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So the eye is like a hollow sphere with a rigid wall, why is the rigid wall important to us?
As the contents of the sphere increase, because of the rigid wall, the intraocular pressure will also increase.
What is the normal IOP?
- is approximately 12-20mmHg.
- Intraocular pressure more than 25 is considered to be pathologic
What are the most important influences on IOP?
- Movement of aqueous humor
- changes in paroidal blood volume
- the central venous pressure
- extraocular muscle tone.
What is glaucoma caused by?
Glaucoma is caused by an obstruction to an outflow of aqueous humor
How does CVP relate to IOP?
- An increase in the volume of blood within the globe will increase IOP
- If the CVP increases, intraocular pressure will increase because there will be decrease aqueous drainage.
- So changes in ABP and ventilation will also affect intraocular pressure.
Where is the aqueous fluid and where is it made?
- The aqueous humor in the anterior and posterior chambers
- Produced by the ciliary processes
What does the aqueous humor look like? what does it do?
- Aqueous humor is clear and a watery solution.
- Provides nutrients for the avascular cornea and the lens
How quickly is aqueous humor made?
The aqueous humor is produced at a rate of 2-3 microliters/min in the posterior chamber.
How much aqueous humor is there at any given time?
At any given time there is 0.3ml within the posterior and the anterior chambers
Why does Diomax help decrease IOP?
2/3 of the aqueous humor is an active secretory product of the carbonic anhydrase enzyme system within the cilliary body of the posterior chamber so we may be asked to give diamox which is a carbonic anhydrase inhibitor in order to decrease intraocular pressure.
After passing though the pupil from the posterior chamber, into the anterior chamber, where does it drain from there?
the aqueous humor will drain into the scleral venous sinus at the aryetnal corneal angle
So the main physiologic determinants of intraocular pressure is the balance between what things????
the production of aqueous humor and it’s elimination via the spaces of fontana and the canal of schlemm at the aryteno-iris corneal angle
The ouflow of aqeous humor from schlemm’s canal follows what law?
poiseuille’s law of fluid flow
How does the size of pupils effect aqueous flow?
The size of the pupil is a major factor affecting the spaces of fontana, if the pupil is dilated the spaces are narrowed and resistance to outflow is increased which would then increase intraocular pressure. Constircting the pupils naturally has the opposite effect
Increased ABP and CVP will cause an increase or decrease in IOP?
INCREASE, and vice versa
Does increase CO2 cause an increase or decrease in IOP?
Because hypercarbia causes vasodilation it will subsequently cause an increase in intraocular pressure.
Coughing can increase IOP to about __-___mmHg
What effect does PO2 have on IOP?
When intubating, what must we ensure?
ETT intubation will increase IOP to a greater extent that Succinylcholine so adequate depth of anesthesia is important
What other intra-op concerns might we have in regards to IOP?
Pressure on the eye from the mask, circuit, prone position, and giving an eye block could increase IOP as well.
IOP: Blinking along raises it by __mmHg, squinting raises it by ___mmHg.
- Blinking: 5mmHg
- Squinting: 26mmHg
How much does Succs increase IOP? For how long?
Succs increases IOP by about 5-10mmHg for about 5-10min following administration
How does Succs increase IOP?
- The mechanism of this is likely to be prolonged contraction of the extraocular muscles.
- Unlike other skeletal muscles the extraocular muscles contain cells that have multiple neuromuscular junctions so repeated depolarization of these cells by succs causes prolonged contracture and the increase in IOP can have varied sequela including spurious measurements of IOP on exam under anesthesia in patients with glaucoma.
What effect do volatile agents and N2O have on IOP?
DECREASE (more w/volatiles)
What effect do Barbs/Benzos have on IOP?
What effect do narcotics have on IOP?
DECREASE (less than Benzos/Barbs)
What effect do non-depolarizers have on IOP?
Why do we give sedation for open surgery?
Sedation is usually given for infiltration of the regional block for cataract, glaucoma or retinal surgery
Why must we be careful not to oversedate the patient?
Careful not to over sedate the patient so the patient falls asleep and then awakens startled and moves suddenly.
Occasionally we’re faced with a patient with a penetrating eye injury d/t trauma when the globe is open during surgical procedures following trauma. The IOP will approach atmospheric pressure, why is this such an issue?
- Any factor that normally increases IOP will tend to decrease intraocular volume according to Boyles law, by drainage of aqueous or extrusion of vitreous through the wound.
- This can be a serious complication that can worsen the vision
What is the retina?
Retina is transparent tissue that permits us to see the images that are focused on it by the cornea and the lens
What happens when the retina detaches?
when the retina detaches the light sensitive lining in the back of the eye peels loose, and then floats freely within the eyes interior and when this happens, sight can be lost
What causes a detached retina?
- Some of the causes of retinal detachment include trauma, diabetes, and inflammation.
- It can also occur spontaneously from changes in the consistency of the volume of the vitreous humor.
- A detached retina is associated with a tear or a hole in the retina and so eye fluids can leak through this hole.
What is the sclerobuckle surgery for a detached retina?
One of the procedures for a large detachment is called sclerobuckle this involves suturing a piece of rubber essentially to the sclera of the eye, it bends the wall of the eye inward so that it meets the hole in the retina.
Why is a detached retina so urgent to repair?
Because a detached retina lacks oxygen leading to cell death, it is really an urgent procedure in order to prevent blindness
How is an internal tamponade help correct a detached retina?
Another strategy is internal tamponade of the retinal detachment, done with the use of an expandable gas like sulfahexalchloride that is injected by the surgeon into the vitreous
How long should we avoid N2O after a intraocular gas injection for a retinal repair?
- It’s also recommended not to use N2O in patients within 5 days of receiving an air injection.
- And within 10days, up to 1M of receiving a sulfahexalchloride injection
Retinal detachment is extraocular unless the surgeon does what??
Retinal detachment operations are considered extraocular but may become intraocular if the surgeon decides to perforate and drains the fluid.
When might we see a posterior victrecomy performed?
posterior vitrectomy are performed occasionally in the diabetic patient who has experienced vitreous hemorrhages, the surgery will removed the bloody gel left from the hemorrhage
Strabismus surgery is done to correct misalignment of the visual axis, how is this manifested?
This may manifest by diplopia, reduction of dimness of vision, loss of depth perception
What are the TWO key points about strabismus surgery?
It is associated with an increased incidence of ocular cardiac reflex and increased incidence of MH
How is strabismus surgery associated w/MH?
- The association with MH is d/t the fact that it may be d/t a myopathy and patients susceptible to MH may have a localized areas of skeletal muscle weakness
- So succs is best avoided d/t risk of triggering MH and also may interfere w/some of the tests the surgeon may want to do.
Why is vomiting after eye muscle surgery common?
Barash refers to the presence of an oculogastric reflex.
What is the sensory information of the eye trasmitted by?
Sensation if the eye is transmitted through the afferent fibers from the cornea and conjunctiva to the 1st branch of the ophthalmic division of the 5th cranial nerve (Trigeminal)
All the muscles of the eye (except the lateral rectus and the superior oblique) are innervated by what nerve?
Motor innervation of the extraocular muscle is by way of the motor fibers of the 1/3 of the oculomotor cranial nerves.
What nerve innervates the lateral rectus muscle of the eye?
innervated by the 6th cranial nerve, the abducer
What nerve innervates the superior oblique muscle of the eye?
innervated by the 4th cranial nerve, trochlear
Which nerve controls the contraction of the orbicularis occuli?
The motor fibers of the 7th cranial nerve (facial)
What is the usual block, peribulbar or retrobulbar?
Usual technique is a regional approach with a peribulbar block
What anesthetic approach is usually used for cataract or glaucoma surgery?
Cataract and glaucoma surgery is most often accomplished with either a peribulbar block or a topical anesthetic.
What anesthetic approach is used for retinal surgery?
Retinal surgery usually requires either a peribulbar or a retrobulbar approach.
Combination of local infiltration of the facial nerve & blockade of motor & sensory branches of the posterior orbit is used for regional anesthesia of the eye.
What will the facial nerve block help with? What about the peribulbar or retrobulbar?
- Blockade of the facial nerve will produce sensory anesthesia at the periorbital area and also motor block of the lid.
- The peribulbar or the retrobulbar part of the block will create the akinesia needed for cataract extraction and other opthomological procedures
What local anesthetic is used for the facial nerve block? What about the peribulbar or retrobulbar?
- For facial nerve blockade, 1% lidocaine is what is usually used.
- For retrobulbar and peribulbar block either 2% Lidocaine or 0.75% Bupivacaine may be used
What is sometimes added to retrobulbar blocks?
The other thing that gets added to retrobulbar injection is something called hyaluronidase, its an enzyme and will promote the spread of the anesthetic through the muscle cone
Blockade of facial nerve (CN 7) produces _______anesthesia of periorbital area & _____ lid block
How is the Modified Atkinson approach for facial block done?
- 1.5in needle is inserted through a skin wheel 2cm in the lateral border of the orbit.
- The needle is advanced first superiorly towards the upper orbital rim, 2-4ml of anesthetic is injected as it’s withdrawn to the insertion point then again it gets redirected to the inferior orbital rim and a repeat injection is made.
- Sensory block of the lid is achieved by a subcutaneous injection of 1% Lidocaine through this single skin puncture.
How is the Van Lint approach for the facial block done?
- It’s slightly more medial at a point 2cm the lateral canthus of the eye.
- 3ml of local anesthetic is injected and again 1% Lido is injected as the needle is withdrawn to the entry point.
- Needle is left in the skin and redirected inferiorly and anteriorly with a similar injection of 3ml on withdrawal.
- The 2 injections should produce a V bordering the eye.
- An additional 2ml could be injected deeper at the apex of the V to provide anesthesia to deeper fibers
What are the goals of the retrobulbar blocks?
- Goal with this block is to immobilize the eye and block the ciliary ganglion.
- 3 As: Akinesia of the eye, Anesthesia of the eye, and Abolishment of the oculocardiac reflex
Describe the process of performing a retrobulbar block
- Prior to beginning the eye block, topical anesthesia, usually 1% Tetracaine is applied to the conjunctile lacrimation, sedated
- the inferior border of the orbital rim is located at a point approximately 1/3 of the distance from the lateral to the medial canthus.
- This point is usually directed inferior to the lateral border of the dilated pupil. The eye is held in neutral forward gaze.
- 1.5in 23G blunt tip needle gets introduced perpendicularly into the skin and advanced directly posterior, parallel to the floor of the orbit. The tip will usually lie opposite the equator of the globe just below the skin.
- After the needle is advanced past the equator of the globe it can be angled supranasally at approximately a 45 degree angle to pass the muscle cone.
- Once the needle passes through the muscle body into the cone there is an abrupt release of traction and the globe springs back to neutral position. After aspiration 3-4ml of anesthetic is injected slowly,
Why is it SO important the patient be able to cooperate for a retrobulbar block?
The eye is held in neutral forward gaze, so helpful if the patient is a bit sedated but able to cooperate so they can look where you want them to look because upward medial deviation can rotate the optic nerve and then the vessels into the path where the needle is going to go so it’s important the patient cooperates here
What does it mean if there is resistance when performing a retrobulbar block?
- There shouldn’t be any resistance to injection if the needle is in the cone.
- If there is resistance there might be intramuscular placement and the needle should be repositioned.
What does it mean if there is pain on injection during a retrobulbar block?
Sclero perforation should be suspected if the patient complains of pain on injection and again want to remember sedation may mask this.
What should we do after a retrobulbar block to ensure good spread of the solution?
Then gentle pressure is applied to the globe for 5min to facilitate the spread of the solution but again this is released Q30sec to preserve retinal blood flow
What is the disadvantage of peribulbar blocks compared to retrobulbar blocks?
The disadvantage to this approach is that there is a slower onset and need for reinjection of 25-35% of the time compared to 10% of the time with a retrobulbar injection.
How is a peribulbar block done?
- Sedation and topical anesthesia of tetracaine.
- 1.5in 25G needle is inserted through the conjunctiva at the inferior temporal area above the inferior orbital rim.
- Needle is advanced in the slight upward direction parallel to the rising orbital floor.
- Without any attempt to enter into the muscle cone and 4-5ml of local anesthetic is injected.
- The needle is reinserted in the superior nasal area just below and medial to the supraturbcular notch and an additional 4-5ml is injected.
- With both injections the needle is only advanced 1inch into the orbit and this is generally enough to reach behind the equator of the globe
The onset is slower for a peribulbar block so when should we assess it?
the block needs to be assessed at 10 min for potential supplementation of the injection
Do we need to inject the facial nerve to get upper lid block in the peribulbar block?
No, 7th CN anesthesia is often obtained by diffusion of the anesthetic into the subcutaneous tissues of the upper lid without the need for the separate injection of the facial nerve
hemorrhage is the most frequent complication of retrobulbar block and occurs as often as __% of cases
Retrobulbar hemorrhage is a serious complication that can interfere with retinal blood supply if the excessive pressure develops. What are the signs?
- Signs include an immediate downward displacement of the eyeball, increased pressure in the globe and the appearance of subconjunctival blood.
- Surgery is cancelled, sometimes there will be drainage via a lateral canthotomy by a surgeon to relieve the pressure.
Brainstem anesthesia can result from the spread of the anesthetic. How often does this occur?
Less common, less than 0.5% but more life threatening, pt will become apnic
How can brainstem anesthesia happen with the eye blocks?
Likely d/t the spread of the anesthetic along the optic nerve to the central brain stem
What are the s/s & treatment of brainstem anesthesia in eye blocks?
Presenting signs, SOB, dysphagia, and ventilation and supportive therapy will suffice until symptoms resolve.
Why can systemic toxicity result from an eye block? What can happen?
- Systemic toxicity is possible because of the proximity of the retinal artery.
- Unintentional injection under pressure into the arterial circulation of the head could produce a rapid high cerebral local anesthetic blood level and seizures.
- Always important to aspirate and utilize incremental injection techniques
What causes the oculocardiac reflex? How is it treated?
- any stretch of the extraocular muscles can produce a reflex bradycardia and this needs to be treated immediately.
- Tx to d/c stimulus and give an anti-muscarininc
Globe perforation can occur even with these blunt tip needles and even with the peribulbar technique. What are the risk factors for this?
Risk factors include an elongated globe, multiple injections, previous sclerobuckle surgery, and the use of long beveled needles.
What would we see if there was globe perforation during an eye block?
Perforation can usually produce pain and restlessness and surgery should be cancelled and appropriate care given
Why is intramuscular injection so bad during eye blocks??
If you’re injecting retrobulbar, you’re injecting into the cone but not into the muscle because injecting directly into the muscle body can produce muscle destruction and ultimate paresis, so it’s rare and can be avoided by stopping the injection if there is any resistance
What are the causes of the oculocardiac reflex? (the list)
- Traction on extraocular muscles
- Pressure on the eyeball
- Orbital hematoma
- Ocular trauma
- Eye pain
What is the mechanism behind the oculo-cardiac reflex?
- a trigemino-vagal reflex
- Trigeminal (CN V) afferent & vagal efferent pathway
What will we see during the oculo-cardiac refllex?
- Bradycardia, nodal rhythm, ectopy, asystole
- Somnolence & Nausea
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