Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Cycloplegics |
273 |
Side effects
-Transient stinging effect.
-Increase in IOP in POAG patients due to decrease in aqueous outflow
-Angle closure glaucoma
Systemic reactions are rare - as decrease in affinity for systemic muscarinic receptors. Safest for systemic hypertension / angina and other cardiovascular patients and in neonates. Patients with hypersensitivity to belladonna exhibit cross sensitivity to tropicamide.
25
Local Anesthetics
LOCAL ANESTHETICS
Local anesthetics are drugs which cause reversible loss of sensory and motor impulses without loss of consciousness. Local anesthetics can be classified as ester linked and amide linked.
Amide linked agents produce more intense, long lasting anesthesia and rarely cause hypersensitivity reaction, whereas the ester linked LAs, have short duration and less intense anesthesia and is used on mucous membrane.
Ester linked LAs—Cocaine, Procaine, Tetracaine. Amide linked LAs—Lidocaine, Bupivacaine, Ropivacaine.
Injectable anaesthesia also can be classified as:
1.Low potency - Procaine, Chlorprocaine.
2.Intermittent potency and short duration - Lidocaine.
3.High potency and long duration - Bupivaccine, Tetracaine, Ropivacaine.
In 1884, Carl Koller for the first time used Cocaine for topical anesthesia. It was used as 1% - 4 % topical solution. Cocaine is epitheliotoxic and used to remove corneal epithelium in severe epithelial edema. Because of its epithelial toxicity it was later discarded. Knapp used Cocaine as retrobulbar anesthesia for enucleation. Van Lint used LAs for orbicualaris akinesia. Nowadays topical anesthesia with Proparacaine, Tetracaine are used for clear corneal phacoemulsification.
Mechanism of Action
LAs cause reversible nerve conduction block by decreasing the Na+ uptake thereby decreasing the depolarization of nerves and slowing the conduction. Sensory and motor fibres are equally sensitive. Sensitivity is determined by diameter of nerve fibres, smaller fibres are more sensitive. Autonomic fibres are generally more susceptible than somatic fibres.
LAs are used either as topical, regional infiltration or specific nerve block in ophthalmology.
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Local Anesthetics |
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TOPICAL ANESTHESIA |
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DRUG |
Maximum dose |
Onset of action |
Duration(min) |
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(in mg%) |
(min) |
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Cocaine( 1% - 4%) |
20 |
1 |
20-30 |
Tetracaine ( 0.5%, 1%) |
5 |
1 |
15 |
Proparacaine ( 0.5%) |
10 |
1 |
15 |
Lidocaine ( 4 %) |
3 |
1 |
15 |
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Cocaine is a natural alkaloid from Erythroxylon coca. It is a good surface anesthetic, which is rapidly absorbed from mucous membranes. It is a protoplasmic poison and produces tissue necrosis. In the periphery it stimulates sympathomimetic effect. It causes constriction of conjunctival vessels, clouding and rarely sloughing of cornea due to drying and tissue toxicity.
Proparacaine is the first synthetic LA used. It has least bactericidal topical activity so it can be used as surface anaesthesia for diagnostic corneal scraping.
Tetracaine ( Amethiocaine ) — is more potent and more toxic due to slow metabolism. It is used both as surface and infiltrative anesthetic. Its absorption from mucous membrane is fast. Tetracaine and Proparacaine are the least toxic to corneal epithelium.
Benoxinate (0.4%) an ester anesthetic, is currently most frequently used as topical agent because of its high degree of safety.
Lidocaine is the most widely used LA. It is used both for surface and infiltrative anesthesia. It produces nerve block within 3 minutes. CNS side effects like drowsiness, mental clouding and tinnitus may occur. Overdose results in twitching, convulsions, arrhythmias and respiratory arrest.
Regional Infiltration
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Onset ( min) |
Duration |
Maximum dose |
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(mg/Kg) |
Lignocaine ( 2%) |
5 |
45 min |
5 |
Lignocaine with adrenaline |
5 |
1-2 hrs |
7 |
Bupivacaine ( 0.25 - 0.75%) |
7 - 10 |
2hrs |
200 |
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Bupivacaine is a long acting local anesthetic. It is used as infiltrative anesthetic which produces a delayed but prolonged effect. It is prone to cause tachycardia and cardiac depression. Ropivacaine, bupivacaine congener is less cardiotoxic and is equally long acting.
Adverse effects of topical ocular anaesthetics are:-
1.Epithelial toxicity—healing is delayed after an epithelial defect. Lignocaine does not affect healing.
2.Alteration of lacrimation and tear film stability.
3.Endothelial toxicity occurs when associated with penetrating trauma and Benzalkonium is used as a preservative.
4.Contact dermatitis can occur and is common with Proparacaine.
5.Surface keratopathy can occur.
6.Systemic effect may occur particularly with cocaine.
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Adrenaline, a vasoconstrictor is used in concentration of 1: 100,000 to 1: 200,000. The advantage is
1.Prolongs the duration of LAs.
2.Reduces the systemic toxicity of LAs.
Disadvantages
1.It makes injection more painful.
2.May raise BP, cause arrhythmias in susceptible individuals.
3.Increases local tissue edema and delays wound healing by reducing the
oxygen supply.
Hyaluronidase is an enzyme that hydrolyzes hyaluronic acid, an essential component of the connective tissue cement substance. As a result, tissue permeability is increased, and injected solutions and local accumulations of fluid spread more rapidly and are absorbed faster. It reduces the amount of anaesthetic used and enhances quick onset of action of drug. Preparations are as follows: lyophilized powder, 150NF(TR)units and 1500NF(TR)units per vial: solution for injection, 150USP units per ml.It is used in concentration of 7.5 to 10units/ml of anaesthetic.
OCULAR ANESTHESIA
Ocular anesthesia can be classified as those for:
1.Intraocular surgery.
2.Extraocular surgery.
Local anesthesia for Extraocular surgery (Fig. 15.1).
Lacrimal Sac
Regional anaesthesia for Dacrocystectomy and Dacrocystorhinostomy is produced by:
1.Anaesthesia of nasal mucosa by packing with ribbon gauze soaked in lignocaine 4 % plus adrenaline 1: 100,000. Injection of lignocaine 0,5 ml is done to raise nasal mucosa from bone.
2.Blocking the nasociliary nerve by injection at junction of roof and medial wall of orbit for 3 cm depth.
3.Another infiltration over anterior lacrimal crest towards the nasolacrimal duct.
Anaesthesia of Upper Lid
Supraorbital nerve is found by palpating superior orbital notch at junction of medial 1/3rd and lateral 2/3rd of superior orbital rim. Infiltration of 1-2 ml is done by pushing the needle backward and slightly downwards piercing the orbital fascia. It is redirected medially and laterally to involve supratrochlear and infratrochlear nerve. Injection of lacrimal nerve completes akinesia of upper lid.
Local Anesthetics |
277 |
Fig. 15.1
A modified single injection by Hildreth and Silver is given by a 4 cm needle injected in midline of orbit following roof and about 0.5 ml is injected. This gives anaesthesia of upper lid except extreme nasal and temporal ends.
Infraorbital nerve block
The infraorbital foramen lies 4-5 mm below lower orbital margin in the same vertical plane as supraorbital notch. About 2 ml of anesthetic is injected at this point and directed laterally and posteriorly.
It can be also done by passing a needle along the junction of floor and lateral wall for 1.5-2.5 cm to reach inferior orbital fissure. Injection of 1-2 ml is made around infra orbital nerve as it enters the orbit. Anaesthesia for intraocular surgery involves akinesia of orbicularis oculi, extraocular muscles and sensory interruption of ciliary nerves.
Akinesia of orbicularis oculi
Orbicularis oculi is supplied by the facial nerve. Facial nerve passes from stylomastoid foramen into substance of parotid gland and divides into temporofacial and craniofacial branches. Temporofacial division passes in front of the neck of the mandible and divides into temporal and zygomatic branch and supplies the orbicularis oculi. Facial nerve and its temporofacial branch
can be blocked in its course by |
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1. |
Nadbath - Ellis |
2. |
O'Brein |
3. |
Atkinson's |
4. |
Van Lint. |
Nadbath-Ellis involves injection at area of stylomastoid foramen where facial nerve emerges. 2526 G, 5/8th inch needle is used and 4 ml of anesthetic is injected perpendicularly in that area. Jugular foramen with vagus, glossopharygeal and spinal accessory nerve is located near stylomastoid foramen and accidental involvement may lead to dysphagia, dysphonia and respiratory arrest.
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O'Brien's block: It is the commonly employed method by blocking the proximal trunk of facial nerve. The condyloid process of mandible is located by palpation just in front of the tragus. 27G, 1 inch needle is inserted upto the periosteum and 3-5 ml of anesthetic is injected. Injection may injure the temporomandibular joint, or external carotid artery which lies at a deeper level. Atkinson's block: Temporofacial branch is blocked as it crosses the zygomatic arch. Injection is made with 23G, 1.5 inch needle from inferior edge of zygomatic bone and directed upwards across zygomatic arch towards the top of the ear. This avoids damage to cervicofacial branch.
Van Lint akinesia: Van Lint block is made by injecting vertically towards temporal fossa from 1cm lateral to lateral canthus. It is then injected subcutaneously along entire extent of both eyelids. Disadvantage is ballooning of lids which can be reduced by adding hyaluronidase which aids in rapid diffusion.
Akinesia of EOM and anesthesia of orbital contents:
RETROBULBAR (INTRACONAL) ANESTHESIA
Injection is made within the posterior muscle cone. It is a blind procedure in which dull tipped 1.25 inch 25-27 G needle is used.
The position of eye during the procedure is important. Patient is asked to look in primary gaze during injection at junction of lateral 1/3rd and medial 2/ 3rd of lower orbital margin. Surgeon should recognize when the needle has passed through orbital septum and then through intermuscular septum into muscle cone. 1.5 - 2 ml of anaesthetic mixture is injected which paralyses the nerves in the muscle cone and the ciliary ganglion. Trochlear nerve which is outside the muscle cone is spared and there is some amount of torsion seen. Depth of 31mm from orbital rim should not be exceeded to prevent injury to optic nerve.
Complications which occur during the procedure
1.Globe perforation.
2.Retrobulbar hemorrhage.
3.Optic nerve trauma.
4.CNS spread by piercing the meninges.
5.Retinal vascular occlusion.
6.Strabismus.
7.Ptosis.
Globe perforation occurs more commonly in myopic and deep set eyes. It results in marked pain, hypotony and poor red reflex. Retrobulbar hemorrhage may be either venous or arterial. Arterial hemorrhage causes proptosis, elevated IOP, massive ecchymosis of lids and conjunctiva, and inability to separate the lids. Increased IOP may affect the perfusion of optic nerve and may result in late optic atrophy. Venous hemorrhage spreads slowly. Surgery should be postponed.
Local Anesthetics |
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Direct spread to CNS through subdural space may occur. This results in mental confusion, vomiting, dysphagia, dyspnea and respiratory depression. This can be prevented by maintaining primary gaze during injection and not penerating more than 31 mm.
Strabismus may occur after injection of anesthetic mixture into muscle. This mostly affects inferior rectus muscle due to progressive contracture of muscle.
PERIBULBAR (PERIOCULAR) ANESTHESIA
Peribulbar anesthesia has largely replaced retrobulbar anesthesia for intraocular surgery. Various methods are described by different texts.
It is given at the junction of lateral 1/3rd and medial 2/3rd of inferior orbital rim using ¾ th inch, 25 - 27 G needle in the peribulbar space. 1 ml of solution is injected in orbicularis muscle, the needle is pierced and 3 ml is injected at the equator and another 3 ml behind the equator. This creates mild proptosis and chemosis of conjunctiva which can be tackled by superpinke or Honan balloon.
Alternative method involves injecting, 5 ml of anaesthetic mixture at junction of lateral 1/3rd and medial 2/3rd of the lower orbital rim either transconjunctivally or transcutaneously into peribulbar space. Another 5 ml may be required as superior injection either temporally or nasally below the superior orbital rim.
Advantages
1.Separate facial nerve block is not required.
2.Less chance of retrobulbar hemorrhage, perforation of globe, optic nerve.
Disadvantages
1.Akinesia is slow.
2.Orbital pressure is more.
3.More amount of drug required.
4.Periorbital ecchymosis, conjunctival chemosis.
SUBTENON ANESTHESIA
The procedure is done with blunt cannula which is inserted into subtenon space in inferotemporal quadrant. The procedure can also be done with 27G needle. The degree of Akinesia is proportional to drug.
Disadvantage: conjunctival chemosis and haemorrhage (due to damage to vortex veins).
Advantage:
1.No rise of IOP.
2.Lack of elevation of blood pressure.
TOPICAL ANAESTHESIA
Topical anaesthesia is gaining popularity for clear corneal phacoemulsification. It requires a very cooperative patient. Midazolam is injected about ½ hour prior to surgery.
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Topical anaesthetic tetracaine 0.5%, 0.75% bupivacaine or 4 % lignocaine can be used by instillation 5-10 minutes before surgery. A piece of cotton impregnated with mixture of lignocaine, bupivacaine and hyaluronidase is placed in the superior and inferior fornix. Topical anesthesia can be combined with intracameral lidocaine (1%).
Intracameral lidocaine provides uveal anesthesia for pressure changes which act at the root of the iris and ciliary body. It is used as 1% isotonic non preserved lidocaine 0.3 ml.
Advantage is, it is cost effective, there is immediate visual recovery and avoidance of complications of retrobulbar and peribulbar anesthesia.
Disadvantage is awake and talkative patient. No Akinesia of eye and if difficulties or problem occur the anesthesia may not be adequate.
Respective advantages of regional and general anesthesia
Regional anesthesia |
General anesthesia |
Simple technique |
Complete control of patient |
Less postoperative nausea and vomiting |
No risk of retrobulbar hemorrhage |
Faster recovery |
No risk of globe perforation |
Postoperative analgesia superior |
No risk of myotoxicity |
Blockade of oculocardiac reflex |
Applicable to all ages |
Absence of respiratory depression |
Preferable for teaching of surgery |
Full mental status retained |
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No loss of 'control' for patient |
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Potential for reduced stress |
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No risk of toxic hepatitis |
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Not contraindicated with low serum K+ |
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No risk of malignant hyperthermia |
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Easily applicable in high altitude |
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Contraindications to regional and general anesthesia (in decreasing order of significance)
Regional anesthesia |
General anesthesia |
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Reversible medical condition, uncorrected |
Reversible medical condition |
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Anesthetist inexperience |
History of serious adverse effect from GA |
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True allergy to LA |
History different airway |
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Emergency surgery (open eye wound) |
Known problems or disease state |
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Prolonged surgery (more than 2 hours) |
c. |
malignant hyperthermia |
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Children upto age of early teens |
d. |
muscle disease: dystrophia |
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Unsuitable psychological status |
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myotonica, myasthenia gravis |
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Mental retardation |
e. |
haemoglobinopathies |
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Senile dementia |
f. |
chronic obstructive pulmonary disease |
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Head movements or tremors |
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caution with |
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Inability to lie flat |
g. |
history of porphyria |
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Intractable cough |
h. atypical pseudocholinesterase |
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Communication barrier |
i. patients on MAO inhibitors |
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a. |
language |
j. |
interactions with regular medications |
b. |
deafness |
k. |
patient on anticoagulants |
moderate to severe arthritis neurological disease claustrophobia
caution with patients on anticoagulants
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DOs and DON'Ts of ophthalmic regional anesthesia
DON’Ts |
DOS |
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Use the ‘up and in’ globe position for inferotemporal intraconal blocking. Attempt without anatomical knowledge.
Fail to check the axial length.
Fail to check for globe anomalies of previous surgical intervention.
Inject with other than tangential alignment
Use the superonasal quadrant
Inject at the orbital apex using long needles
Use dull coarse needles
Inject into extraocular muscles
Use unnecessarily high concentrations of anesthetic agents.
Use primary gaze position, or consider 'down and out' and 'down and in'.
Use a precise technique based on sound anatomical knowledge.
Measure axial length (thinner sclera with long ovoid myopic eyes).
Take extra caution in patients known to have staphyloma, coloboma or scleral buckle.
Align needles tangentially to the globe for all injections.
Use avascular injection sites.
Inject no deeper than 31 mm from the inferior orbital rim.
Use fine sharp needles in combinations with good anatomical knowledge for better tactile discrimination, patient comfort and a lower incidence of tissue trauma and hemorrhage
Avoid extraocular muscles.
Use a well - thought technique, volume and concentration of chosen anesthetic agent.
26 Fluorescein Sodium
and Other Dyes
FLUORESCEIN
It is a yellow acid dye made of xanthene. It is generally available as sodium salt with molecular weight of 376 Kd. Straub in 1888 first clinically used fluorescein for detection of corneal ulcers and in 1910. Burk reported use of fluorescein to detect retinal disease.
Depending on the pH of the solution it exists in various ionic states.
pH less than 2- cationic form predominates with weak blue-green fluorescence. pH 2 - 4 - cation dissociates to neutral molecules.
pH 7 - negative ions predominate with brilliant yellowgreen fluorescence.
Factors Altering the Fluorescein
1.Concentration.
2.pH of the solution.
3.Presence of other substances.
4.Intensity and wavelength of absorbed light.
Clinical Use
It is available as topical drops, fluorescein impregnated filter paper strips which was developed by Kimura, and as injection form for IV use. Pseudomonas aeruginosa grows easily in presence of flourescein. Preservatives are used to prevent growth of microorganism in fluorescein solutions (like chlorbutanol, thiomersal).
Topical Preparations
2% freshly prepared fluorescein is commonly used. Uses are-
1.For assessment of ocular surface integrity. Because of its high degree of ionization at physiological pH, fluorescein neither penetrates intact corneal epithelium nor forms a firm bond with any vital tissue. Breakdown of epithelium allows stromal penetration. When observed with cobalt blue filter of slit lamp the epithelial defects appear outlined in vivid green fluorescence because the yellow orange dye diffuses freely through the intercellular spaces and accumulates in the defect.