General Concepts

Action of Extraocular Muscles from Primary Position

Muscle Primary Secondary Tertiary
Medial Rectus Adduction
Lateral Rectus Abduction
Inferior Rectus Depression Extorsion Adduction
Superior Rectus Elevation Intorsion Adduction
Inferior Oblique Extorsion Elevation Abduction
Superior Oblique Intorsion Depression Abduction
  • Normal muscle positions from limbus: Medial 5.5 mm, , Inferior 6mm, Lateral 7mm, Superior 8mm (Spiral of Tillaux)
  • Surgical results tend to drift back to original position
    • Wait longer to reoperate if there is an overcorrection as it may disappear
    • An undercorrection is less likely to disappear-reoperating sooner is justified. If there is a persistent undercorretion at six weeks post-op, plan to reoperate
  • To improve accuracy, especially in congenital ET, measure from the limbus before cutting the muscle off the sclera.  Place muscle based on limbus measurement.
  • To avoid crippling muscle action: avoid moving the medial beyond 13mm from limbus, the laterals can go back to 16 mm from the limbus. Sometimes you want to cripple the muscle (Kestenbaum surgery for nystagmus) You may need very large recessions in Graves- crippling of the muscle is less likely in this setting.
  • Don’t forget the refractive error
    • Adjust for high plus and high minus (> +/- 5.00)
      • High plus lenses: acts like base out prism for ET and base in for XT. Prism measurement is underestimated, increase surgical numbers,  see: Glasses Tables
      • High minus lenses: act like base in prism for ET and base out for XT. Prism measurement is overestimated , decrease surgical numbers  see: Glasses Tables
    • Correct myopia in XT and hyperopia in ET (>+1.50) it will often help
  • Don’t forget to check for A or V pattern and Near/distance disparities
    • Pattern = upgaze measument vs downgaze measurement
    • Check for oblique dysfunction: if present,  oblique surgery should be considered (oblique surgery will have very little effect in primary position)
      • V pattern: look for inferior oblique overaction/superior oblique palsy
      • A pattern: look for superior oblique overaction
    • Vertical displacement of horizontal rectus muscles
      • V pattern: move medal down, lateral up
      • A pattern: move medial up, lateral down
      • < 15 Δ no treatment, 15-25Δ: ½ tendon width, 25-35Δ: ¾ tendon width, >30Δ may combine with oblique surgery
  • Remember AC/A ratio: see ET and XT below
    • AC/A Gradient Method – fixation distance is fixed. The deviation is measured with and without a modifying lens. This lens changes the amount of accommodation used to create a clear image and thus changes the resultant deviation. AC/A = change in deviation / change in accommodation
  • Measure by stimulating accommodation
    • Measure ocular misalignment while fixating at 6 meters, then remeasure with a -1.00 D lenses in front of both eyes
    • The difference between the two measurement is the AC/A ratio
  • Measure by relaxing accommodation
    • Measure misalignment with target at 0.33 meters, then remeasure with +3.00 D lenses over both eyes
    • The difference is divided by 3 to get the AC/A ratio

Normal range of AC/A ratio (Gradient Method) average is 3.7:1

  • range is 0.9 – 9.8 ; Low – 0-2.0; Normal – 2.5 – 5.0; High – >5.0
  • Remember stacking prisms in same direction gives more prism power than the sum of the prism values
  • In the presence of amblyopia or poor vision: operate on the amblyopic eye
    • More surgery will be done than usually acceptable
    • Crippling of muscle action often seen (desired)

Esotropia

  • Congenital ET is more likely to need multiple surgeries
    • Four muscle surgery may be more likely to have overcorrections
  • Acquired ET: use Recess & Resect whenever possible for basic type (N=D) up to 35 Δ- good for partially accommodative ET
  • If near > distance: increase surgical numbers
    • Consider operating for near angle if difference is <20Δ
    • Consider posterior fixation sutures if more
    • Can do prism adaptation for near angle
  • Prism Adaptation for partially accommodative and acquired non-accommodative ET
    • This is done if you are planning surgery to increase precision
    • Place Fresnel prism for largest angle of deviation
    • Remeasure 1 week: if peripheral fusion present (Worth 4 Dot) and ortho or less than 8 PD
    • if angle of alignment increases, reapply new prism- and remeasure in 1 week
    • If fusion occurs at larger angle operate for new angle
    • If angle grows > 60, or no fusion occurs then operate for near angle

See Esotropia Surgical Tables

Exotropia

  • Remember to identify types based on Duane’s Classification
    • Basic Type (N=D): Recess & Resect
    • Divergence Excess (True or Pseudo): Bilateral Lateral Rectus Recessions
    • Convergence insufficiency: Include resection(s) of medial(s)
    • High AC/A: minus lens therapy
  • Late undercorrections (>6 weeks) more common than in ET

see Exotropia Surgical Tables

Vertical Deviations

  • 1 mm = 3 Prism Diopters
  • Large recessions of the Inferior rectus are prone to late slippage (can use permanent suture, 6-0 polyester, WE Scott recommends good cleaning of the muscle belly that opposes the sclera and no polyester)
  • Vertical rectus muscles are attached to eyelids (especially inferior)
    • Inferior Rectus Recessions will pull lid down (>4mm): need to clean muscle back at least 20 mm. Consider reattaching lower lid retractors to muscle at original position (15mm).
  • DVD can be treated with “supramaximal” recession of superior rectus
    • Consider bilateral surgery if any DVD of fellow eye
    • If inferior oblique dysfunction, anterior displacement of inferior oblique can treat DVD

See Superior Oblique Tables  and DVD Surgical Table

Head Turns (Nystagmus)

  • If able, delay this surgery until little or no risk of inducing amblyopia for accidental post-op ET or XT (7-8 years old)
  • If strabismus present proceed with surgery
  • Use fixating eye for nystagmus treatment and fellow eye to correct induced strabismus
  • Recurrence of head turn (progressive undercorrections) common
  • If esotropia associated with Nystagmus- this may be the mechanism for dampening and bilateral medial rectus recessions will help both problems

See Nystagmus Tables