Opthal Flashcards
What are the extra-conal eye blocks?
Peri-bulbar block
Sub-tenon block
What is the option for an intra-conal eye block?
retro-bulbar block
What are the nerves blocked by an intra-conal block?
oculomotor
abducen’s
Optic
nasociliary
What are the nerves blocked by an extra-conal block?
lacrimal
frontal
trochlear
Which eye muscles are innervated by which cranial nerves?
Oculomotor nerve (III) - inferior oblique, superior rectus, inferior rectus, medial rectus
Trochlear nerve (IV) - superior oblique
Abducens (VI) - lateral rectus
Absolute contraindications for a peribulbar block?
patient refusal, LA allergy, localised infection
Describe the process of a peribulbar block
supine position
25G 25mm needle, ~6-10ml of LA
Eye sterilised with providone-iodine 5%
Eye drops of 0.5% tetracaine, 5-10mins prior
Injection point is in the inferotemporal corner
Retract eye lid and inject
Medial cantus top up, shallow ~15mm, ~3-4 ml
Describe the LA recipe for eye blocks
Add 10ml of 10% lignocaine to 10ml syringe
Dissolve 1500 units hulas, making it 150u/ml
in 5ml syringe, drop up 4ml of 0.75% rope
Add 1ml of lignocaine solution to make 5ml total
Solution then contains 0.6% ropi + 2% lignocaine +30 units / ml of hylase
What does hyaluronidase do in an eye block?
Improve spread of anaesthetic by
- Increase tissue permeability
This is by depolymerisation of hyaluronic acid and chondroitin sulphate in the extracellular matrix
What are the special equipments required for a sub-tenon block?
Moorfield’s non-toothed forceps
Westcott Scissors
Subtenon’s cannula
What is the specific contra-indication for a sub-tenon block?
scleral buckle
Describe the process of a sub-tenon block
Supine, position self at the patient’s head
Lid speculum in
Deep bite of the conjunctiva in the inferomedial quadrant, using a non-toothed forceps
Small incision with Westcott Scissors
Curved subtenon’s cannula, along the surface of the globe, aim for vertical syringe
Deposit 5ml of LA
What are the specific side effects of a sub-tenon block?
subconjunctival haemorrhage
Chemosis
What is chemosis
Swelling of the conjunctiva
What are some of the specific complications of an eye block?
Ocular
- Globe perforation
- Retrobulbar haemorrhage
- Optic nerve damage
- Muscle palsies, chemosis, corneal abrasion
Systemic
- LAST
- Optic nerve sheath injection (acts as a subdural / subarachnoid injection)
- Oculocardiac reflex
What are the periop issues of managing a penetrating eye injury?
Other traumatic injury
Emergency procedure: fasting, urgency to procedure, limited time for optimisation
Prevent intra-ocular pressure rise
- Normally 10-22mmHg
- Avoid coughing / straining
- Venous drainage to eye = valveless. Cough = increase IOP
- Can use Mannitol 20% to reduce IOP
Oculocardiac reflex due to extrinsic ocular muscle retraction
Prevent secondary injuries
PONV risk high -> adequate prophylaxis
Shared airway procedure, limited airway access
What are the secondary injuries that can occur with penetrating eye injury?
Infection
Bleeding - correct coagulopathy
Eye protrusion from high IOP
measures to reduce IOP?
acetazolamide 500mg IV
Mannitol 1mg/kg IV
paralysis
hyperventilate
BP control
What is the Tenon’s capsule?
a fibroelastic connective tissue layer from the corneal limbus anteriorly to the dura of the optic nerve posteriorly
What’s the sub tenon’s space?
A potential episcleral space between the capsule and the sclera
What syndromes are associated with strabismus?
Down syndrome
Prader-Willi
DiGeorge
Turner
Why is sux avoided in strabismus surgery?
Increased incidence of malignant hyperthermia in patients with a squint
What is echothiophate?
treatment of specific type of strabismus
- Reduce IOP
- Increase drainage of IO fluid
- Facilitates accommodative convergence response - allowing eye to focus
Advantages of a sub-tenon block
- Avoids complication of sharp needle
- Safer in a myopic eye (long axial length)
- Safer in patients on anti-coagulation
- Less painful cf peribulbar/retro block
- Good analgesia and akinesia
Disadvantages of a sub-tenon block
Technically challenging with specialised equipment
Contraindicated in scleral buckle
Risk of chemosis and subconjunctival haemorrhage (cosmetic issue)
Needs cooperation, lying still
Akinesia less consistent - abducen’s nerve often not blocked
What are the anterior and posterior boundaries of tenon’s capsule?
anteriorly - fusion with conjunctiva at the limbus
posteriorly - fusion with dura of the optic nerve
All six extra ocular muscles pierce the Tenon’s capsule
What neurovascular structures can be found within the subtenon’s space?
short posterior ciliary arteries and nerves
Veins draining the choroidal circulation
Describe the orbit of the eye
Shaped like a bony pyramid, apex posteriorly
Measuring 42-54mm from apex to the base (cornea and lids)
Globe is located in the anterior half of orbit, measuring 24mm in axial length
Why is inferotemporal quadrant the preferred location for a peri-bulbar block?
it is relatively avascular
Where is the peribulbar space in relation to the ocular muscles?
ocular muscles form a cone around the globe
Peribulbar space is outside the muscle cone
What are the intraocular contents that determine IOP?
Aqueous humour - fixed production and drainage via canal of Schlemm. This is the primary determinant of IOP.
Choroidal blood volume
Vitreous humour - affected by mannitol
What are the extra ocular components that affect the orbital pressure?
extra ocular muscles - contraction can increase IOP (sux)
Deposit of LA (peribulbar block)
What conditions can increase IOP due to more aqueous humour?
Reduced drainage - glaucoma, venous congestion, mydriasis
How does autonomic nervous system affect the aqueous humour production?
Beta SNS and PNS stimulation increase aqueous humour flow
Alpha-2 activation decreases aqueous humour production. Sustained SNS stimulation also decreases production
What drugs can reduce IOP?
Beta blocker - reduce SNS stimulation = reduce aqueous humour flow
Alpha agonist - brimonidine
Acetazolaimide - reduce production
Cholinergics - pilocarpine, increases aqueous outflow
Intra-op risk factors for post-op visual loss?
Hypotension, anaemia
Direct pressure on globe
Increased CVP
- Reduced venous drainage from head down position, or head turned.
- Abdominal pressure
Fundoscopic findings for
- Ischaemic optic neuropathy
- Retinal artery thrombosis
optic disc oedema, peripapillary haemorrhage
retinal pallor, cherry red spot.
