Ophthalmology Flashcards
Main goal of ophthalmic anesthesia
prevent unwanted increases in IOP
Increased IOP = pressure on optic N = vision loss
Also want to prevent sudden increases in IOP in patients with partial/imminent loss of globe integrity to avoid complete rupture ie desmetocele, trauma, deep corneal ulcer
Aqueous Humor
similar to blood plasma but low protein, fills space in front of eyeball btw lens and cornea, maintains IOP, provides nutrients to eye
Comprises anterior, posterior chamber
Vitreous humor
clear gel that fills space btw lens, retina; provides nutrients to eye and helps eye hold shape
Dorsal Rectus M
CNIII
Elevation, medial rotation of globe
Intraconal
Deficit: ventrolateral strabismus
Medial Rectus M
CNIII
Adduction of Globe
Intraconal
Deficit: ventrolateral strabismus
Ventral Rectus m
CNIII
Depression, lateral rotation of globe
Intraconal
Deficit: ventrolateral strabismus
Superior elevator palpebral/levator palpebrae superioris
CNIII
Retracts superior eyelid
Intraconal
Deficit: Ptosis
Retractor Bulbi m
CNIII
Pulls globe into socket
Intraconal
Deficit: exophthalmus
Ventral Oblique m
CNIII
Elevation, lateral rotation
Intraconal
Deficit: VL strabismus
Dorsal Oblique m
CV IV
Medial Rotation of Globe
Intraconal
Deficits: Rotational strabismus, looking down
Lateral Rectus m
CN VI
Abduction
Intraconal
Deficits: Medial Strabismus
What else is supplied by CN III?
Parasympathetic visceral motor innervation to pupillary constrictor m anisocoria
CN V - branches
ophthalmic
maxillary
ciliary
Ophthalmic Br of CN V
exits via orbital fissure: lacrimal N, nasociliary, frontal N
Br of nasociliary = infratrochlear = medial canthus
Maxillary Br of CN V
exits via round foramen: zygomaticofacial N = lateral canthus
Sympathetic Innervation of Eye
T1-T3 SC segments -> vagosymapthetic trunk –> cranial cervical ganglion –> ophthalmic br CN V
Blood Supply to the Eye
Ocular perfusion pressure determines blood supply to retina, optic nerve
OPP = MAP – IOP
Cats: no collateral circulation, exclusively maxillary a
PLR
(II in, III out): assessment of parasympathetic pathway
- CN II from retina –> optic tract +/- decussation at optic chiasm
- pretectal nucleus
- parasympathetic nucleus CN III (CB)
- project via CN III to ciliary body
- postganglionic neurons in ciliary body project to pupillary constrictor m to mediate constriction of pupil
Pupil Dilation
- Autonomic centers in brainstem
- lateral tectogemento-spinal tract
- synapse SC segments T1-T3
- Vagosympathetic trunk
- cranial cervical ganglion
- pupillary dilator m
Palpebral Reflex
Maxillary: lateral (zygomaticofascial)
Ophthalmic branch: medial (infratrochlear)
Palpebral: trigeminal N to trigeminal sensory nucleus –> facial motor nucleus –> CN VII to orbicularis oculi
Menace Response
(II in, VII out)
Not a reflex: learned behavior, requires pathways involving cerebral cortex, cerebellum
Medial retina (optic nerve); continuing through the contralateral geniculate nucleus, motor cortex, pontine nucleus; to cerebellum; terminating at both facial nerves
Corneal Reflex
mediated via nasocillary n (ophthalmic br of trigeminal), same pathway as palpebral
IOP
Depends on balance btw inflow, outflow of aqueous humor
Also affected by extraocular m tone, choroidal blood flow, CVP
Goldman Equation: IOP = (AH formation rate/AH outflow rate) + episcleral venous pressure
AH Flow
Produced by Ciliary Bodies
Conventional Outflow Pathway
Unconventional Outflow Pathway (Uveoscleral)
Conventional AH Outflow Pathway
AH enters venous vascular system via scleral venous plexus (analogous to Schlemm’s canal in humans)
drains into vortex veins
orbital vasculature
episcleral venous system
Unconventional AH Outflow Pathway
HORSES
Exits anterior chamber via diffusion through iris stroma, CB musculature
* Flows caudally to enter suprachoroidal then scleral/choroidal vasculature
* Involves ciliary m, superciliary regions, choroidal spaces
From there, drains either through scleral pathway or vortex pathway
Scleral Pathway for Unconventional AH Outflow
drains across sclera to be reabsorbed by orbital vessels
Vortex pathway for Unconventional AH Outflow
AH enters choroid itself to drain through vortex vessels, less dependent on IOP
Choroid Blood Flow
arterial BPs, CVPs
Intraocular (choroidal) blood volume determined by arterial inflow, venous outflow, tone of intraocular vasculature
Autoregulation of choroidal blood flow minimizes effects of systemic ABP on choroidal blood volume, IOP
Hypoxemia, hypercapnia effect on IOP
induce VD, increase intraocular blood volume, increase IOP
Hyperventilation may not decrease IOP DT effects of IPPV on CVP
not demonstrated in horses DT differences in aqueous flow
Effect of Resp Alkalosis, hyperbaric oxygen
induce VC –> decreased AH formation via decreased CAH activity, decreased choroidal blood volume, IOP
Normal IOP: dog
Dog 10-26mm Hg
Normal IOP: cat
Cat 12-32mm Hg
Normal IOP: horse
Horse 23.5-28.6mm Hg
Normal IOP Cattle
Cattle 16-39mm Hg
Consequences of Increased IOP
lens/vitreous prolapse, choroid hemorrhage, subsequent retinal detachment
Physical Causes of Increased IOP
Pressure on eyelids (facemask)
ET Intubation
Excessive restraint, struggling
Jug vein pressure
Head below body
Cataract sx
Physiologic Causes Increased IOP
Vomiting
Pre-existing glaucoma
Coughing
Tenesmus, straining
Hypoxemia
Hypercapnia
Pharmaceutical Causes Increased IOP
Succinyl choline
Ketamine (??) - dose dependent
Etomidate (myoclonus)
Positive Inotropies/VPs
Most Anesthetics Effect on IOP
anesthetic drugs decrease IOP, likely via multiple mechanisms: m relax, decreased venous and arterial BP, increased aqueous outflow, central depression of diencephalic centers controlling IOP
Pupil Size - mammals
smooth muscle units, autonomic innervation
* Sympathetic –> iris dilator m –> mydriasis = pupil dilation
* Parasympathetic –> iris constrictor m –> miosis = pupil constriction
Pupil Size - birds, most reptiles
striated pupillary muscles
* VOLUNTARY ACTIVE CONTROL OF PUPIL DILATION
* Unresponsive to topically applied parasympatholytic/sympathomimetic agents
WILL respond to NMBA
Globe, Pupil Position for Ophthalmic Anesthesia
Most anesthetics will constrict pupil - use atropine or epi, important to facilitate cataract sx
Globe usually needs to be central: ketamine, NMBAs
Tears
Protects, mechanical removal of debris and bacteria from ocular surface, lubricates cornea to maintain transparency, nourishes cornea
Primary O2 source for avascular cornea
- Reflex Tears
- Basal Tears
Tear Production and Anesthesia
Produced by Lacrimal Gland
Depressed produced of both types of tears by anesthesia EXCEPT IM ketamine (increased d tear production) for up to 24hr
PSNS: increased tear production
SNS: decreased tear production
reflex tears
produced IRT irritants by optic nerve (bright light), trigeminal nerve (wind, temperature changes, conjunctival/corneal irritation)
Basal Tears
impt for normal tear film function, produced constantly
Schrimer Tear Test
Quantitative eval of tear function
Normal: 15-20mm/min in most species
* STT Type 1: reflex tears
* STT Type 2: basal tears utilizing topical ax, drying of ventral conjunctival fornix
Tear production decreases with age
Oculocardiac Reflex (Ashner’s Reflex)
Triggered by: globe pressure or traction, retrobulbar block, ocular trauma/pain, traction on extraocular muscles
Possibly more likely to occur when p hypercapnic, quickly changing/high levels of globe/m traction
Bradycardia, ectopic beats/dysrhythmias, asystole, vfib
More acute onset and more sustained pressure/traction, more likely OCR is to occur
Afferent Pathway of Oculocardiac Reflex
ciliary nerves to ciliary ganglion
ophthalmic br of trigeminal n (CNV)
sensory nucleus of trigeminal N/motor nucleus of Vagus in fourth ventricle
Efferent Pathway of OCR
vagal nucleus into efferent vagal fibers/vagal cardiac depressor n –> negative dromotrophy, inotropy
Treatment of OCR
dc stimulation, +/- atropine if needed
Atropine admin to tx/prevent OCR in people controversial
Can be effective if OCR persists, but dosage/timing of atropine affects ability to block reflex
Topicals: cholinergic agonists
Tx glaucoma by increasing aqueous outflow
Direct agents mimic ACh, indirect = anti ACh-E
SE: systemic absorption –> bradycardia, AV block, bronchoconstriction
Ex: pilocarpine (direct), few to no systemic effects
Potential Effect of Indirect Cholinergic Agents
Anti AChE: additive effect with organophosphates,
Potential Effect of Indirect Cholinergic Agents
Anti AChE: additive effect with organophosphates, prolong duration levels of succinylcholine - d/c 2-4wks prior to succ use
Topicals: Cholinergic Antagonists (eg atropine)
Induce mydriasis (pupil dilation) – paralyze pupillary sphincter
Topical atropine will increase IOP, effect of atropine IV ???
No effect of systemic glyco on IOP/pupil size
Tachycardia, ileum (horses) if systemic absorption
Topical Adrenergic Agonists
peripheral VC, mydriasis
* Systemic hypertension, tachycardia
* Subconjunctival phenylephrine: hypertension, pulmonary edema in horses
topical a2s will decrease IOP: MOA not well understood
Timolol used in past
Osmotic Agents for Ophtho Patients
PO, IV –> fluid shift –> decrease vol of vitrous body, allows for better drainage by opening iridocorneal angle , decrease IOP
Use: emergency tx, short term control of glaucoma
Ex: Glycerol, mannitol
Topicals: carbonic anhydrase inhibitors
Ex: acetazolamide, dorzolamide
Decrease IOP via decreased AH production
Important to remember where carbonic anhydrase is
* Systemic effects: may cause renal chloride retention; K/HCO3 excretion (decreased HCO3 resorption) = metabolic acidosis, hyperchloremia, hypokalemia
Topical CAIs: minimal systemic effect when used short term in dogs, cats, horses
Glycerol
Osmotic agent used for emergency glaucoma tx
* PO admin, slower onset
* Relatively non toxic, emesis reported
* Metabolized to glucose = caution in diabetic patients
Mannitol
Emergency glaucoma tx
- Not metabolized to significant degree –> urine excretion, osmotic diuresis so will decrease urine output
- Rapid expansion of EC volume, overloading of CV system
may precipitate formation of pulmonary edema in patients with CV dysfunction, patients under GA, patients with renal dysfunction
–PPV during, immediately after admin of mannitol may help prevent formation of PE vs SpV
Topical Prostaglandin Analgoues
Ex: latanoprost
Most commonly used now for glaucoma tx in dogs
* Increase uveoscleral (unconventional) outflow of AH
* Minimal to no systemic effects
Topical, subconjunctival corticosteroids
prednisolone acetate
May incur systemic effects of hepatopathy, alopecia, marked decreases in cortisol production
* Dose, duration dependent
Implicated in late term abortions in llamas when applied late gestation
NSAIDS! Can become Cushingoid
Topicals: NSAIDS (diclofenac)
Increased IOP, likely decreased effectiveness of topical prostaglandin analogue glaucoma tx
Topically, may delay wound healing, cause corneal irritation
For overall analgesic purposes, systemic admin usually more effective
Systemic effects possible with longer duration
* Cats: decreased GFR after 7d admin topical 0.1% diclofenac = caution using these agents in at-risk population
Effect of Inhalants on IOP
Historically, methoxyflurane = inhalant of choice for ophthalmic px DT greater ocular m relaxation, hypotonic/centrally rotated eye, slower recovery
Consensus: decreased to no effect
Effect of Inhalants on Tear Production
Tear production decreased: duration depends on study (up to 24h), may be related to time under GA
Effect of N2O On ophthalmic sx
DO NOT USE IF INTRAOCULAR INJ OF GAS BUBBLE
Diffusion of nitrous oxide into gas bubble will cause it to expand, increase IOP = loss of vision DT central retinal artery occlusion
Prior to inj, N2O should be discontinued for 15-20’
For repeat ax episodes, recommended that N2O not be administered for at least 5 days after intraocular air inj, 10d after sulfur hexafluoride inj
Barbiturates IOP
Decrease
Propofol IOP
Mixed effects on IOP - increases then decreases
Alfax IOP
Mostly indicate increases in IOP, decreased tear production, miosis lasting approx 10’ post induction
Etomidate IOP
Humans: mydriasis, decreases IOP
If etomidate-myoclonus occurs, increases IOP
Current recommendation: admin benzo prior to etomidate induction to patients at risk of globe rupture
Dissociatives and IOP
Ketamine may cause increased IOP DT extraocular m ctx, effects = variable
Tiletamine alone = muscular clonus
Telazol did not induce extraocular myoclonus
Alpha 2s and IOP
In general, tend to decrease IOP
Mattos-Junior et al 2021 (VAA)
dexmedetomidine alone or in conjunction with torb, meperidine, methadone, nalbuphine or tramadol resulted in decreased IOP in dogs for 120’
Phenothiazines - Effect on IOP, tear production
No changes to decrease
Signs of Ocular Pain
Blepharospasm, discharge, photophobia, rubbing of eyes, eye/facial guarding, avoidance
Topical Corneal Anesthesia
Proparacaine
If topical locals used long term, can delay healing; also cause pain when administered
Intracameral inj of PF lido: no AE on IOP, corneal thickness
Succinylcholine Effect on IOP
Increases IOP
ctx extraocular muscles, distortion of globe with axial shortening, choroidal vascular dilation secondary to increase arterial pressure, ctx orbital smooth muscle
Effects of non-depolarizing NMBA on IOP?
no effect, decreases IOP
Pupil Dilation in Birds, Reptiles
Topical vecuronium +/- atracurium showed to be effective mydriatic agents in raptors, psittacines, vultures
Vecuronium: fewest SE when admin to 3 species of psittacines and most consistent/greatest pupil dilation vs pancuronium, d-tubcurarine
Intracameral inj of d-tubocurarine in pigeons, pancuronium = effective but not preferred bc more AE (apnea, salivation) than topical applications
Topical Local Anesthetic
Preservative-free formulations preferred: preservatives can damage corneal epithelium
SE: Irritating, transient conjunctival hyperemia, damage corneal epithelium, delay wound healing, mask signs of dz/discomfort
Best for diagnostic use: DO NOT USE LONG TERM
Which locals are used for ophthalmic topical preparation?
Proparacaine, bupivacaine (potentially less toxic, shorter acting), tetracaine 4x more toxic/more irritating
LAST possible in small patients but unlikely
Auriculopalpebral NB
Terminal branch of facial N (CN VII)
Motor innervation to orbicularis oculi
* NO SENSORY! Blockade = eliminates forceful blepharospasms
Horses: no effect on tear production, IOP
Crosses dorsal dorsal aspect of zygomatic arch midway btw lateral canthus and base of ear
Lacrimal NB
Br ophthalmic CN V
Lateral 1/3 of upper eyelid, lacrimal gland, local CT, temporal angle of orbit
Dorsal rim of orbit medial to lateral canthus
Supraorbital NB
Supraorbital foramen, br ophthalmic CN V
Successful completion of block will desensitize forehead, middle 2/3 upper eyelid, +/- some terminal branches of auriculopalpebral N
Horses: thumb at medial canthus, middle finger at lateral canthus – first finger falls into supraorbital foramen
Enucleation, SPL placement, palpebral lac repair
Infratrocholar NB
Br ophthalmic CN V
Medial canthus, partially responsible for innervation of third eyelid, lacrimal gland, connective tissue
Horses: notch in orbital rim just above medial canthus
Zygomaticofacial NB
Br maxillary CN V
Lateral ¾ of lower eyelid, lower 2/3 lower eyelid, skin, CT
Lateral cantos
Retrobulbar Block
Indications: intraocular sx, corneal surgery, evisceration/enucleation
* DT risk of optic N damage, primarily used for enucleation
Structures Blocked with Retrobulbar
cornea, uvea, conjunctiva via blockade of ciliary nerves
II, III, IV, V, VI
Densitizes globe/palpebrae, akinesia, transient vision loss, pupil dilation, decreases IOP
Prevents globe movement, OCR
Limitations of the Retrobulbar Block
Does not block orbicularis oculi or lids
some branches of ophthalmic, maxillary br of CN V pass extraconally
Retrobulbar Block Approaches
- Cats - DM (1mL)
- Dogs - VL (2-3mL)
- Horses - spinal needle perpendicular to skin 0.25” behind bony orbit, needle directed ventrally until “pop” + enter retrobulbar space, 10-12mlL
- Large ruminants - 4pt, 5-10mL/site
- Calves, SR - 2pt, 2-3mL/site
What is the indication of a successful retrobulbar block?
PROPTOSIS = INDICATION OF SUCCESSFUL BLOCK
Risks Retrobulbar Block
retrobulbar or orbital hemorrhage, inadvertent arterial inject (acute sz), damage to optic n, intrathecal inj (acute CNS toxicity, death), globe rupture, brainstem ax, cardiac arrest, chemosis, corneal abrasions, ecchymosis
Contraindications to Retrobulbar Block
orbital infection/severe inflammation, excessive movement, +/- coagulopathy, hypersensitivity to LA, space-occupying lesion in orbit
Peterson Block
Cattle; less reliable DT need for careful needle placement
Efficacy depends on accurate placement of injected anesthetic at site of emergence of nerves from foramen orbitorotundum (ventral to optic foramen)
Structures Blocked with Peterson Block
block CN II, III, VI; maxillary br of CN V (zygomatic N, zygomaticofacial br to get lower eyelid innervation); pterygopalantine and infraorbital N (ax nasal passages, noses)
Approach for Peterson Block
needle just in front of rostral border of coronoid process of mandible, caudal to notch formed by zygomatic arch and supraorbital process
* Direct needle slightly ventrally, posteriorly for length of needle or until strike bone
Can also use technique in calves, SR
SE Peterson Block
accidental inj into CSF = death
Peribulbar Block
Extraconal, requires 2-4x vol of RB blocks
* Large vol required for adequate intraconal distribution may exceed max LA dose in small dogs, cats
Block placed in space btw boney orbit and ophthalmic m
Approaches: single VL, single medial, double inj of DM/VL, DM single (cats)
Sub-Tenon’s Technique
Indications: cataract sx, corneal/intraocular sx
Sterile insertion of blunt cannula along curvature of sclera into Tenon’s space via small incision in conjunctiva, Tenon’s capsule several mm from limbus
Indications for Sub-Tenon’s
Blocks short ciliary n (pupil dilation), long ciliary n (analgesia)
Complications of Sub Tenon’s
chemosis, ecchymosis, retrobulbar hemorrhage, globe perforation, central spread of LA
Bartholomew, Smith, Bentley and Lasarev 2020 (VAA)
use of retrobulbar bupivacaine for enucleation in dogs not assoc with increased d risk major, minor complications
Scott, Vallone, Olsen, et al 2020 (VAA)
Preop RB block 0.75% ropi inj (1mL/10kg) provided analgesia in dogs following enuc at extubation - intraop, postop pain control did not differ from placebo inj with saline
Lack of differences btw groups may have been influenced by sample size limitations
Rabbogliatti, De Zani et al 2021 (VAA)
cadaver study, complete regional ax seems more likely using combined ventrolateral/dorsolateral peribulbar techniques with 20mL/ea (40mL total)
Greco, Costanza, Senatore et al 2021 (VAA)
CT-based method for assessment of canine RB cone volume for ophthalmic ax, larger retrobulbar cone volume with brachycephalic, dolicocephalic than mesocephalics. Weight = strongest predictor
Horner’s Syndrome
Loss of sympathetic innervation to the head
miosis, enophthalmos, protrusion of third eyelid, prolapsed third eyelid
Ddx Ptosis
Anything that causes globe to be smaller or recede into orbit
Problems with CN VII in LA, Horner’s, dehydration, oculomotor deficits
Electroretinogram
measures electrical response of light-sensitive cells in eyes, most commonly used before cataract sx
a, b, c wave; b/a ratio
ERG: a wave
generated by cones, rods in outer photoreceptor layer
* Amplitude measured from baseline to trough
ERG: b wave
generated by bipolar, Müller cells of inner retina
* Amplitude measured from trough of a wave to peak of b wave
ERG: c wave
not usually included in animal protocols, technical challenging in normal dogs
* Generated by retinal pigment epithelium
ERG: b/a ratio
index of inner to outer retinal function
ERG: implicit time
Assoc with b wave
time btw stimulus onset, maximum amplitude
Anesthesia Effect on ERG
Generally decreased amplitude, increased implicit time for rod and cone driven responses
Also affected by hypoxemia, hypercapnia
How Anesthetists can minimize effects on IOP
Premedication with no vomiting, retching, struggling
Careful restraint not to put pressure on eye or occlude jugular veins
If using mask, ensure not pressing on eyes
Body position can also affect IOP
* Head below heart level vs head above or at heart level
Can place temporary tarsorrhaphy or tape lids shut to protect eyes
Eye Lubricant
aqueous-based formulations preferred
Petroleum-based ointments gain access to intraocular structures = severe uveitis, further compromise vision/comfort
Important Considerations in Rodents
(mice, rats, hamsters
lens opacification during prolonged sedation, ax
Transient, caused by lack of blinking, subsequent evaporation of fluid from shallow anterior chamber
Mitigated by application of eye ointment
