and experimental studies on human corneas confirmed these results [23, 25–29], at least for lidocaine. Other compounds, like mepivacaine, seem more toxic [30]. Following the sensation of some amaurosis during uncomplicated surgery, Hoh et al. [31] investigated the speed of visual recovery after phacoemulsification with intracameral lidocaine: they found prompt return to normal vision after 4 h.

Clinical Application of Topical/Intraocular Anaesthesia

The current application schedule of topical anaesthesia has some variations among surgeons, yet common features can be indicated.

Patient Selection. At first, topical anaesthesia was not appreciated as a universal procedure, but it was felt that patient selection was mandatory. Grabow [32] was one of the first addressing difficulties in applying topical anaesthesia to some patients, like foreigners and those affected by deafness, dementia and uncontrolled eye movements. In addition, many surgeons avoided topical anaesthesia in patients unable to cooperate during tonometries or A-scan measurements [33], and in younger patients. At present some of these contraindications remain, but some have been overcome by the confidence both of surgeons and of patients in topical anaesthesia.

Patient Instructions. As patients now expect to be operated under topical anaesthesia, few instructions have to be given before surgery. On the contrary, too many details could increase the patient’s anxiety. We only tell the patient that anaesthesia will be present, although with no needle injection; that anaesthesia can be increased at any time during surgery; that the lack of burning on eyedrop instillation is the proof of analgesia.

Bilateral Instillation. Most surgeons apply the anaesthetic agent to both eyes, to prevent blinking and Bell’s phenomenon, elicited by the non-operated eye. This practice allows the patient to keep his eyes open without effort during surgery, but often causes some vision impairment in the postoperative, due to corneal epithelial toxicity of the drugs and lack of hydration.

Instillation Schedule. Lidocaine 4% unpreserved eyedrops are instilled in the 10–60 min preceding surgery, according to the local protocol. We prefer 6 instillations at 10-min intervals, that assure steady analgesia for 10–15 min. The great variations in instillation schedule probably reflect more the characters of the local population than the precision of the surgical technique.

Intraocular Irrigation. When intraocular irrigation of 1 or 0.5% unpreserved lidocaine is added to the anaesthesia schedule, it is usually performed immediately after the first corneal incision, or at hydrodissection. The first method requires fewer anaesthetic eyedrops, but adds one passage to surgery; the second method looks somewhat simpler, but capsulorhexis has to be performed under topical anaesthesia alone. The intraocular irrigation can be repeated in prolonged or complicated surgeries, because lidocaine is rapidly removed from ocular tissues by irrigating BSS [10]. Every surgeon should check the pH and the osmolarity of injected solutions.

Preferred Surgery. There are some modifications to be made to our surgical technique to adjust for topical anaesthesia. The corneal surface rapidly dries during surgery, and must be frequently irrigated. The light of the microscope is frequently a cause of discomfort, especially in young patients, and microscopes with lights eccentric to the fovea should be preferred. We had to eliminate toothed forceps to grasp the conjunctiva, in favour of notched forceps. Conjunctival flap should be avoided, as diathermy of ocular surface. Should they be necessary,

additional eyedrop instillation after conjunctival opening must be considered. The eye is better stabilized by a second instrument within a side port incision rather than grasping the sclera. Cataract extraction should be made with phacoemulsification, because the manoeuvres required for manual fragmentation could be more traumatizing for the eye. IOL implantation should not stretch the incision, as at that point analgesia is lower than at the beginning of the procedure. The lids must remain free from trauma because they are not anaesthetized, a condition evident on draping removal.

Complications Management. Topical anaesthesia is not associated with a higher complication rate in published studies, still complications occur and have to be managed with safety and efficacy. Prolonged surgeries can be managed by repeating instillations or irrigations. Iris touch is painless if a sufficient amount of drug is present in the anterior chamber. Posterior capsule rupture and anterior vitrectomy cause no pain. Scleral fixation can be achieved with intraocular irrigation of unpreserved lidocaine [9]. Probably the only manoeuvre requiring additional peribulbar anaesthesia – to block the eye – is incision enlargement.

Postoperative Instructions. One of the advantages of topical anaesthesia is the rapid recovery of sensation. Patients should be aware that ocular burning will be perceived by most of them, but ocular pain is not expected in uncomplicated cases. Burning sensation and vision impairment can also be observed in the fellow eye, if the anaesthetic agent was applied bilaterally. Following intraocular anaesthetic irrigation, vision in the operated eye is usually low at the end of surgery, with many patients speaking of ‘wonderful colours’ that probably reflect some retinal anaesthesia. When vitrectomy is added in aphakic eyes, a deep amaurosis lasting 4–6 h can be expected.

Variations in Topical Drug Delivery

Although eyedrops and intraocular irrigation are the most popular methods of drug delivery, other procedures have been developed to prolong the contact between the applied agent and ocular surfaces.

Lidocaine Gel. A single application of lidocaine 2% gel into the conjunctival sac has been found as effective as repeated eyedrop instillation in providing anaesthesia for cataract surgery [34–37], with the advantage of less burning on application and less corneal dehydration. The lidocaine gel seems to be an efficacious alternative to eyedrop instillation, with the advantage of simplicity.

Anaesthetic Sponges. The use of sponges soaked with an anaesthetic agent in contact with ocular surfaces to obtain analgesia was advocated by Bloomberg and Pellican [38] and Rosenthal [39] in 1995. Bloomberg [38] proposed his

anaesthetic ring, that was left in place for 10 min before surgery and during surgery itself, if no contraindication emerged. Since then, similar devices appeared sometimes in literature, but they were not widely used. Recently they were again proposed for Lasik and Intacs procedures, where deeper anaesthesia of the conjunctiva has to be obtained than in phacoemulsification.

Clinical Experience of Small Incision Cataract Surgery with Topical Anaesthesia

The first clinical reports about phacoemulsification with topical anaesthesia date back to 1993 [32, 40, 41]. Those early works had already recognized some of the main features of topical anaesthesia: the obvious lack of risks typical of needle injections, the efficacy of the obtained analgesia, the relative loss in corneal transparency during surgery, the steady rate of surgical complications as compared with peribulbar, the good acceptance from patients’ perspectives. As topical anaesthesia spread over the operating theatres, many variations were proposed for the treatment schedule. Most of surgeons incorporated some oral sedation in their protocol, especially during transition or when comparison studies were carried out [11, 42–44]. Using sedation in selected cases, Dinsmore [45] demonstrated the feasibility of topical anaesthesia in almost 100% of patients. Interestingly, some investigations demonstrated that in peribulbar cases sedation was necessary more for needle anaesthesia delivery than for surgery itself [42, 46], and at the same time Fichman [47] found that patients’ objective anxiety for the surgical procedure was very low. In other studies subconjunctival or periocular injections of anaesthetics were added [48, 49], all these adjuncts to topical anaesthesia reflecting in part the difficulties in changing the surgeon’s mentality and in establishing new relations with the patient in the operating room [50]. Not surprisingly, patients proved less attached to retrobulbar or peribulbar anaesthesia than surgeons. All the published studies report as anecdotal the preference for needle injections, the most feared part of cataract surgery itself, although discomfort is frequently greater during surgery with topical anaesthesia [51]. At present there is a push from patients towards topical anaesthesia, which is now regarded as the standard technique in many areas. This change in mentality will probably further increase success rates and will extend topical anaesthesia to other procedures on the anterior and posterior segment [52, 53].

The intracameral irrigation with unpreserved 1% lidocaine proposed by Gills et al. [7] was the event that helped change surgeons’ mentality [28]. Intraocular levels of drug are about 100 times the level after topical instillations [12], completely eliminating pain and discomfort coming from intraocular

Table 3. Example of topical/intraocular anaesthesia for phacoemulsification

structures. In his study on 1,000 subjects, Koch [8] had only 4 failures. In our study on 1,442 operations from 1996 to 1998, we had 14 partial failures (would prefer peribulbar injections because of discomfort) and 11 total failures (peribulbar injection selected before or during surgery): a percentage of 1.7% [9]. The current schedule of anaesthesia for phacoemulsification as preferred by us is reported in table 3. Intracameral irrigations with anaesthetic agents gained widespread popularity, and even allow to teach surgery without going back to heavier types of anaesthesia. Nevertheless, some concern rose about their necessity in topical anaesthesia, that seems sometimes questionable [54]. We believe that intraocular irrigation probably offers little advantage to uncomplicated cases performed by experienced surgeons, but also in these circumstances it adds to our surgery the confidence not to cause pain even with sudden or unwanted movements. Patient reaction to acetylcholine irrigation with and without intracameral irrigation does fully explain this concept.

Conclusions

Small incision cataract surgery is a surgical procedure controlled step by step at the operating microscope. It looks quite obvious that the types of anaesthesia that were developed for other kinds of cataract extractions – intracapsular or extracapsular – have to be revised. Topical anaesthesia will rapidly become the preferred method in all countries for the reasons of simplicity, safety and cost. Although there are some drawbacks, the advantages by far exceed the disadvantages. Intraocular irrigation of unpreserved lidocaine induced many uncertain surgeons to abandon peribulbar injections, and helped in reducing the number and toxicity of instillations. It will probably last if signs of intraocular side effects do not emerge. The feasibility of intraocular irrigations of anaesthetics is a challenge for other types of ocular surgery. An increasing number of

anterior and posterior segment procedures can now be performed with topical anaesthesia, in all or in selected cases. Again, cataract surgery is stimulating other types of ocular surgery to evolve.

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