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New discoveries about extraocular muscles have widened the range of diagnoses and surgical techniques for strabismus.
Reviewed by Joseph L. Demer, MD, PhD
Los Angeles-There is nothing simple about ocular muscles and sorting out the causes of diplopia and nystagmus. However, recognition of another ocular structure-i.e., pulleys-might make the job a little easier and shift the attention away from the nerves in the orbit.
When physicians learned about how the extraocular muscles (EOMs) are “hooked up” anatomically, they might have assumed that the muscle paths are fixed in the orbit. Therefore, the belief would be that the lateral rectus (LR) muscle always pulls laterally, which would be incorrect, because that belief implies that the muscle sideslips over the globe as it rotates vertically, said Joseph L. Demer, MD, PhD.
“What is correct is that connective tissue pulleys exist in the orbit at the level of posterior Tenon’s fascia that cause the paths of the muscles to change systematically with gaze,” said Dr. Demer, the Leonard Apt Professor of Ophthalmology and professor of neurology, David Geffen Medical School, and director, Ocular Motility Laboratory, Stein Eye Institute, University of California, Los Angeles.
This is important to ocular motility and it is at least as important as certain neurologic features of the EOMs, he noted.
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Every muscle rotates the eye toward its pulley. In cases in which a pulley is displaced, the muscle can pull the eye in directions other than that intended by a normal pulley. Therefore, Dr. Demer mentioned, not all imbalances of the oblique EOMs result from the oblique muscles.
“So many common forms of strabismus that were presumed to have neurologic origins are actually mechanical,” he said.
When strabismus results from a mechanical problem, the nervous system is not to blame and the deviation is easier to fix surgically, he pointed out.
Dr. Demer demonstrated on a coronal section of the orbit that connective tissue sheaths surround the scleral collagen and are convex toward the orbit. These tissues can be dissected during strabismus surgery without a great deal of effect on the EOMs. The functional pulleys are located in the mid-orbit posterior to the center of the globe. Complete rings of collagen are present around the LR, superior rectus (SR), medial rectus (MR), and inferior rectus (IR) muscles-which pass through the rings.
“This is the area that is functionally important to the pulling direction of the EOMs,” he said.
The EOMs also have orbital layers and global layers. There is no muscle cone posterior to the globe, but only loose connective tissue among the EOMs. This indicates that there is nothing in the deep orbit that constrains the paths of the EOMs.
“The pulleys determine everything about posterior muscle paths,” Dr. Demer said.
Some strabismus occurs as the result of degeneration of orbital connective tissue. This is especially true of the LR-SR band, which suspends the LR pulley, prevents the inferior oblique muscle from pulling it down, and forms an interconnection with the SR pulley. This may be responsible for from 15% to 20% of adult cases of strabismus.
Abnormal pulleys can be congenital or acquired, and certain mislocations of the former are often responsible for characteristic patterns of incomitant strabismus, Dr. Demer explained. A-pattern strabismus typically exhibits under-elevation in adduction and over-depression in adduction.
External clues can be helpful during the examination to discern the orientation of the pulleys based on the eyelids.
If a line is drawn between the medial and lateral canthi in both eyes and then the lines that are perpendicular are draw, the letter A is enscribed, which corresponds in most cases to a patient’s pattern of incompetence, Dr. Demer noted.
The inverse to this is the V pattern characterized by over-elevation in adduction and under-depression. When coronal imaging is done at the level of the pulleys, line can be drawn that interconnect the corresponding pulleys.
Dr. Demer highlighted a case in which the MR muscle is also a supraductor as well as a traditional adductor; the LR muscle is also an infraductor and as well as a traditional abductor; the IR muscles are adductors as well as infraductors; and the SR muscles are abductors as well as supraductors. When lines are drawn between the medial and lateral canthi and the perpendicular lines are drawn to them, the letter V is enscribed.
Acquired malposition of the rectus pulleys resulting from age-related degeneration of the LR-SR band can be seen frequently in neuro-ophthalmology.
Dr. Demer demonstrated in cadaver orbits that at 17 months and 4 years of age, the LR-SR band is robust, while later at 57 and 93 years, the band is attenuated and ruptured, and the residual remnants are bulged superiorly and laterally.
In addition, the LR muscle is shifted inferiorly, indicating that the muscle might have infraducting and abducting functions, i.e., the sagging eye syndrome that can occur in 15% to 20% of strabismus in adults in the United States.
“If this process of LR-SR degeneration occurs bilaterally symmetrically, there is a downward shift of the LR muscle that is similar in both eyes,” Dr. Demer said.
Some of the adduction action of the LR muscle is converted symmetrically to infraduction so no vertical strabismus results, but the patient loses some abducting action. There is no slowing of the saccades, but the patient will have age-related distance esotropia, which is synonymous with divergence paralysis.
If the degeneration is asymmetric, with more downward LR muscle sagging in one eye than the other, there is greater imbalance vertically. Then the eye with the greater downward shift of the LR pulley will be hypertropic and excyclotropic.
In contrast to this LR-SR muscle degeneration is the heavy eye syndrome, which is characterized by both axial high myopia due to hypotropia and esotropia that is associated with shifting of the IR muscle nasally.
EOMs each have two parts that are independent of one another and do different things. The orbital layer displaces the EOM's pulley posteriorly upon muscle contraction; this layer contains 40% to 50% of the total muscle fibers and inserts on the inside of the pulley.
In contrast, in the other layer, the global layer, with 50% to 60% of the muscle fiber, the fibers go through the pulley to tendon where they insert on the globe. The orbital layer does not rotate the globe, while the global layer does, Dr. Demer said.
Most of EOMs have two other functional compartments-the horizontal rectus muscles have superior and inferior compartments, and the superior oblique muscle has medial (torsional action) and lateral (mostly vertical action) compartments.
Pulleys are now important considerations in eye muscle surgery. Dr. Demer advised that surgical dissection of the intermuscular membranes and tissues around the pulleys should be minimized unless manipulation of pulleys is a specific surgical objective.
For example, in esotropia that is greater at near than distance, action of the medial rectus muscle can be selectively reduced at near by pulley posterior fixation, suturing the muscle margin to the pulley entrance without scleral suturing at that location.
However, Dr. Demer noted that in acquired downshift of the lateral rectus pulley due to sagging eye syndrome in older patients, direct pulley surgery is usually not necessary.
For example, in age-related distance esotropia, he recommends medial rectus recession that is conveniently done under topical anesthesia.
Dr. Demer also recommends partial tenotomy of vertical rectus muscles for small-angle hypertropia. This technique under topical anesthesia permits reliable correction of hypertropia in the range of 2 to 6 prism D.
Joseph L. Demer, MD, PhD
This article was adapted from Dr. Demer’s presentation at Neuro-Ophthalmology Subspecialty Day at the 2015 meeting of the American Academy of Ophthalmology. Dr. Demer has no financial interest in the subject matter. He reported that some surface coils used were not FDA approved.