Eye surgery Flashcards
What anaesthetic factors influence intra-ocular pressure?
IOP is determined by balance between aqueous humour formation in ciliary body and drainage through canal of Schlemm. Globe is of a fixed volume and poorly compliant so changes in chorioidal blood volume or aqueous humour volume will result in a steep change in pressure; while ↑ muscular tone surrounding globe will also cause ↑ pressure. Factors increasing IOP: external compression of globe by facemask; laryngoscopy (pressor or straining); suxamethonium and LA placed in orbit Factors decreasing IOP: induction agents (↓ BP); NDNMB (↓ muscle tone); head-up tilt and hypocapnia (constricts choroidal vessels).
What drugs can acutely ↓ IOP in an emergency?
• Mannitol 0.5g/kg IV – withdraws fluid form vitreous humous • Acetazolamide 500mg IV – ↓ aqueous humour production
What reflexes are triggered by pressure on the globe?
• Oculocardiac: bradycardia • Oculorespiratory: respiratory arrest • Oculoemetic: vomiting Preventable through use of LA abolishing afferent arc (long/short cilliary nn), avoiding ↑ CO2 and prophylactic anti-muscarinic.
6 main questions required from pre-anaesthetic assessment to safely give an eye block
• Axial length • INR / APTT if anticoagulated • BSL if diabetic • Ability to lie flat (heart failure, OSA, arthritis) • Ability to cooperate (hearing intact, dementia) • Ability to tolerate oxygen
Risks and benefits of GA vs regional for eye surgery
Regional usually preferred due to minimisation of physiological disturbance in pts with many comorbidities and cheaper (quicker throughput of pts). GA preferable when pt refuses LA, unable/unwilling to lie flat or still (children, movement disorders), major/long procedures (oculoplastics and vitreoretinal sx).
What modalities of providing anaesthesia exist for eye surgery?
Akinetic anaesthesia • Needle injection of LA Into muscle cone (retrobulbar or intraconal) External to muscle cone (peribulbar or extraconal) • Cannula infusion of LA beneath Tenon’s capsule • GA Kinetic anaesthesia • Topical LA eye drops • Subconjunctival injection
Pros and cons of peribulbar vs retrobulbar block
Retrobulbar intraconal block • Steeply angled and deeply placed low volume (1-3ml LA) • Rapid akinesia and analgesia but may require facial nerve block for lid akinesia • More complications: globe perforation, optic nerve sheath injection (brainstem anaesthesia and trauma), intravascular injection and retrobulbar haemorrhage. May lead to permanent visual loss. Peribulbar extraconal block • Minimally angled and shallow placed high volume (4-10ml) • Gradual onset akinesia and analgesia; does not require facial nerve block for lid akinesia due to large volume LA spread • Complications less likely with single inferolateral injection and avoid superior aspect of orbit; may supplement with medial canthus block
Prerequisites for regional eye block
• IV access and monitoring • 0.5-2.0mg midazolam or propofol 10-30mg • Topical LA drops (tetracaine 1%) • Ask pt to lie supine and look straight ahead
Technique for peribulbar block
• Technique: o 25mm 25G needle on 10ml syringe inserted transconjuncitvally or trans cutaneously o Landmark is groove at junction b/w medial 2/3 and lateral 1/3 inferior orbital rim o Insert needle shallowly (<25mm) and parallel to globe and perpendicular to all planes observing that globe does not move o Aspirate and inject 4-10ml. Stop injecting if proptosis or upper eyelid swelling (indicates retrobulbar injection which requires lower volume) o Compression balloon to dissipate LA and normalise IOP o Consider medial canthus injection – pass needle medial to caruncle with bevel facing globe towards medial orbital wall: 3-4ml
Contraindications to peribulbar block (4)
All are relative • Axial length > 26mm or myopic staphyloma → ↑ risk globe perforation • Perforated or infected eye • Inability to lie flat and still • Anticoagulation
3 major complications of eye blocks, 3 systemic complications and steps to prevent/manage them
• Globe perforation – pain on injection or hypotonic eye during surgery → risk retinal haemorrhage/detachment requiring urgent laser retinopexy or vitrectomy. Minimise by ensuring axial length < 26mm, observing for globe movement while inserting needle, using inferolateral approach and avoiding technique in pts with myopic staphyloma. • Retrobulbar haemorrhage – often innocuous but if severe bleeding with rapid orbital swelling needs urgent retinal artery assessment and lateral canthotomy to relieve IOP. Minimise risk by observing for ecchymosis and proptosis. • Optic nerve damage/injection – minimise risk by restricting needle length to 25mm Systemic complications * Intra-arterial injection to ICA and midbrain structures –> cardiac arrest; convulsions * Optic nerve sheath injection –> subarachnoid injection –> total spinal * Oculocardiac reflex –> bradycardia, asystole (trigeminal nerve afferent, vagus efferent)
LA mixture for sharp needle eye block
• 1:1 mixture lignocaine 2% and ropivacaine 0.75% • Hyaluronidase 30-150 units per ml
Indications and contraindications for subtenon eye block
Deposition of LA into tenon’s capsule for cataract, vitreoretinal, trabeculectomy and strabismus surgery. Indications Long eye – risk of globe perforation with sharp needle Anticoagulation – risks of haemorrhage more significant with sharp needle; this block allows direct cautery Contraindications (relative) Previous sub-Tenon’s block in same quadrant Previous extensive vitreoretinal/strabismus surgery Eye trauma Conjunctival disease (eg pemphigoid) Infection Surgery requiring complete akinesia (viscocanalostomy)
Equiment required for subtenon block
• Eyelid speculum • Moorfield’s forceps • Westcott’s blunt tipped scissors • Subtenons cannula (flattened distal portion) • Ropivacaine 0.75% and lignocaine 2% in 1:1 ratio - 3-5 ml per pt (volumes > 4 ml for akinesia)
Describe procedure for subtenon block
• Instil tetracaine 1% drops – intranasal portion • Monitoring; sedation is relatively contraindicated as cooperation is required • Prep with 5% aqueous povidone iodine solution ensuring some present in conjunctiva • Attach subtenon cannula to LA syringe • Insert eye speculum and ask pt to look up and out (use infranasal quadrant as no muscle insertions there and rarely used by surgeons) • Rest both hands on pt to minimise tremor • Push into globe with forceps to pick up conjunctiva and tenon’s capsule • Cut perpendicular to globe heading radially out from globe • Dissect posteriorly with closed scissors and keep forceps in place holding up conjunctiva and Tenon’s capsule • Inject • Apply gentle pressure post injection of LA
