David L. Guyton

Exaggerated Traction Test for the Oblique Muscles

Retroplacement and torsional manipulation of the globe during forced duction testing provide an exaggerated traction test of the oblique muscles, allowing graded evaluation of superior oblique and inferior oblique tightness. On a scale from 0 to 4 +, normal superior oblique tightness averages 1.5+, and normal inferior oblique tightness averages about 1+. The degree of superior oblique or inferior oblique tightness correlates well with clinical overaction of these muscles, allowing better distinction between oblique overaction and dissociated vertical deviation. Also, the effect of partial superior oblique tenotomy can be monitored, and a complete tenotomy can be confirmed with certainty.

Ipsilateral Hypertropia after Cataract Surgery

BACKGROUND Reports of acquired strabismus caused by injection of local anesthetics during cataract surgery have increased recently. The authors proposed a mechanism to explain the occurrence of strabismus with apparent overactive muscles after cataract surgery. METHODS The authors studied 19 patients in whom strabismus developed after cataract surgery. Prism and cover test in the diagnostic positions of gaze and forced-duction testing were used to identify the affected muscles. RESULTS The deviation was greater in the field of action of the presumed tight muscle in 16 of 19 patients. An ipsilateral hypertropia with superior rectus muscle overaction subsequently developed in two patients with an initial hypotropia. An overaction of the ipsilateral lateral rectus muscle causing an exotropia developed in one patient with initially limited abduction. CONCLUSIONS Myotoxicity from direct injection of local anesthetics into an extraocular muscle probably causes transient paresis followed by segmental contracture of the involved muscle. Mild contractures result in strabismus with a motility pattern of an overactive muscle. Larger amounts of contracture lead to restrictive strabismus. The risk of strabismus may be decreased by administering the local anesthetic into sub-Tenon space using a blunt-tipped cannula when performing cataract surgery.

Ophthalmic prisms: Measurement errors and how to minimize them

Variable results of strabismus surgery may be due in part to errors in prism measurement. The amount of deviation neutralized by an ophthalmic prism is variable depending on how the prism is held. For example, a 40 delta glass prism with the posterior face held in the frontal plane gives only 32 delta of effect. Glass prisms are calibrated for use in the Prentice position. Plastic prisms are calibrated for use in the frontal plane position. Surprisingly large errors in prism measurement are produced when adding a small prism to a large prism. For example, adding a 5 delta glass prism to a 40 delta glass prism gives not 45 delta of effect, but 59 delta. This error can be minimized but not eliminated by holding one prism in front of each eye. The error can also be calculated so that the appropriate correction can be made.

Optimal Astigmatism to Enhance Depth of Focus after Cataract Surgery

A small amount of myopic astigmatism can enhance the depth of focus of the pseudophakic eye, optimally providing at least 20/30 visual acuity for both near and distance fixation. For given spherocylindrical refractive errors and fixation distances, the cross-sectional area of Sturms conoid at the retina was calculated for a schematic eye. These data were used to determine the optimal astigmatic error needed to obtain maximum depth of focus and least theoretical blur for any given spherical equivalent refractive error. Optimal depth of focus was obtained when the plus cylindrical component equaled negative sphere - 0.25 diopters. The near and distance visual acuities of ten pseudophakic patients with induced refractive errors were highly correlated with this model. Low myopic astigmatism after cataract surgery may represent an alternative to multifocal intraocular lenses by providing spectacle independence.

Prescribing cylinders: The problem of distortion

Astigmatic spectacle corrections produce distortion of retinal images. Under binocular conditions, small amounts of monocular distortion may significantly alter the sense of spatial localization, producing symptomatic binocular distortion. From examination of the nature and sources of monocular distortion, a rational basis may be developed for methods to minimize distortion when necessary. Adaptation to distortion occurs both by interpretive and physiological mechanisms, with the physiological mechanism appearing to be a type of rotational anomalous retinal correspondence. Through better knowledge of distortion and adaptive mechanisms, a revised set of practical guidelines for prescribing cylinders is presented.

Rapid Determination of Intraocular Lens Tilt and Decentration Through the Undilated Pupil

The apparent optical axis of an implanted intraocular lens (IOL) can be located by alignment of the examiners hand light with the third and fourth Purkinje images from the front and back surfaces of the IOL. Tilt of the IOL can be estimated (or measured with an arc perimeter) by the angle between the apparent optical axis and the patients line of sight (actual tilt approximately 0.85 x apparent tilt). Decentration of the IOL is easily detected, equal to the distance of the IOL optical axis from the center of the pupil. This simple technique can be used through the natural pupil with posterior chamber IOLs, providing the optical axis of the malpositioned IOL still passes through the pupil.

Reoperation rate in adjustable strabismus surgery

Adjustable suture strabismus surgery may reduce the frequency of reoperations by reducing immediate postoperative over- and undercorrections. We reviewed 290 strabismus procedures performed with an adjustable suture technique. Thirty-five patients required additional surgery, for a reoperation rate of 9.7%. Reoperation frequencies for patients undergoing non-adjustable procedures average approximately 20% in our experience and in reported series. Our low reoperation rate with adjustable strabismus surgery supports our clinical impression that more accurate results are possible with this technique in appropriate patients.

Anesthetic myotoxicity as a cause of restrictive strabismus after scleral buckling surgery

Purpose: To explore the possibility that anesthetic myotoxicity may play a role in restrictive strabismus following scleral buckling procedures. Methods: The authors performed a retrospective study of patients who presented with strabismus following scleral buckling procedures. Details were sought regarding the scleral buckling procedure, including type and route of anesthesia. The types of strabismus were compiled, as were relevant findings at strabismus surgery. The contributing vitreoretinal surgeons were surveyed regarding the usual type and route of anesthesia used for their scleral buckling procedures. Results: Over 90% of scleral buckling procedures resulting in significant strabismus were performed under local anesthesia. Of the 17 patients on whom strabismus surgery was performed, 14 had positive forced ductions. A hypodeviation of the buckled eye was the most common presentation. Conclusion: Based on the types, patterns, and amounts of strabismus encountered after scleral buckling procedures, and the similarity of these findings to cases of strabis‐ mus following retrobulbar anesthesia for cataract procedures, the authors propose that local anesthetic myotoxicity is often the primary cause of strabismus occurring after scleral buckling procedures for retinal detachment.

Automated detection of foveal fixation by use of retinal birefringence scanning

Foveal fixation was monitored in normal subjects remotely and continuously by use of a noninvasive retinal scan. Polarized infrared light was imaged onto the retina and scanned in a 3 degrees annulus at 44 Hz. Reflections were analyzed by differential polarization detection. In all 32 eyes studied, the detected signal was predominantly 88 Hz during central fixation (within +/-1 degree) and 44 Hz during paracentral fixation. Phase shift at 44 Hz correlated with the direction of eye displacement. Potential applications of this technique include screening for eye disease, eye position monitoring during clinical procedures, and use of eye fixation to operate devices.

Dissociated vertical deviation: Etiology, mechanism, and associated phenomena ☆ ☆☆

Purpose: The etiology and mechanism of dissociated vertical deviation (DVD) are explored. Methods: In 6 young adults with DVD, the simultaneous horizontal, vertical, and torsional eye movements for both eyes were recorded by using dual-coil scleral search coils. Analysis of the simultaneous vertical and torsional movements that occurred during the DVD response identified the primary muscles acting in the vergences and versions involved. Results: Typically, both horizontal and cyclovertical latent nystagmus developed upon occlusion of either eye. A cycloversion/vertical vergence then occurred, with the fixing eye intorting and tending to depress and the covered eye extorting and elevating. Simultaneously, upward versions occurred for the maintenance of fixation, consisting of various saccades and smooth eye movements, and this led to further elevation of the eye behind the cover. The cyclovertical component of the latent nystagmus became partially damped as the DVD developed. Conclusions: In patients with an early onset defect of binocular function, the occlusion of one eye, or even concentration on fixing with one eye, produces unbalanced input to the vestibular system. This results in latent nystagmus with a cyclovertical component, sometimes only seen with magnification. A normal, oblique-muscle-produced, cycloversion/vertical vergence then comes into play, occurring in an exaggerated form in the absence of binocular vision, probably as a learned response. This cycloversion/vertical vergence helps damp the cyclovertical nystagmus (a cyclovertical “nystagmus blockage” phenomenon), aiding vision in the fixing eye. But this mechanism also produces unavoidable and undesirable elevation and extorsion of the fellow eye, which we call DVD. (J AAPOS 2000;4:131–44) J AAPOS 2000;4:131–44