Anesthesia for Ophthalmic Surgery

Primary goals: Safety, akinesia of the eye and eyelids, analgesia, hemostasis, and minimizing the Oculocardiac Reflex (OCR).
Maintenance of Intraocular Pressure (IOP) and smooth emergence from general anesthesia are critical.
Demographics: Typically involves elderly patients with comorbidities or pediatric patients.
Patient-centered issues: Assess for claustrophobia, anxiety, and the ability to cooperate.
Positioning: Patients are supine with the bed turned $90^\circ$ or $180^\circ$ ( $90^\circ$ counterclockwise is common), creating "field avoidance" where the airway is less accessible.

Acetazolamide: Interferes with carbonic anhydrase to decrease aqueous humor formation and lower IOP.
Echothiopate: Long-acting anticholinesterase miotic; may prolong the action of succinylcholine.
Timolol: Nonselective beta blocker used to lower IOP.
Prostaglandin F2a analogs: First-line glaucoma agents including Bimatoprost, Latanoprost, and Travoprost.
Phenylephrine, Epinephrine, Pilocarpine, and Atropine are frequently encountered.
Effects on IOP:
- Increase: Succinylcholine, Etomidate (due to myoclonus), Ketamine (nystagmus), and hypoventilation leading to hypercapnia.
- Decrease: Propofol, barbiturates, benzodiazepines, inhalational agents, narcotics, and nondepolarizing neuromuscular blockers (NMB) (provided normocapnia is maintained).

Retrobulbar Block: Local anesthetic (LA) is injected into the intraconal space. Provides quick onset and complete akinesia but requires a separate facial nerve block.
Peribulbar Block: Extraconal injection where LA diffuses into the conal space. Requires more volume but blocks the facial nerve; onset is slower than retrobulbar.
Sub-tenon’s Block: Blunt needle cannula injects LA into the space between Tenon's capsule (enveloping the sclera) and the sclera.
Complications:
- Brainstem anesthesia ( $1/300$ to $1/500$ ): Direct spread of LA along the optic nerve sheath; results in contralateral blindness, apnea, hemiplegia, and potential cardiac arrest.
- Retrobulbar hemorrhage: Occurs in approximately $1/700$ cases.
- Extraocular palsies: Myotoxicity of LA may lead to post-operative strabismus.

Pathway: Trigeminovagal reflex involving the afferent limb (Cranial Nerve $5$ ) and efferent limb (Cranial Nerve $10$ ).
Triggers: Pressure on the globe or traction on extraocular muscles (common in enucleation or strabismus surgery).
Manifestation: Sinus bradycardia.
Management: Request the surgeon to cease manipulation; HR typically returns to baseline within $20$ seconds. If persistent or serious, administer IV Atropine ( $0.02\,mg/kg$ in pediatrics) or Glycopyrrolate ( $0.01\,mg/kg$ ).

Factors increasing IOP: Direct globe pressure (face mask), laryngoscopy, coughing, vomiting/retching, hyperthermia, and hypercapnia ( $PaCO_2$ ) or hypoxia ( $PaO_2$ ).
Factors decreasing IOP: Head of Bed (HOB) elevation to $30^\circ$ , hyperventilation (leading to decreased $PaCO_2$ ), and hypothermia.
Emergence Strategy: Aim for a "smooth emergence" without coughing or bucking. Utilize IV Lidocaine, narcotics, and Orogastric tubes for gastric decompression to minimize PONV risks.

Retinal Detachment (Pars plana vitrectomy/PPV): Involves injecting expandable gas (Air, Sulfur Hexafluoride, or Perfluorocarbons).
- $N_2O$ caution: Nitrous oxide must be discontinued $15$ minutes prior to gas injection to avoid rapid expansion and increased IOP.
- Subsequent $N_2O$ avoidance: $5$ days for Air, $10$ days for Sulfur Hexafluoride, and $30$ days for Perfluorocarbons.
Strabismus Surgery: High risk for OCR and PONV. Propofol infusions and multiple antiemetics (e.g., Dexamethasone) are recommended.
Open Globe Injury: Considered a medical emergency and a "full stomach" case.
- Management: General anesthesia with Rapid Sequence Induction (RSI) is required.
- Contraindications: Regional anesthesia is avoided. Succinylcholine is contraindicated; use Rocuronium $1.2\,mg/kg$ instead.
- Induction Adjuncts: Lidocaine $1\text{--}2\,mg/kg$ , Fentanyl $1\text{--}2\,\mu g/kg$ , and Midazolam $2\,mg$ to blunt the stress response to intubation.