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Affections of the Eye: Veterinary Surgery Reference

Note: The library contains human-focused ophthalmology textbooks (principally Kanski's Clinical Ophthalmology, 10th edition). The anatomy, pathology, and surgical principles described below are directly translatable to veterinary practice - dogs, cats, horses, and other animals share the same fundamental ocular anatomy. Where veterinary-specific differences exist, they are noted.

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1. GENERAL OCULAR EXAMINATION

Anatomy Overview

Visual Acuity Assessment

• Counting fingers (CF) - patient can count fingers at a specified distance (usually 1 metre)
• Hand movements (HM) - able to detect hand motion only
• Perception of light (PL) - can discern light only. In pure media opacity (dense cataract), the patient/animal can still determine direction of the light source
• No perception of light (NPL)

Slit Lamp Biomicroscopy

• Eyelids and lid margins
• Conjunctiva (palpebral and bulbar)
• Cornea - surface, stroma, and endothelium
• Anterior chamber depth and presence of flare/cells (Tyndall effect)
• Iris and pupil
• Lens - capsule, cortex, and nucleus using retroillumination and oblique beam

Fluorescein Staining (Corneal Examination)

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2. CONJUNCTIVITIS

Anatomy of the Conjunctiva

• Palpebral conjunctiva - firmly attached to the tarsal plates
• Fornical conjunctiva - loose and redundant
• Bulbar conjunctiva - covers the anterior sclera; continuous with corneal epithelium at the limbus. The palisades of Vogt at the limbus are the likely reservoir of corneal stem cells.

Symptoms and Signs of Conjunctival Inflammation

• Lacrimation, grittiness, stinging, burning
• Itching is the hallmark of allergic disease
• Visual acuity is usually unaffected
• Significant pain, photophobia, or foreign body sensation suggest corneal involvement

• Conjunctival injection - superficial vessels (bright red, movable), vs. ciliary injection (deep purple-red, fixed - indicates intraocular inflammation)
• Chemosis - conjunctival oedema
• Papillae - raised, vascularized folds with a central fibrovascular core; seen in bacterial and allergic conjunctivitis
• Follicles - avascular lymphoid aggregates without a central core; typical of viral and chlamydial conjunctivitis
• Membranes/pseudomembranes - in severe bacterial (especially gonococcal) or adenoviral conjunctivitis
• Subconjunctival haemorrhage - adenoviral, gonococcal, mechanical

Types of Conjunctivitis

Bacterial Conjunctivitis

• Onset: Acute; usually bilateral (one eye 1-2 days before the other)
• Signs: Eyelids stuck together on waking, mucopurulent discharge
• Organisms: Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae (in children), Moraxella catarrhalis
• Hyperacute purulent discharge is a red flag for Neisseria gonorrhoeae or N. meningitidis - requires urgent Gram stain (Gram-negative intracellular kidney-shaped diplococci) and culture on chocolate/Thayer-Martin agar
• Superficial corneal punctate epithelial erosions are common
• Peripheral corneal ulceration may occur in gonococcal/meningococcal infection and can rapidly perforate
• Treatment: Topical antibiotics (chloramphenicol, fusidic acid, fluoroquinolones); systemic for gonococcal/chlamydial

Viral Conjunctivitis

• Usually caused by adenovirus (epidemic keratoconjunctivitis - EKC)
• Features: Watery discharge, preauricular lymphadenopathy, follicular reaction
• Often self-limiting; highly contagious

Allergic Conjunctivitis

• Itching is the dominant symptom
• Types range from acute allergic (seasonal/perennial) to vernal keratoconjunctivitis (VKC) and atopic keratoconjunctivitis (AKC)
• VKC: giant cobblestone papillae on upper tarsal plate; Trantas dots at the limbus
• Treatment: Antihistamines, mast cell stabilizers (sodium cromoglicate, lodoxamide), topical steroids for severe cases

Chlamydial Conjunctivitis

• Adult: sexually transmitted Chlamydia trachomatis (serotypes D-K)
• Trachoma (serotypes A-C): leading infectious cause of blindness worldwide
• Chronic follicular conjunctivitis with pannus (fibrovascular invasion) and corneal scarring

Veterinary Note

• Dogs/cats: Bacterial (Staphylococcus, Streptococcus), viral (feline herpesvirus-1 in cats is a major cause), Chlamydophila felis (cats), Mycoplasma
• Horses: Bacterial, viral (Equine Herpesvirus), traumatic
• Cattle/sheep: Infectious bovine keratoconjunctivitis (Moraxella bovis - "pink eye")

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3. CORNEAL EXAMINATION AND CONDITIONS

Normal Cornea

1. Epithelium (with Bowman's layer beneath)
2. Stroma (90% of corneal thickness)
3. Descemet's membrane
4. Endothelium

Examination Techniques

• Diffuse illumination: Overall survey
• Focal/direct illumination: Detect opacities, vascularization
• Retroillumination: Detect early corneal haze, subtle opacities, endothelial changes
• Specular reflection: Examine endothelial cell morphology
• Fluorescein staining + cobalt blue light: Epithelial defects (ulcers, abrasions) stain bright green
• Rose Bengal staining: Devitalized epithelial cells and mucous filaments (dry eye)

Key Corneal Conditions (relevant to surgery)

Dry Eye / Keratoconjunctivitis Sicca (KCS)

• Punctate epithelial erosions that stain with fluorescein
• Filaments: mucus/cell strands attached at one end to the corneal surface (stain with rose Bengal)
• Mucous plaques
• Complications: epithelial breakdown, corneal melting, perforation, bacterial keratitis
• In dogs, KCS is a common and important surgical/medical condition (Schirmer tear test used for diagnosis)

Corneal Ulceration

• Superficial ulcers: Epithelial defects - visible with fluorescein; managed medically
• Stromal ulcers: Deeper; risk of perforation
• Descemetocele: Only Descemet's membrane remains - surgical emergency (corneoconjunctival transposition, conjunctival flap, or keratoplasty)
• Corneal perforation: Iris prolapse; treated with corneal suture + conjunctival flap or graft

Band Keratopathy

• Calcium deposition in the superficial cornea (Bowman's layer)
• Associated with chronic uveitis, hypercalcemia, or silicone oil tamponade in the eye

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4. CATARACT

Anatomy

• Capsule (outermost - elastic basement membrane)
• Epithelium (germinative zone anteriorly)
• Cortex
• Nucleus (oldest, most central fibres)

Classification by Location and Morphology

Causes of Cataract

• Age-related (senile): Most common in humans and dogs
• Diabetic: Sorbitol accumulation from glucose via aldose reductase pathway; cortical fluid vacuoles; "snowflake" pattern in young; accelerated nuclear sclerosis in older patients
• Traumatic: Penetrating injury (capsule disruption), blunt trauma (flower-shaped opacity), electrical/radiation injury; intralenticular foreign body
• Secondary to systemic disease: Myotonic dystrophy, hypocalcemia (fine lamellar cataract)
• Steroid-induced: Posterior subcapsular cataract from topical/systemic corticosteroids; significant glare symptoms
• Uveitic: Anterior plaques from fibrin, posterior synechiae causing iris-lens adhesion with sector opacification; glaukomflecken (focal lens epithelial infarcts) from acute angle-closure glaucoma
• Inherited/Congenital: Common in certain dog breeds (Labrador Retriever, Golden Retriever, Boston Terrier, etc.)
• High myopia: Associated with PSC and early nuclear sclerosis

Indications for Cataract Surgery

1. Visual improvement - opacity sufficient to impair daily activities (or functional vision in animals)
2. Medical indication - phacolytic glaucoma (lens proteins leak through hypermature capsule causing macrophage-mediated outflow obstruction), phacomorphic glaucoma (intumescent lens causing angle-closure), lens-induced uveitis

Surgical Management - Phacoemulsification

• Biometry (A-scan ultrasonography) to determine intraocular lens (IOL) power
• Systemic medications reviewed (alpha-blockers associated with intraoperative floppy iris syndrome - IFIS)
• Anticoagulant management per local protocol
• Diabetic patients: assess/photograph the retina before surgery (macular oedema may worsen postoperatively)
• Contact lens wearers: discontinue soft lenses 1 week, rigid gas-permeable (RGP) lenses 6 weeks before biometry to achieve corneal stability
• Previous refractive surgery (LASIK, PRK): standard IOL formulas are inaccurate; use modified formulas (Haigis-L, Masket, contact lens method)

1. Clear corneal incision (2.2-2.8 mm self-sealing, temporal or superior approach)
2. Continuous curvilinear capsulorhexis (CCC) - circular opening in the anterior lens capsule (key step; prevents capsule radial tears)
3. Hydrodissection - fluid wave under the capsule to free the lens nucleus
4. Phacoemulsification - ultrasonic fragmentation and aspiration of the lens nucleus and cortex
5. Cortex aspiration (irrigation/aspiration - I/A)
6. IOL implantation into the capsular bag (standard foldable acrylic IOL)
7. Wound closure (usually self-sealing; suture if necessary)

• Target refraction: usually emmetropia (±0.25 D myopia as safety margin)
• Monovision option: non-dominant eye targeted at ~1-2 D myopia for near
• Multifocal IOLs: provide near/intermediate/distance but may cause glare and reduced contrast sensitivity
• Postoperative refraction within 1.0 D of target in ~90% of cases

• Posterior capsular opacification (PCO) - commonest late complication; "after-cataract"; Elschnig pearl formation from residual lens epithelial cells; treated by Nd:YAG laser capsulotomy
• Cystoid macular oedema (CME)
• Endophthalmitis - rare but devastating; prophylaxis with intracameral cefuroxime
• Posterior capsule rupture intraoperatively - risk of vitreous loss and dropped nucleus
• IFIS (from tamsulosin/alpha-blocker use) - floppy, billowing iris; managed with iris hooks or Malyugin ring

• Dogs are the most common veterinary patients for cataract surgery
• Preoperative electroretinography (ERG) is performed to confirm retinal function before surgery (dogs cannot communicate visual symptoms)
• Intravitreal gentamicin is NOT used (retinal toxicity); topical/systemic antibiotics used
• Diabetic dogs develop cataracts extremely rapidly (days to weeks) due to high aldose reductase activity
• Feline cataracts are less common and often secondary to uveitis
• Horses: cataract surgery is more technically challenging due to globe size and vitreal liquefaction

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Epidural Anaesthesia - Comprehensive Reference

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1. DEFINITION AND OVERVIEW

• Single-shot technique (limited duration)
• Catheter technique - allows continuous infusion, intermittent boluses, or patient-controlled epidural analgesia (PCEA)

• Slower onset (10-20 minutes vs. 2-5 minutes for spinal)
• Larger drug volumes required
• More gradual sympathectomy - better haemodynamic control
• Ability to extend or prolong the block via catheter
• Segmental block possible - anaesthesia can be confined to specific nerve root levels

"Continuous epidural anesthesia is a neuraxial technique offering a range of applications wider than single-dose spinal anesthesia."

• Morgan & Mikhail's Clinical Anesthesiology, 7e

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2. ANATOMY

The Epidural Space

• Surrounds the dura mater posteriorly, laterally, and anteriorly
• Extends from the foramen magnum to the sacral hiatus
• Boundaries:
  • Posterior: vertebral laminae + ligamentum flavum (key landmark)
  • Anterior: posterior longitudinal ligament and vertebral body periosteum
  • Lateral: pedicles and intervertebral foramina

Contents of the Epidural Space

• Fatty connective tissue (loose fat - more in infants and children up to 6-8 years)
• Batson's venous plexus (rich epidural veins - risk of intravascular injection)
• Lymphatics
• Nerve roots travelling to exit via intervertebral foramina
• Connective tissue septa/bands (may explain occasional unilateral block)

Spinal Levels and Tuffier's Line

• Tuffier's (intercristal) line - connects the two iliac crests, crosses the spinous processes at L4-L5 in adults and older children (crosses L5-S1 in infants up to 1 year)
• Spinal cord ends at L1 (conus medullaris) in adults - lower lumbar interspaces are safest
• Thoracic epidurals are technically more difficult due to acute downward angulation (30-50°) of spinous processes

Layers Traversed (Midline Approach, Posterior to Anterior)

1. Skin and subcutaneous fat
2. Supraspinous ligament
3. Interspinous ligament
4. Ligamentum flavum (fibrous, yellow, 3-5 mm thick in lumbar region) → entry into epidural space
5. Epidural space
6. Dura mater (avoid piercing)
7. Subdural space
8. Arachnoid mater
9. Subarachnoid (intrathecal) space

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3. INDICATIONS

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4. CONTRAINDICATIONS

Absolute

• Patient refusal
• Coagulopathy / anticoagulation (risk of epidural haematoma)
• Infection at the insertion site
• Raised intracranial pressure (risk of brainstem herniation if dura accidentally punctured)
• Severe hypovolaemia (profound hypotension from sympathectomy)

Relative

• Sepsis / bacteraemia (risk of epidural abscess)
• Anatomical abnormalities of spine (scoliosis, previous spinal surgery, trauma)
• Pre-existing neurological disease (medicolegal risk)
• Demyelinating conditions
• Severe aortic stenosis or fixed cardiac output states
• Uncooperative patient

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5. EQUIPMENT

Epidural Needles

Epidural Catheters

• 19-20 gauge catheter passed through 17-18 gauge needle
• Advanced 2-6 cm into the epidural space
  • Too short (<2 cm): risk of dislodgement
  • Too long (>6 cm): risk of unilateral block, knotting, venous penetration, or exit through foramen
• Multi-orifice (closed tip + side ports) preferred - lower intravascular penetration rate, fewer paresthesias
• Spiral/spring-wire reinforced: kink-resistant
• Tunnelled subcutaneously for prolonged placement (>1 week)

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6. PATIENT POSITIONING

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7. TECHNIQUE - STEP BY STEP

A. Preparation

1. IV access established, resuscitation equipment and drugs (vasopressors, atropine, intralipid) at hand
2. Standard monitoring: ECG, NIBP, SpO2
3. Pre-loading with IV crystalloid (500-1000 mL) to attenuate sympathectomy-induced hypotension

B. Needle Placement (Midline Approach)

1. Position patient (lateral or sitting), identify interspace (L2-L3, L3-L4, or L4-L5 for lumbar)
2. Sterile preparation; infiltrate skin and subcutaneous tissue with local anaesthetic
3. Insert epidural needle (stylet in place) through skin → supraspinous ligament → interspinous ligament (increased resistance felt)
4. Remove stylet; attach syringe (glass syringe with saline or air for loss-of-resistance)
5. Advance needle with continuous or repeating pressure on plunger

C. Confirming Epidural Space Entry

Loss of Resistance (LOR) Technique - preferred

• Needle within ligament: resistance prevents injection
• As tip enters epidural space: sudden loss of resistance - injection becomes easy
• Can use saline (avoids air embolism; LOR more reliable) or air (helps identify nerve injection but risks air embolism and pneumocephalus)

Hanging Drop Technique

• Hub of needle filled with saline so a drop hangs at the opening
• As needle enters epidural space, negative pressure sucks the drop in
• Less reliable if needle becomes plugged

D. Catheter Insertion (Catheter Technique)

1. Confirm epidural space
2. Thread catheter through needle, advance 2-6 cm beyond needle tip
3. Hold catheter, withdraw needle over catheter
4. Aspirate through catheter (check for blood = intravascular; CSF = intrathecal)
5. Secure catheter to skin

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8. TEST DOSE AND INCREMENTAL DOSING

Test Dose

• Standard: 3 mL of 1.5% lidocaine + epinephrine 1:200,000 (15 mcg)
• If intrathecal injection: 45 mg lidocaine produces rapid dense spinal block
• If intravascular injection: 15 mcg epinephrine produces ≥20% heart rate increase within 1 minute
• Limitations: false positives/negatives possible (uterine contractions, beta-blockers)
• Aspiration alone is insufficient - false-negative aspirations are common

Incremental Dosing

• Inject in 5 mL increments (allows early detection of inadvertent intravascular injection before seizure dose reached)
• Observe for: tinnitus, metallic taste, perioral numbness, slurred speech between doses
• Total dose typically given over 5-10 minutes

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9. DRUGS USED IN EPIDURAL ANAESTHESIA

Local Anaesthetics

Large volumes needed - 10-20 mL for lumbar, up to 35 mL for caesarean section. Always in incremental doses.

Opioids (Adjuvants)

Other Adjuvants

• Epinephrine 1:200,000 (5 mcg/mL) - prolongs block, reduces systemic absorption, acts as marker in test dose; use cautiously in thoracic epidurals (risk of spinal cord ischaemia)
• Clonidine (α2-agonist) - enhances analgesia; dose 1-2 mcg/kg
• Sodium bicarbonate - alkalinises LA solution → increases non-ionised free base → faster onset (1 mEq/10 mL lidocaine)
• Neostigmine - epidural use reduces opioid requirements (investigational)

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10. PHYSIOLOGICAL EFFECTS

Sympathetic Blockade

• Preganglionic sympathetic fibres (smallest, B fibres) are blocked first
• Vasodilation in blocked segments → hypotension (most common significant side effect)
• Onset is more gradual than spinal → better haemodynamic control
• Blocked T1-T4 (cardiac accelerator fibres) → bradycardia
• T6-L2 blockade → bowel relaxation (facilitates abdominal surgery)

Sensory Blockade

• C and Aδ fibres blocked → pain and temperature loss
• Aβ fibres blocked → light touch, pressure
• Sensory block is the primary effect required

Motor Blockade

• Aα motor fibres - largest, most resistant to local anaesthetics
• Degree of motor block depends on concentration:
  • Low concentration (0.0625-0.125% bupivacaine + opioid): sensory block only, minimal motor block - ideal for labour analgesia and postoperative pain
  • High concentration (0.5% bupivacaine): dense motor block - for surgical anaesthesia

Block Height and Spread

• Volume of injectate (most important for epidural)
• Level of injection
• Speed of injection
• Patient positioning (gravity less important than with spinal)
• Age (reduced spread in elderly due to decreased epidural space compliance and narrowed foramina → reduce dose in elderly)
• Pregnancy (engorgement of epidural veins reduces space → reduce dose in pregnancy)
• Obesity (same effect)

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11. SPECIAL APPLICATIONS

Labour Epidural Analgesia

• Dilute bupivacaine 0.0625-0.1% + fentanyl 2 mcg/mL (or sufentanil) as continuous infusion ± PCEA
• Provides excellent pain relief with minimal motor block (patient can push/walk)
• Maintenance: 10-15 mL/h continuous infusion, PCEA bolus 5-10 mL with 10-15 min lockout

Epidural for Caesarean Section

• Lidocaine 2% + epinephrine 1:200,000 (15-35 mL) most common in USA
• Bupivacaine 0.5% (15-25 mL) - widely used globally
• Add fentanyl 50-100 mcg or sufentanil 10-20 mcg for dense block
• Achieve T4 sensory level (nipple line) before proceeding
• Inject in 5 mL increments to avoid high block
• Epidural morphine 3-5 mg at end of surgery provides 6-24 h postoperative analgesia

Thoracic Epidural Analgesia (TEA)

• Inserted at T4-T8 for upper abdominal/thoracic surgery; T7-T10 for lower abdominal surgery
• Benefits: Excellent pain control; early extubation; reduced pulmonary complications; better bowel recovery; reduced cardiac ischaemia risk
• Uses: oesophagectomy, thoracotomy, nephrectomy, aortic surgery, major bowel surgery
• Infusion: dilute bupivacaine 0.125-0.25% + fentanyl at 5-10 mL/h

Combined Spinal-Epidural (CSE)

• "Needle-through-needle" technique: spinal needle through the epidural needle into the subarachnoid space; spinal dose given; needle withdrawn; epidural catheter placed
• Combines rapid dense onset of spinal with flexibility/prolongation of epidural
• Used for: labour (initial spinal injection for rapid relief, then epidural), caesarean section, major joint replacement surgery
• Caution: epidural drugs may pass through dural puncture → potentiated effect; titrate carefully

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12. COMPLICATIONS

A. Cardiovascular

B. High / Total Block

• Cause: Excessive dose, inadvertent intrathecal injection of epidural dose, or exaggerated cephalad spread
• Signs: Dyspnoea → upper limb numbness → Horner syndrome → unconsciousness → apnea
• Apnoea more often from medullary hypoperfusion than phrenic nerve block (C3-C5)
• Treatment: Reassure; supplemental O2; intubate and ventilate; vasopressors + fluids; atropine for bradycardia

C. Post-Dural Puncture Headache (PDPH)

• Occurs after "wet tap" (accidental dural puncture with epidural needle) or unrecognised dural puncture
• Incidence: up to 20-50% following wet tap with large Tuohy needle in young pregnant women; <1% with 27G pencil-point spinal needle in elderly men
• Characteristics:
  • Bilateral, frontal/occipital, extends into neck
  • Postural: worse sitting/standing, relieved lying flat (hallmark)
  • Onset 12-72 hours after procedure
  • Associated with photophobia, nausea, tinnitus, diplopia (VI nerve traction)
• Mechanism: CSF leak from dural defect → intracranial hypotension → meningeal traction
• Risk factors: large needle gauge, cutting-point needle, female sex, young age, pregnancy, history of prior PDPH
• Conservative treatment:
  • Recumbent positioning, oral/IV fluids, caffeine (vasoconsticts intracranial vessels + stimulates CSF production)
  • NSAIDs, paracetamol, opioids
  • Sphenopalatine ganglion block (intranasal LA application)
• Epidural blood patch (EBP): 15-20 mL autologous blood injected at the level of puncture; stops CSF leak by tamponade/clot formation; ~90% success rate with single patch; 90% of initial non-responders respond to second patch; generally not recommended prophylactically

D. Nerve Injury

E. Other Complications

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13. EPIDURAL HAEMATOMA AND ABSCESS - KEY SAFETY POINTS

Epidural Haematoma

• Most feared complication of neuraxial anaesthesia
• Risk factors: anticoagulant use, platelet disorders, traumatic placement, indwelling catheter
• ASRA anticoagulation guidelines for timing of neuraxial block:
  • Unfractionated heparin: hold 4-6 hours, check APTT
  • LMWH (prophylactic): 12 hours before; 24 hours after removal
  • LMWH (therapeutic): 24 hours before
  • Warfarin: INR ≤1.5 before insertion/removal
  • Aspirin/NSAIDs: not a contraindication alone
  • Direct oral anticoagulants (DOACs): 48-72 hours (drug-specific)

Epidural Abscess

• Incidence: 1:1000-1:10,000 epidural catheter insertions
• Symptoms develop over hours to days: fever, back pain, meningism, then neurological deficit
• Surgical emergency if neurological deficits present

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14. EPIDURAL vs. SPINAL ANAESTHESIA - COMPARISON

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15. SUMMARY OF CLINICAL PEARLS

• A segmental thoracic epidural provides excellent upper abdominal analgesia while sparing cervical and lumbar nerve roots - invaluable for major thoracoabdominal surgery
• Dilute LA + opioid = analgesia without motor block; concentrated LA = surgical block
• The Batson venous plexus in the epidural space means intravascular catheter placement is common - always aspirate and always test dose
• Pregnancy and obesity reduce the epidural space volume - reduce doses by 25-30%
• A blood patch cures ~90% of PDPHs; do not patch prophylactically
• Epidural haematoma presents as severe back pain with leg weakness - requires emergency MRI and decompressive laminectomy within 6-8 hours to avoid permanent paralysis
• Epidural morphine provides 6-24 hours of excellent post-surgical analgesia but carries risk of delayed respiratory depression (monitor for 24 hours)

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