Robert C. (Roy) Hamilton, MB, BCh, FRCPC
Contents
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Desirable Prerequisites |
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Anatomy and Applied Anatomy |
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Ophthalmic Regional Block Anesthesia |
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CHAPTER HIGHLIGHTS |
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Advances in surgical techniques, especially small-incision phacoemulsification, have lessened the universal demand for akinetic anesthesia using regional blocks. Other methods, including sub-Tenon's, subconjunctival, and solely topical corneoconjunctival anesthesia, have been introduced. However, solid regional block anesthesia including muscle akinesia is still the preferred choice of anesthesia for many cataract surgeons. Although peribulbar blocks were popularized in 1986, claiming to avoid serious complications of the retrobulbar method,[1] a recent survey of the annual American Society of Cataract and Refractive Surgeons with input from 1342 members indicates 30% using retrobulbar and 24% using peribulbar blocks.[2]
Desirable prerequisites
Knowledge of the basic science disciplines (pharmacology of ocular and local anesthetic drugs, physiology of the eye, anatomy of the orbit and its contents) is essential to safe practice of orbital regional anesthesia, including retrobulbar block.[3] Observation of and subsequent initial supervision by personnel with wide clinical experience and knowledge are recommended. The goal for each practitioner is to build up an experiential database from which increasingly good judgment can result. Even when the blocking practitioner is an ophthalmologist, a strong argument can be made for the routine presence of an anesthesiologist.[4,][5] Noninvasive blood pressure, electrocardiographic, and oxygen saturation monitoring should be routinely used before and during the induction of anesthesia and intraoperatively.
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Anatomy and applied anatomy
In this chapter the adjective, retrobulbar, refers to the conical compartment within the confines of the four rectus muscles and their intermuscular septa. Compared with the peripheral orbit where fat is more dense, the retrobulbar cone contains fat that is arranged in large globules, which permit free movement of the intraorbital portion of the optic nerve in the various duction positions of the globe. A matrix of connective tissues, which supports and allows dynamic function of the orbit contents, controls the spread of local anesthetic solutions.[6]
Motor nerves enter the muscle bellies of the four rectus muscles from their conal surface, 1–1.5cm from the apex of the orbit. For conduction block of nerves and the resulting akinesia of their supplied muscles to occur, local anesthetics in blocking concentration have to reach and diffuse to the core of an exposed 5–10mm segment of each of these motor nerves in the posterior retrobulbar space. Retained activity of the superior oblique muscle is often seen after retrobulbar local anesthetic injection because its motor nerve, the trochlear, runs outside the muscle cone. Total blockade of the smaller-diameter sensory and autonomic nerves, including the ciliary ganglion, on the other hand, is more easily achieved. Corneal and perilimbal conjunctival sensory innervation, along with the superior-nasal quadrant of the peripheral conjunctival sensation, are mediated through the nasociliary nerve, which lies within the retrobulbar space. The remainder of the peripheral conjunctival sensation, however, is supplied through the lacrimal, frontal, and infraorbital nerves coursing outside the muscle cone.[7] Because of this, intraoperative pain may be experienced in the peripheral orbit following a solely retrobulbar block.[8]
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Ophthalmic regional block anesthesia
Traditional retrobulbar block and its inherent problems
In 1934 Atkinson[9] described a technique that evolved into the traditional method of retrobulbar blockade. In his article:
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[With the patient's gaze directed] upward and inward, [a 35mm needle entered percutaneously] a short distance below the inferior-temporal margin of the orbit…the skin is moved upward with the needle so that the point just clears the inferior orbital margin. The needle is then directed upward and inward, midway between the external and inferior recti muscles, and advanced toward the apex of the orbit for a distance of from 2.5 to 3.5cm. |
Although Atkinson did not use such a directive in his text (however, illustrations in his article may have led to the interpretation), traditional teaching regarding the inferior-temporal needle entry point has been to locate it at the junction of the medial two-thirds and lateral third of the inferior orbital rim. Generations of ophthalmology residents were trained in this way as a result; in fact, the technique continues to be reproduced in ophthalmology and anesthesiology texts even though “there is now no doubt that [it] is unsafe and there are medicolegal implications.”[10] Unsöld, Stanley, and DeGroot[11] demonstrated conclusively in a cadaver model that the Atkinson “up-and-in” globe position places a stretched and taut optic nerve and the posterior pole of the globe in line for potential damage from the tip of the needle approaching from the inferior-temporal quadrant. In addition, tangential puncture of the optic nerve sheath can occur, leading to injection of anesthetic agent into the subarachnoid space resulting in brainstem anesthesia.[12] Pautler et al.[13] reported two cases of optic nerve trauma with resultant catastrophic loss of vision from long and sharp needles injected toward the orbital apex with the globe in the up-and-in position. Using information gained from the Unsöld paper, they recommended that for retrobulbar blocks patients should fixate in primary gaze and that needle length be reduced to 32mm (11/4 inch) and directed toward an imaginary point behind the macula rather than aiming for the orbital apex. They reported that in the primary gaze globe position the optic nerve lies in a nontaut manner on the nasal side of the sagittal plane passing through the visual axis, in which location and state the risk of nerve damage is much reduced (Figure 8-1).
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Figure 8-1 Plane of the iris and midsagittal plane of the globe in primary gaze; view from above. Fine dashed line indicates the plane of the iris (useful in gauging depth of needle advancement); coarse dashed line indicates the midsagittal plane of the eye and the visual axis through the center of the pupil. The optic nerve lies on the nasal side of the midsagittal plane of the eye. Note how the temporal orbital rim is set back from the rest of the orbital rim at or about the globe equator, making for easy needle access to the retrobulbar compartment. |
Katsev et al.[14] analyzed the dimensions of 120 orbits from 60 human skulls related to the length of needles used for retrobulbar anesthesia. The distance from the inferior orbital rim to the optic foramen ranged from a maximum of 58mm to a minimum of 42mm. Because a 38mm (11/2-inch) needle fully inserted toward the posterior orbit had the potential of damaging vital structures in fully one-fifth of the orbits examined (i.e., those of smaller dimension), they recommended that depth of needle penetration into the orbit be limited to a maximum of 31.5mm (11/4inch) from the inferior orbital rim. This would avoid damage to the tightly packed important structures (nerves, blood vessels, muscles) at the orbital apex.
Liu, Youl B, Moseley[10] repeated Unsöld's cadaver experiment in vivo using magnetic resonance imaging and confirmed the findings of a taut optic nerve in up-and-in position and of a sinuous loose nerve in the primary gaze position.
Complications of ophthalmic regional block anesthesia
Optic Nerve Injury
Injection at the orbital apex, as was advocated in the distant past[15] and is now outmoded, has the potential of frank optic nerve injury. Katsev et al.[14] recommended that needle length introduced beyond the orbital rim for both intraconal and periconal injections should not exceed 31mm (11/4 inch) to avoid damage to the optic nerve in all patients. In the execution of orbital blocks, it is possible for the needle tip to enter the optic nerve sheath and produce not only brainstem anesthesia, as described below, but also tamponade of the retinal vessels within the nerve and/or the small vessels supplying the nerve itself either by the volume of drug injected or by provoking intrasheath hemorrhage.[13,][16–19]
Brainstem Anesthesia
Brainstem anesthesia is caused by direct spread of local anesthetic to the brain from the orbit along submeningeal pathways. It is the eyeblock complication most likely to warrant cardiopulmonary resuscitation. An essential prerequisite in all locations where regional ocular anesthesia is performed is the provision of oxygen saturation monitoring in the room where the block is done and in the operating room,[12] along with equipment to provide respiratory support and cardiopulmonary resuscitation.[5] The incidence has been reported as 1 in 350–500 retrobulbar injections.[12]
Globe Penetration and Perforation
The incidence range of globe penetration (solely entrance wound) and perforation (entrance and exit wounds) has been reported as low as 0 in a series of 2000 peribulbar blocks[1] to 1 in a series of 1000 retrobulbar blocks.[20] In myopic patients the incidence may be as high as 1 in 140 blocks.[21] The true incidence is not known because many cases are not reported;[22] more than 50% of cases go unrecognized at the time of their occurrence.[23] A rare and devastating complication, ocular explosion, has been reported several times;[24,][25] it results from excessively high pressure being applied to the injecting syringe following unrecognized ocular penetration by the needle. Ultimate visual outcome is very poor.
Extraocular Muscle Malfunction
Because extraocular muscle malfunction can result from local anesthesia agent myotoxicity or needle trauma,[26,][27] it is important to choose a block technique in which the needle placement avoids needle contact with muscle. The most common muscles affected, in order of frequency, are the inferior rectus muscle (Figure 8-2),[28–31] the inferior oblique muscle (including injury and trauma to its motor nerve) (Figure 8-3),[28] the superior rectus muscle (Figure 8-4),[32] and the medial rectus muscle (Figure 8-5).[33]
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Figure 8-2 Inferior rectus muscle trauma, lateral view. A straight 31mm (11/4-inch) needle being advanced from the inferior-temporal quadrant in an attempt to enter the retrobulbar compartment has failed to adequately rise from the orbit floor. The needle tip has entered the belly of the inferior rectus muscle. Hemorrhage into the muscle with subsequent fibrosis, or intramuscular injection of local anesthetic with subsequent myotoxicity, may result in prolonged or permanent imbalance between the superior and inferior rectus muscles and vertical diplopia.[31] |
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Figure 8-3 Risks of injection from the traditional entry point. Right orbit: A, view from above; B, view from in front with the globe removed. Observe the proximity of the needle path to the inferior oblique muscle belly, its motor nerve, and the lateral border of the inferior rectus muscle. One or more of these three structures can easily be damaged by a traditionally placed retrobulbar needle. |
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Figure 8-4 Superior rectus muscle trauma, lateral view. A straight 38mm (11/2-inch) needle being advanced from the inferior-temporal quadrant through the retrobulbar compartment too deeply has entered the belly of the superior rectus muscle. Hemorrhage into the muscle with subsequent fibrosis, or intramuscular injection of local anesthetic with subsequent myotoxicity, may result in prolonged or permanent imbalance between the superior and inferior rectus muscles and vertical diplopia.[32] |
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Figure 8-5 Medial rectus muscle trauma, view from above. A straight needle being advanced in a sagittal plane from the extreme medial end of the palpebral fissure (on the nasal side of the caruncle) has traversed the medial compartment on the nasal side of the medial rectus muscle and entered into the belly of the medial rectus muscle. Hemorrhage into the muscle with subsequent fibrosis, or intramuscular injection of local anesthetic with subsequent myotoxicity, may result in prolonged or permanent malfunction of the medial rectus muscle. |
Hemorrhage
Retrobulbar hemorrhages vary in severity. Some are of venous origin and spread slowly. Signs of severe arterial hemorrhage are rapid and taut orbital swelling, marked proptosis with immobility of the globe, and massive blood staining of the lids and conjunctiva.[34] Serious impairment of the vascular supply to the globe may result.[35,][36] By constant vigilance and keen observation of the signs immediately following needle withdrawal, bleeding may be minimized and confined by rapid application of digital pressure over a gauze pad placed on the closed lids. The incidence of serious retrobulbar bleeding was reported to be 1% to 3% in one paper[16] and as 0.44% in a series of 12,500 cases.[37] A strong argument can be made in favor of fine disposable needles over those of larger gauge,[8,][13,][38] on the grounds that if a vessel is perforated, less bleeding occurs through a small rent and the bleeding is less precipitous. Because the orbital apex contains the largest vessels entering and exiting the orbit, the depth to which needles are inserted should be strictly limited. When serious bleeding occurs in this area, there is the problem not only of general increase in orbital pressure, making surgery difficult, but also of the potential for obstruction to the blood supply to and from the globe.
The anterior orbit generally has smaller vessels than exist posteriorly. Two anterior orbital locations, which are relatively avascular and frequently used as sites for needle placement, are the inferior-temporal quadrant and the compartment directly on the nasal side of the medial rectus muscle. Needle placement into the superior nasal compartment should be avoided because the end vessels of the ophthalmic artery system are located there, as are some large veins and the complex trochlear mechanism of the superior oblique muscle.
In intraocular surgery it is considered advantageous if the intraocular pressure is low and pressure fluctuations are kept to a minimum.[39] The attainment of a “soft eye” in the avoidance of complications, particularly suprachoroidal hemorrhage,[40,][41] was more important in a former era. Phacoemulsification techniques, which require a smaller surgical incision, are associated with smaller swings in intraocular pressure than the older intracapsular or extracapsular methods. At the completion of retrobulbar and peribulbar injections, mechanical orbital decompression devices[42–45] are commonly used to promote ocular hypotony and a reduction in vitreous volume,[46] especially when larger volumes of orbital injectate have been used (as in peribulbar blockade).
General Comment on Complications
The occurrence or avoidance of the complications mentioned previously is directly influenced by block technique. Elimination of known hazards (e.g., inappropriate globe position during block, inappropriate choice of needle path, inappropriate depth of needle placement) is the key to successfully avoiding complications.
Comparison of retrobulbar with peribulbar blockade
As discussed earlier, the peribulbar technique was introduced in 1986 as a less hazardous alternative to retrobulbar anesthesia in response to a concern about complications of the latter.[1] The rationale was that peribulbar technique, by avoiding needle placement within the rectus muscle cone, would avoid optic nerve damage and globe perforation. However, after initial enthusiasm, a significant number of globe perforations were reported.[47–49] Higher volumes of injectate were required to achieve akinesia, onset time of blockade was much slower than with retrobulbar, and repeat injections (each with inherent risk of complication) were more frequently required. Although there are proponents of both retrobulbar and peribulbar techniques, safe anesthesia can be accomplished by both methods; likewise, serious complications can arise with both if carried out incorrectly. Two published large series preferred the more dependable outcome of retrobulbar needle placement.[8,][50] Therefore, rather than condemn the retrobulbar technique outright, it merits revisitation and revision in the light of better understanding of the causes of various complications.[51]
Revised retrobulbar block
Site and Depth of Injection
The inferior-temporal orbital quadrant is the preferred location for retrobulbar needle placement because it provides easy access to the retrobulbar cone compartment (see Figure 8-1). To avoid complications (hemorrhages, optic nerve trauma, brainstem anesthesia, muscle damage), needles must never be inserted deeply to the orbital apex.[14] Injectate placement in the anterior retrobulbar compartment is much safer; from there, posterior spread occurs to achieve motor nerve blocking concentration at the apex of the cone.[52]
Needle Type and Syringe Size
Traditional teaching favored dull-tipped, intermediate-gauge needles with the supposed advantages that blood vessels were pushed aside rather than traumatized and that tissue planes could be more accurately defined. Although a commonly held belief among ophthalmologists,[47] it is not true that it is more difficult to penetrate the globe, the optic nerve sheath, or blood vessels with a blunt needle.[3] Larger dull needles, compared with fine disposable ones, cause more serious damage if the globe is penetrated.[3] Because disposable cutting needles produce minimal tissue distortion, little or no pain results. Tactile discrimination is progressively reduced with increasing needle size.[38] The use of blunt-tipped, wider-gauge needles should be abandoned.[53] Special attention should be paid to the length of needle entering beyond the orbital rim; 31mm as measured from the orbital rim should never be exceeded to rule out optic nerve impalement.[14] In regional block techniques (both retrobulbar and peribulbar), all needles should be orientated tangentially to the globe with the bevel opening faced toward the globe.[8] Because less force has to be exerted, a change in resistance to injectate flow is more easily detected by the injecting hand when using a needle mounted on a smaller syringe as compared with a larger size. This ability to more easily detect change in resistance to injection is important in avoiding complications, as is the regular use by all practitioners of standard sets of needles and syringes so that they become familiar with the normal flow resistance characteristics of their equipment. In addition, an “inject-as-you-advance” technique provides added safety.[54]
Advantage of Minimal or No Sedation
Fully conscious or minimally sedated patients on whom regional ophthalmic blocks are done painlessly can accurately report symptoms or demonstrate signs that may indicate onset of undesirable block complications. Thus, by being conscious they act as their own monitors. For example, the devastating complication of ocular explosion described previously[24,][25]is not likely to occur in a conscious patient because the pain experienced by the patient would be so great. Elderly patients require less pharmacologic support for anxiety at the time of surgery than do young patients and take the discomforts of life more easily “in their stride.” For a small percentage of elderly patients who benefit from preoperative sedation, fine judgment is required to select the correct drug dosage to produce a calm patient who remains alert and cooperative. The advantages of regional anesthesia can be negated rapidly with excessive use of sedation.[55] A recent multicenter study confirmed that intravenous anesthetic agents administered to reduce pain and anxiety are associated with an increased incidence of side effects and adverse medical events.[56] Incomplete regional anesthesia is best managed with block supplementation until complete; operating in the presence of obvious block failure subjects the patient to an unpleasant and stressful experience; and use of intravenous sedation to cover gross block inadequacy is hazardous and inappropriate.
Painless block techniques are achievable through the use of fine, sharp disposable needles and precision placement methods. The author strongly recommends a preblock transconjunctival injection of local anesthetic diluted 10 times with sterile balanced salt solution, which renders the percutaneous injection to follow totally painless (Figure 8-6).[57] This transconjunctival injection is carried out through conjunctiva previously rendered anesthetic with topical local anesthetic eye drops.
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Figure 8-6 Injection of “painless local” in inferior-temporal quadrant, lateral view. After instillation of topical anesthesia drops in the inferior conjunctival fornix, the lower eyelid is gently retracted with a finger. A 30-gauge 12mm needle enters transconjunctivally in the inferior-temporal area just posterior to the inferior tarsal plate with the shaft of the needle arranged tangentially to the globe. Following test aspiration, the initial injection is of 1 mL painless local* to a depth of 1cm from the conjunctiva. The needle has easily and painlessly penetrated the conjunctiva, and deep to it the capsulopalpebral fascia. The needle entry point is at the lower end of the lateral orbital rim (small insert). After an interval of 3–4 min, the skin overlying the site of injection (lateral third of lower lid) will be anesthetic. |
Preblock Assessment
A safe prerequisite to regional anesthesia of the orbit is to know the axial length measurement of the eye before the block to warn of the higher risk in longer-than-average eyes.[58] In cataract surgery a precise axial length measurement is usually available because it is required for intraocular lens diopter power calculation. In the presence of high myopia, peribulbar block or even general anesthesia, as opposed to retrobulbar block, may be more prudent. Similar caution would apply when a pre-existing scleral buckle exists from an earlier retinal operative procedure.
The axial length of the globe to be blocked is noted, as is the position of the globe in the orbit (enophthalmos versus exophthalmos), by observing the plane of the iris and the location of the globe equator relative to the temporal orbital rim.
Recommended Block Technique
The author, with an experience of 27,500 retrobulbar blocks and 5,700 peribulbar blocks over the past 18 years, has adopted a rational approach to safe retrobulbar blocking that stresses the importance of aiming the retrobulbar needle (27-gauge sharp disposable, 31mm length) “midway between the inferior and lateral rectus muscles”[30] from an inferior-temporal entry point at the junction of the temporal and inferior orbital rims (Figure 8-7). This modified entry point allows easy and safe access to the retrobulbar space because the temporal orbital rim is set back from the rest of the orbital rim (see Figure 8-1).
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Figure 8-7 Traditional and modified needle entry positions. The outline of the globe is superimposed on a template of the orbital rim. Traditional inferior block injection site is just inside the orbital rim at T. The author's modified injection site is inferior-temporal, just inside the orbital rim at M. |
The inferior-temporal rim of the orbit is palpated and the desired entry point chosen just inside the orbital rim at the 7:30 position for the right eye (Figure 8-8A) or the 4:30 position for the left eye. With the patient's eyes in primary gaze, the needle is advanced in a sagittal plane with a 10° upward inflection from the transverse plane, at first invaginating the skin while being directed safely between the globe and temporal orbit wall. It very soon penetrates the skin and can then be advanced to the depth of the globe equator before being redirected upward and inward toward an imaginary point behind the pupil, approaching but not passing the midsagittal plane (see Figures 8-1,8-8, and 8-9). The globe is continuously observed during needle placement to detect globe rotation that would indicate engagement of the sclera by the needle tip. During this latter action the circumference of the globe can be “palpated” with the shaft of the needle as it passes around (Martin Livingston, MD, personal communication). The modified entry position provides safer access to the orbit because there is more physical space here compared with the traditional entry point (see Figure 8-7). In addition, the modified technique avoids possible needle damage to the inferior rectus and inferior oblique muscles and to the motor nerve supply to the inferior oblique (see Figure 8-3).[28] A percutaneous as opposed to a transconjunctival needle entry point is used because it avoids having to combat the orbicularis tone often present in the inferior eyelid or the problems created when there is a narrow palpebral fissure and wide lateral canthal fold. Slow needle advancement following first penetration of the skin is ideal, with injections of minidoses of anesthetic solution at multiple intervals. Having reached the desired final needle-tip location, and after checking by aspiration for inadvertent intravascular placement, a slow injection of the desired volume of anesthetic solution is made. This interval method of needle advancement provides not only patient comfort but also constantly updated information about tissue resistances along the needle path. Should the needle tip penetrate the globe, the next minidose injection will announce itself loud and clear as severe pain (provided, of course, there has not been use of excessive sedation to render the patient beyond being able to act as his or her own monitor). A globe penetration picked up accurately and early in this fashion is far better than rapid needle placement to full depth in one swift motion with the possibility of globe perforation and its late diagnosis. Final depth of needle penetration of the orbit is gauged by observing the hub-shaft junction of the 31mm needle in relation to the plane of the iris (Figures 8-1 and 8-8), instead of measuring from the inferior orbital rim as in the traditional technique; thus, in dealing with enophthalmic and exophthalmic globes, there is automatic correction for the anomaly. In dealing with a globe of average axial length (23.5mm), when the midpoint of the 31mm needle is at the plane of the iris, the point of the needle will already have passed the globe equator. In like manner, ovoid globes in myopic patients (greater axial length measurement) will require a longer section of the advancing needle to guarantee passage beyond the globe equator before redirection into the retrobulbar compartment. The final desired needle-tip position lies between the lateral rectus muscle and the optic nerve, as depicted in the cadaver dissection (see Figure 8-9).
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Figure 8-8 Revised inferior-temporal retrobulbar block. A and D, Frontal views; B and E, lateral views; C and F, views from above. The inferior-temporal rim of the orbit is palpated and the desired entry point (*) chosen just inside the orbital rim at the 7:30 position for the right Eye (A) or the 4:30 position for the left Eye. With the patient's Eyes in primary gaze, the 27-gauge 31mm (11/4-inch) sharp disposable needle is advanced in a sagittal plane (C) with 10° upward inflection from the transverse plane (B), at first invaginating the skin while being directed safely between the globe and temporal orbit wall (C). It very soon penetrates the skin and can then be advanced to the depth of the globe equator (B and C). (If the needle were further advanced in the sagittal plane, contact with the lateral wall of the orbit would occur.) Revised inferior-temporal retrobulbar block. The needle is then redirected with medial and upward components (D and E) toward an imaginary point behind the pupil, approaching but not passing the midsagittal plane (F). The needle enters the retrobulbar space by passing through the intermuscular septum between the lateral and inferior rectus muscles (E). The globe is continuously observed during needle placement to detect globe rotation that would indicate engagement of the sclera by the needle tip. During needle placement, continuing observation of the relationship between the needle-hub junction and the plane of the iris establishes an appropriate depth of orbit insertion (E and F). In a globe with normal axial length as illustrated here, when the needle-hub junction has reached the plane of the iris, the tip of the needle lies 5–7mm beyond the hind surface of the globe (E and F). Following test aspiration, up to 4 mL of anesthetic solution is slowly injected. |
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Figure 8-9 Cadaver dissection with final needle position in retrobulbar compartment; photograph of the left orbit. The anterior orbital contents have been removed as far back as 5mm behind the posterior pole of the globe. Note the optic nerve stump and the amputated bellies of the four rectus muscles and the superior oblique muscle. The inferior oblique muscle has been removed along with the globe itself. A 27-gauge sharp disposable needle of 31mm length has entered the retrobulbar compartment. It has passed between the lateral and inferior rectus muscles. Its tip lies between the lateral rectus muscle and the optic nerve. Note the medial and upward angling of the needle required for it to access its final desired location. |
Injectate Mixture and Volume
The selection of anesthetic agent with additives depends mainly on the desired duration of effect. Concentrations up to, but not exceeding, 2% lidocaine (or agent of equivalent potency) are appropriate. Admixture with epinephrine is commonly used to prolong block duration and to increase block solidity, but it may be contraindicated if orbital vascular pathology is present; a concentration of 1:200,000, given the volume of injectate used in ophthalmic regional anesthesia, is devoid of systemic effects.[59] Hyaluronidase, a highly purified bovine testicular enzyme that hydrolyzes extracellular hyaluronic acid,[60] is a desirable component for promotion of spread within the orbit and for hypotony.[61,][62] Recently the product has been in short supply and, in fact, is no longer being produced.[60] Anecdotal reports have linked its absence from local anesthetic mixtures with a higher rate of complications, notably diplopias resulting from toxic levels of anesthetic in the extraocular muscles.[63] Having attained the desired safe depth of placement in the anterior retrobulbar compartment, and following a negative test aspiration for possible intravascular penetration, a volume of up to 4mL of the chosen mixture is slowly injected. Younger adults present more of a challenge in achieving akinesia than the elderly because of more dense connective tissues, hindering the access of anesthetics to the motor nerves of the extraocular muscles.[64]
Complementary medial block
Because peripheral orbit sensation may be retained following retrobulbar block, as mentioned earlier (anatomy paragraph), a small-volume peribulbar local anesthetic injection provides an excellent complement. The site of choice is injection into the peribulbar fat compartment on the nasal side of the medial rectus muscle (Figure 8-10).[33] In addition to peripheral orbital anesthesia, this complemental injection provides effective blockade of the central fibers of orbicularis oculi, thus avoiding the need for van Lint or other type of facial nerve blockade. Injection into this compartment at a depth of 25mm will contribute useful extraocular motor-nerve blockade, whereas more superficial placement (for which a 12mm needle may be chosen) will provide excellent central orbicularis muscle blockade. The patient's eyes are directed in primary gaze. With the bevel facing the medial orbital wall, the needle is directed toward the interaural line and toward the midline of the skull at the occiput[65] and inserted to the desired depth. The volume injected can be within the range of 1–5mL of local anesthetic solution, depending on the desired effect.
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Figure 8-10 Complementary peribulbar block. Medial pericone block: needle entry point (*) is on the medial side of the caruncle at the extreme medial angle of the palpebral fissure (A and B). The patient's eyes are directed in primary gaze. With the bevel facing the medial orbit wall, the needle is directed toward the interaural line and toward the midline of the skull at the occiput;[65] that is at about 5° toward the medial orbit wall (B to D). Continuing observation of the relationship between the needle-hub junction and the plane of the iris controls appropriate depth of insertion (D). In a globe of normal axial length, the 25mm needle tip will be at the depth of the hind surface of the eye. The eye in the drawing is 23.5mm in diameter, and the needle is 25mm long. Injection at a depth of 25mm will contribute useful extraocular motor-nerve blockade, whereas more superficial placement will favor blockade of the central fibers of the orbicularis muscle. Volume injected can be within the range of 1–5 ml of local anesthetic solution, depending on the desired effect. |
Peribulbar block
The adjective peribulbar refers to that location external to the confines of the four rectus muscles and their intermuscular septa. In the technique known as peribulbar block, local anesthetic agents or mixtures are deposited within the orbit but do not enter within the geometric confines of the cone of rectus muscles. The mechanism whereby it works was elucidated by Koornneef,[6] who demonstrated that the intermuscular septum between the rectus muscles was incomplete and permitted anesthetic deposited outside the cone of rectus muscles to spread centrally. Introduced as a safer method than intraconal blocking to avoid serious complications,[1] these nevertheless have been reported.[47–49] Knowledge of orbital anatomy is just as important as with the older method, and there are disadvantages to using periconal blocking. Davis and Mandel[1] in 1986 were first to publish a paper on the peribulbar block method. Calling their block technique posterior peribulbar, they used two intraorbital needle placements outside the muscle cone, one above and one below the cone, each to a depth of 3.5cm with a total of up to 10mL of solution injected.[1] Bloomberg[66] championed the cause of shorter needle peribulbar regional anesthesia and called his technique periocular block. In Bloomberg's method a 2.5-cm, or a 25- or 27-gauge needle entered the inferior-temporal orbital quadrant and was directed “deliberately toward the orbit floor” to a depth of 2cm; a single 8- to 10-ml injection was given. He stated that only 5% of patients required supplemental blocking. Other authors report up to 50% failure to achieve akinesia with periconal blocking.[5,][67] Onset of akinesia is considerably slower than with intraconal block,[5,][68–70] volume requirement is greater,[71] postinjection orbital pressure is greater,[72] and the supplementation rate to achieve total akinesia is higher.[5,][73,][74] The incidence of periorbital ecchymoses[68] and conjunctival chemosis is also greater.[67,][69,][75] Of the many variations of the peribulbar technique, a common one is placement in two locations, one inferior-temporal and the other in the superior nasal orbit (a site that is vascular and, therefore, prone to hematoma formation). For those who wish to practice peribulbar blocking, the author suggests a two-needle technique: the first being an injection in the inferior-temporal quadrant, as described in Figure 8-11, and the second an injection into the medial fat compartment on the nasal side of the medial rectus muscle (complementary block as described in the previous section). Up to 5 mL of local anesthetic solution is injected at each site. Because there is insufficient space between the lateral rectus and inferior rectus muscles, and the lateral and inferior walls of the orbit, respectively, these areas cannot be used without risking extraocular muscle injury.
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Figure 8-11 Peribulbar block, inferior-temporal injection. A, Frontal view; B, view from above; C and D, lateral views. The inferior-temporal rim of the orbit is palpated and the desired entry point (*) chosen just inside the orbital rim at the 7:30 position for the right eye (A) or the 4:30 position for the left eye. With the patient's eyes in primary gaze, the 27-gauge 25mm sharp disposable needle is advanced in a sagittal plane (B) with 10° upward inflection from the transverse plane (C and D), and passes the globe equator to a depth controlled by observing the needle-hub junction reach the plane of the iris (B). Percutaneous needle entry is the preferred technique (C); however, the transconjunctival route is also possible (D). |
Parabulbar (sub-tenon's) block
Anesthesia for cataract surgery produced by injection beneath Tenon's capsule of small volumes of local anesthetic was first described by Swan[52] in 1956. He indicated that the sub-Tenon's method produced better iris and anterior segment anesthesia than did subconjunctival injection. Since 1990 the sub-Tenon's injection technique has been extensively used.[76]This injection technique evolved into anesthesia produced by blunt cannula insertion[77] after surgical dissection into the sub-Tenon's space.[77–79] Onset of anesthesia is rapid;[81] the degree of abolition of extraocular muscle movement is proportional to the volume of injectate. Following placement of local anesthetic by cannula beneath Tenon's capsule, spread occurs into the anterior retrobulbar space.[82] Disadvantages of the method are an increased incidence of conjunctival chemosis and hemorrhage, and the potential of damaging one of the vortex veins.[77] Conjunctival hemorrhage is common if diathermy is not used.[82] Peripheral orbital anesthesia may be incomplete; supplemental local anesthetic injections may be necessary to achieve patient comfort.[83] Repeat sub-Tenon's injections can be performed simply in the presence of incomplete anesthesia.[53] Unlike topical corneoconjunctival anesthesia, sub-Tenon's, retrobulbar, and peribulbar techniques easily abolish iris and ciliary body sensation, and can be used to produce globe akinesia.[83]
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