July 16, 2016
A research team in Ophthalmology at Mayo Clinic's campus in Rochester, Minnesota, has developed a new method for quantifying intraoperative torsional forced ductions and validated the test by comparing patients with oblique muscle dysfunction and controls. The torsional forced duction test enables quantitative assessment of superior oblique (SO) and inferior oblique (IO) tightness and laxity, and is useful for intraoperative surgical planning.
"This simple method more precisely quantifies SO and IO tightness or laxity by measuring the number of degrees of allowable torsional movement," says Jonathan M. Holmes, M.D. "The new test was validated by comparing the quantitative values obtained between patients with abnormal oblique muscles and controls, and now can be used to guide the surgeon in planning and performing oblique muscle surgery." The study was published in Ophthalmology in September 2015.
The new torsional forced duction test is performed in the anesthetized patient using a commercially available Mendez ring, 0.3-mm forceps and a skin-marking pen.
At the start of surgery, while the patient is under deep general anesthesia, the surgeon marks the 12 and 6 o'clock positions at the limbus. Then the globe is maximally excyclorotated without retroplacement until the first resistance is felt. Then the surgeon reads the angle of rotation (in degrees) on a Mendez ring and a photograph can be taken. The procedure is repeated for incyclorotation.
The team studied 33 eyes with oblique dysfunction — nine with presumed congenital superior oblique palsy (SOP), 13 with acquired SOP, seven with Brown syndrome and four with inverted Brown syndrome — and 31 controls. They also studied six eyes after SO disinsertion and two eyes after inferior oblique disinsertion.
Patients with a history of strabismus surgery, ocular trauma, or any restriction of the horizontal or vertical rectus muscles, based on standard intraoperative forced duction tests, were excluded, except for cases of suspected inverted Brown syndrome.
The team also evaluated 31 patients as controls (ages 8-77 years). One eye was randomly selected from each control patient (n = 31 eyes) for analysis. The control subjects:
- Had no restriction of any horizontal or vertical rectus muscles
- Had negative Guyton's exaggerated forced duction tests for SO or IO tightness or laxity
- Were scheduled to undergo horizontal muscle surgery to correct purely horizontal strabismus
Researchers also measured maximal excyclorotation after complete SO disinsertion, before reattachment, in six eyes (one eye with presumed congenital SOP, one eye with acquired SOP and four eyes with Brown syndrome) and maximal incyclorotation after complete IO disinsertion, before reattachment, in two eyes with suspected inverted Brown syndrome.
Main outcome measures were maximal excyclorotation and maximal incyclorotation in each oblique dysfunction condition and in controls by both surgeon's report and photographic assessment. "Using this new quantitative torsional forced duction test, we found greater maximal excyclorotation in presumed congenital SOP and lower maximal excyclorotation in Brown syndrome compared with controls," says Dr. Holmes. "We also found reduced maximal incyclorotation in cases of suspected inverted Brown syndrome compared with controls. Photographic assessment showed excellent test-retest reliability, and the surgeon's report showed excellent agreement with photographic assessment."
The new torsional forced duction test offers several advantages over current test procedures:
- The new test complements the qualitative exaggerated forced duction test described by David L. Guyton, M.D., published in Ophthalmology in 1981, because the new test is quantitative, yielding a value in degrees and performed in a more physiologic field of rotation.
- The new test extends the qualitative test described by Burton J. Kushner, M.D., published in Archives of Ophthalmology in 2007, because the new test provides a value in degrees and allows values in one eye to be compared with control values and compared with values in the fellow eye. The new test also allows quantitative monitoring of stepwise effects during the surgical procedure.
- The new test procedures expand on the test described by Irene H. Ludwig, M.D., and others in research published in Pediatric Ophthalmology: Current Thought and a Practical Guide in 2009 and Journal of the American Association for Pediatric Ophthalmology and Strabismus in 2013, who studied maximal excyclorotation and maximal incyclorotation under general anesthesia, but did not describe how they quantified the degree of rotation. The new test quantifies the amount of torsion using a Mendez ring and seemed to be less variable in control subjects.
"This new test is particularly useful in the diagnosis of congenital SOP, Brown syndrome and inverted Brown syndrome, and may guide intraoperative surgical planning in these conditions or when these conditions are suspected," says Dr. Holmes. "It is also useful in confirming complete or incomplete intraoperative disinsertion of the SO tendon or IO muscle, and may help prevent unwanted surgical undercorrection when disinsertion or recession of the obliques is indicated."
For more information
Jung JH, et al. Quantitative intraoperative torsional forced duction test. Ophthalmology. 2015;122:1932.
Guyton DL. Exaggerated traction test for the oblique muscles. Ophthalmology. 1981;88:1035.
Kushner BJ. Superior oblique tendon incarceration syndrome. Archives of Ophthalmology. 2007;125:1070.
Wilson ME, et al., eds. Principles and management of complex strabismus. In: Pediatric Ophthalmology: Current Thought and a Practical Guide. Berlin, Germany: Springer-Verlag Berlin Heidelberg; 2009.
Ludwig IH, et al. New strabismus surgical techniques. Journal of the American Association for Pediatric Ophthalmology and Strabismus. 2013;17:79.