Onset
Duration
Potency
Toxicity
Ester-type hydrolyzed by plasma cholinesterase (liver)
Allergic reaction more likely (formation of PABA)
Procaine
S
S
L
L
Tetracaine
S
I
I
I
Cocaine
S
L
H
H
Benoxinate (oxybuprocaine)
S
I
I
H
(cardiotoxic)
Chloroprocaine
F
I
I
L
Amide type
Lidocaine
F
S/I
I
L/I
Mepivacaine
F
S/I
I
I
Bupivacaine
S/I
L
H
H
Ropivacaine
S/I
L
I
I
Etidocaine
F
L
H
H
New drugs
Articaine
S/I
L
I
L
Levobupivacaine
S/I
L
H
H/I
Proxymetacaine
F/I
I
I
L
Table 2
Typical combination of anesthetics for different modalities
Anesthetic modality | Agent |
|---|---|
Topical | Tetracaine (hydrochloride) 0.1 % + oxybuprocaine hydrochloride 0.4 % |
Intracameral | Preservative-free lidocaine 1 % or a mixture of preservative-free lidocaine 1 % and preservative-free bupivacaine 0.5 % |
Regional (Retrobulbar, peribulbar, sub-Tenon’s block) | Lidocaine 2 % – short-duration procedures Lidocaine 2 % + bupivacaine 0.5–0.75 % [or ropivacaine 1 %] – 1 h duration Mixture (50 %–50 %) of mepivacaine 2 % and bupivacaine 0.5–0.75 % – 1 h or longer duration |
Sedatives may be used in local anesthesia but can result in a confused uncooperative patient in the middle of an operation. If sedation is not used, small amounts of clear fluids can be permitted 2 h before surgery. Monitoring should include ECG and pulse oximetry. Intravenous access may become necessary and therefore should be considered in the plan. The most commonly used sedatives are midazolam (benzodiazepine with a half-life of 2 h), propofol (short-acting phenol with rapid recovery, it causes respiratory depression and fall in blood pressure), fentanyl (potent short-acting – 30 min – narcotic analgesic), and remifentanil (ultrashort-acting analgesic metabolized by esterases, with half-life of 3–10 min, causes fall in heart rate and blood pressure).
Regional Anesthesia
Retrobulbar Anesthesia
This procedure achieves good ocular akinesia and anesthesia by delivery of 3.5–5 ml of anesthetic into the retrobulbar space (see Fig. 1). A shorter round-tipped needle than classical (31–38 mm) is introduced with precise placement between the inferior and lateral rectus muscles (point between medial two thirds-lateral one third of the inferior orbital rim) while the patient is in primary or slight upgaze. After the resistance of the orbital septum is encountered, the posteriorly directed needle is directed toward the apex of the orbit, until it meets the resistance of the intermuscular septum. When the latter is passed, the retrobulbar space is reached. While retrobulbar anesthesia is used by 9 % of surveyed surgeons of the ASCRS in phacoemulsification [2], this percentage is surely larger in manual SICS.
Fig. 1
Insertion of needle (35 mm length) for retrobulbar block at the junction of the middle and lateral third of the inferior eyelid above the inferior orbital rim. The needle is directed toward the orbital apex (medial and lateral orbital walls make an angle of 45°)
Pearl
Avoid placing the needle “far from the globe” to prevent perforation. It usually leads to peribulbar placement of the anesthetic. After initial resistance of the orbital septum, a small rotational or vertical movement of the globe is observed when resistance of the intermuscular septum is found.
Complications
Potential complications of retrobulbar block include retrobulbar hemorrhage, perforation of the globe, retinal vascular obstruction, and subarachnoid injection with possible cardiovascular consequences.
Retrobulbar hemorrhage occurs in less than 5 % of retrobulbar injections [1]. There are various possible sources of bleeding. The vortex veins leave the globe 4 mm posterior to the equator, so shearing forces during needle insertion could well affect them and the inferior ophthalmic and central retinal vein. In cases of sudden onset of proptosis, chemosis, hemorrhage, and immobility of the globe, arterial bleeding is presumed. Candidates are the posterior ciliary arteries supplying the choroid and ophthalmic artery branches (including the central retinal artery) or the ophthalmic artery near the optic foramen. Use of needles longer than 31 mm is not recommended (25G, 25-mm long is the most frequently employed) [3].
In most cases, retrobulbar hemorrhage resolves without complication, and surgery should be postponed for 3–4 weeks, with topical or general anesthesia depending on cooperation and availability. If vision is compromised as a result of closure of the central retinal artery due to increased intraorbital or intraocular pressure, a lateral canthotomy is indicated. Damage to smaller vessels could also occur. Anterior chamber paracentesis and orbital decompression are other therapeutic options. Computed tomography is usually undertaken before orbital decompression to help localize the blood and rule out the possibility of bleeding within the optic nerve sheath, which might also require decompression.
Perforation of the globe is a sight-threatening complication, and highly myopic eyes are particularly susceptible to this occurrence. The scleral perforation should be repaired as soon as possible, using cryopexy, laser treatment, scleral buckling, or pars plana vitrectomy. Inadvertent injection of lidocaine into the vitreous cavity may be tolerated, but could cause elevation of the intraocular pressure and rapid opacification of the cornea.
Retinal vascular obstruction has also been reported after retrobulbar anesthesia. Central retinal artery obstruction frequently reverses spontaneously and reperfuses within several hours. Possible mechanisms are spasm of the artery, trauma to the vessel from the needle, and external compression by an injected solution or blood. Anterior chamber paracentesis may help by lowering intraocular pressure, but its efficacy is questionable. When there is combined obstruction of central retinal artery and central retinal vein, a cherry red spot is seen with intraretinal hemorrhages and dilated retinal veins, due to direct trauma of the retinal vessels or compression from fluid or blood injected into the optic nerve sheath. The prognosis in these cases is dull, and hemorrhage or hematoma of the optic nerve sheath may be detected by computed tomography, leading to decompression of the nerve sheath. Neovascularization of the iris may develop, which could be avoided by the use of intravitreal anti-VEGF injection and/or panretinal photocoagulation. Apparently, retrobulbar injection with the eye in primary position could help prevent this complication, whereas when the eye is looking up and in, the optic nerve and retinal vessels are placed more easily in the pathway of the needle.
Inadvertent injection into the subarachnoid space or diffusion from the surrounding orbit could have respiratory or cardiovascular consequences. Optic atrophy and blindness are very infrequent complications of retrobulbar block.
Peribulbar Anesthesia
The peribulbar technique achieves good ocular akinesia and anesthesia using several injections external to the muscle cone (usually 2), but anesthetic effect is frequently of poorer quality than in the retrobulbar technique. The efficacy of peribulbar blockade using short (15.0 mm), medium (25.0 mm), and long (37.5 mm) needles depends on the proximity of the deposition of the local anesthetic solution to the globe or the orbital apex. Orbital hemorrhage and globe perforation are less frequent than in retrobulbar anesthesia. Peribulbar anesthesia could be classified as circumocular (sub-Tenon episcleral), periocular (anterior, superficial), periconal (posterior, deep), and apical (ultra deep). A potential effect of this modality of anesthesia is reduction in pulsatile ocular blood flow and IOP, which in certain operations could be beneficial [4].
Sub-Tenon Anesthesia
The Tenon capsule is a fascial layer of connective tissue surrounding the globe and extraocular muscles and attached anteriorly to the limbus of the eye, extending posteriorly to fuse with the dura that surrounds the optic nerve.
In this technique, after instillation of topical anesthetic, the conjunctiva and Tenon’s capsule are incised with Westcott scissors and dissected in the inferior nasal quadrant, and a blunt cannula is introduced into the sub-Tenon space to administer 3–4 ml of anesthetic agent, to produce anterior segment and conjunctival anesthesia. Globe akinesia follows after a few minutes of rapid induction of anesthesia. This technique requires skill to dissect into the sub-Tenon space and to correctly place the anesthetic agent using a blunt irrigating cannula. The anesthetic is placed in the intraconical space in an attempt to affect the ciliary ganglion. Conjunctival bleeding and especially chemosis are the main complications that should be avoided.
The incidence of conjunctival swelling in the sub-Tenon block is around 39.4 % [5, 6]. It could be attributable to anterior leakage of the anesthetic. Chemosis is exaggerated if the solution is administered into the anterior compartment of sub-Tenon space or the subconjunctival space. The reported incidence of subconjunctival hemorrhage is 32–56 %, which might be reduced with cauterization of the conjunctival incision to reduce hemorrhage. Retrobulbar hemorrhage could occur when we perform the dissection, introduce the cannula through the tunnel, or use a continuous infusion with catheter.
Retrobulbar and peribulbar procedures produce equally good akinesia, in theory, and sub-Tenon procedures produce slightly less akinesia, but there is insufficient evidence for this conclusion.
Pearl
A frequent mistake is subconjunctival injection of the anesthetic, because dissection into the sub-Tenon’s space reaching the sclera is not correctly made, leading to conjunctival chemosis.
Topical Anesthesia
Cataract patients are often managed with topical anesthesia to facilitate recovery and rehabilitation after surgery. It can be supplemented with intracameral anesthesia or oral/IV sedation [7]. A particular advantage of topical anesthesia is that visual recovery is almost immediate or at least faster than after retrobulbar or peribulbar anesthetic technique.
Topical anesthetic agents (Tables 1 and 2) block trigeminal nerve endings in the cornea and the conjunctiva only, leaving the intraocular structures in the anterior segment unanesthetized. Manipulation of the iris and stretching of the ciliary and zonular tissues during surgery can irritate the ciliary nerves, resulting in discomfort, and should be avoided when this anesthesia is used. Consequently, the addition of intracameral anesthesia is frequently used to enhance the effect of topical anesthetics.
Topical anesthesia consists in the use of eye drops or gels. Allergies to local anesthetic agents determine the suitability of anesthetics for choice in a particular case. Patient discomfort and epithelial toxicity are also important factors when topical anesthesia is used. Toxicity to the corneal epithelium might lead to corneal clouding and more difficult surgery due to poor visibility, as well as corneal erosion and prolonged wound healing. Tetracaine (an ester-type anesthetic agent) is the most irritating of the commonly used anesthetic eye drops and should be avoided in patients allergic to this particular family of agents [8]. Proparacaine does not metabolize to p-aminobenzoate, although an ester type, and therefore may be used safely in patients allergic to other ester-type anesthetic drugs.
Viscous lidocaine gel may be used as an alternative or adjunct for topical application of anesthetics. The gel is often mixed with dilating medications and antibiotic and nonsteroidal anti-inflammatory agents, for example, lidocaine gel 2 % mixed with tropicamide, cyclopentolate 1 %, and phenylephrine 10 % or 2.5 % (with or without moxifloxacin and ketorolac applied to the operative eye twice before surgery to achieve good mydriasis and anesthesia) [8]. Topical anesthesia with 2 % lidocaine gel keeps the cornea moistened and increases the contact time with the ocular surface, with sustained diffusion and prolonged anesthetic effect. In my experience, it has some epithelial toxicity that could impair visibility during the cataract surgery procedure, but it is also true of topical anesthesia drops when employed several times before starting surgery.
A non-pharmacological topical anesthesia technique, named cryoanalgesia, consists in using cool fluids but no anesthetic drops [9]. Only selected patients can cooperate for a manual cataract surgery technique with this approach. Before surgery, the patient will feel a cold sensation in his eye. It is necessary to cool all fluids to be used during surgery to approximately 4 °C (excluding povidone, which remains at normal temperature). An eye mask of cold gel is placed over the eye for 10 min before surgery. A cold methylcellulose drop is instilled into the eye also before placing the ophthalmic drape and before starting surgery. The cornea is cooled with continuous irrigation of cold BSS®. Cold viscoelastic material (e.g., Viscoat®) is injected into the anterior chamber, and the cornea is continuously cooled with BSS® throughout the procedure.
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