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    Strabismus much more than orbital pulleys and muscles

    Causes also include connective tissue disorders, nuances of functions of muscle subregions


    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.

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    “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.

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    The problematic pulley

    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.

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    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.

    Recent: OCT + OCTA capable of imaging both structure and function

    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.

    Neuro-ophthalmology findings

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