Anatomy of the Extraocular Muscles and Important Adnexa (Part 1)

1

Adult

The normal adult has frontally placed eyes with an interpupillary distance of approximately 65mm and a palpebral opening 28mm wide and 10mm in vertical dimension.  The horizontal corneal diameter is 12.5mm, and an intercanthal distance depending on race, is equal to one half of the pupillary distance or less.  The infant palpebral opening is 18mm wide and 8mm in vertical dimension.

Infant

2. The shape of the palpebral opening depends on racial characteristics.  Since alignment of the eyes is noted by a centered pupillary light reflex, the shape of the palpebral opening should have no influence of apparent strabismus.

Mongoloid   Level   Anti-mongoloid

Fissure Characteristics Fissure contrary to usual racial characteristic may indicate pathology.  For example: mongoloid fissure in caucasian often associated with "A" pattern and myelomeningcele

3.  A nasally centered pupillary light reflex gives the appearance of exotropia and is physiologic.  A temporarily displaced pupillary light reflex gives the appearance of esotropia and is uncommon, occurring in some cases of very high myopia.

Positive angle kappa - simulates exotropia

Centered pupil light reflex - aligned appearance

Negative angle kappa - simulates esotropia

4.  As the globe enlarges gradually the anterior - posterior axial lengths increases.

Newborn 17mm

1 yr. old 19.5 mm

Emmetropic Adult 23.5 mm

5. The rectus muscles insert in sclera gradually farther from the limbus beginning with the medial rectus at 5.5mm (range 3.0 to 6.0mm), inferior rectus 6.5mm, lateral rectus 6.9mm and superior rectus 7.7mm.  The line of insertion is called the spiral of Tillaux which is also the line of insertion of posterior Tenon's capsule which then proceeds to the limbus as the episclera fused with the underlying sclera.

Spiral of Tillaux

6. Each of the rectus muscles has two anterior ciliary arteries except the lateral which has one.  A long posterior ciliary artery travels from the back of the eye in sclera beneath the horizontal recti.  The oblique muscles do not contribute to the blood supply of the anterior segment of the eye.

2 anterior ciliaries in IR, MR, SR - one in LR

Long Posterior Ciliary

7.  The superior oblique (S.O.) is made up of a 30mm muscle, and 30mm tendon with the trochlea between.  The S.O. has its functional origin in the trochlea.  This unique structure allows an 8mm increase in the trochlear insertion distance in upgaze and an 8mm decrease in distance between the S.O. insertion and trochlea in downgaze.  In a normal trochlea-superior oblique tendon relationship, these values are constant.  In cases of congenital superior oblique underaction (palsy), these relationships can be altered because of laxity and/or malposition of the tendon insertion, usually causing S.O. underaction, (S.O. "palsy") with hypertropia.  In some cases an abnormality of the trochlea tendon limits increase in the trochlea - insertion distance resulting in a hypotropia in the involved eye especially looking up in adduction. (Brown syndrome)