Intraoperative intorsional traction test of the inferior oblique.

PurposeWe present a novel variation of the traction test of the inferior oblique (IO) muscle. We demonstrate the correlation between the traction test and clinically graded IO overaction and describe the utility of this test to confirm IO weakening.MethodsWe performed a retrospective chart review on all patients who underwent IO surgery and intraoperative intorsion traction tests by a single surgeon over a 10-year period. We compared the traction test results, in 'clock hours' of freedom, before and after IO surgery. We correlated the torsion test at start of surgery with clinical observed IO overaction (scale 0 to +4) in 67 IO operations (56 myectomies, 6 anterior transpositions, 4 myotomies, and 1 recession) and compared to a control group of 23 eyes with minimal or no IO overaction.ResultsThe mean intorsion freedom in the eyes undergoing IO surgery was less than in control eyes (1.63 vs 1.89 clock hour; P<0.00005). There was a significant inverse relationship between grading of clinical IO action and the intorsion test result (Pearson rank coefficient, (r=-0.45; P<0.00001)). Myectomy produced the greatest change in torsion freedom (mean 1.32 clock hour), with all myectomies showing at least 1 clock hour extra freedom after the surgery.ConclusionsThe intorsion traction test confirmed that the IO stiffness correlated with pre-operative IO overaction grade. While it can be helpful in confirming that the entire IO muscle was weakened, it does not substitute for the careful inspection at the end of surgery to ensure there are no remaining IO fibers.

Horizontal and vertical angle kappa.

BACKGROUND: The aim of this study is to report the geometric range of angle kappa formation in patients with and without strabismus. PATIENTS AND METHODS: This is a retrospective study of three patients with angle kappa in different planes. Routine eye examinations, including visual acuity, slit-lamp examination, and ophthalmoscopy, were performed. A thorough orthoptic examination revealed a notable difference between the prism and alternate-cover test and the Hirschberg measurements. RESULTS: The first patient exhibited a bilateral vertical angle kappa into opposite directions due to retinochoroidal scars. Two other patients presented with horizontal angle kappa deviations. In one patient a true accommodative esotropia was exaggerated by a right negative angle kappa. The other patient had a pseudoexotropia due to bilateral positive angle kappa. Macular ectopia was noted in all cases. CONCLUSIONS: The patients herein reported demonstrate a marked variability of angle kappa occurrence in the horizontal and vertical plane. The angle kappa can exaggerate or conceal the size of the true heterotropia.

Unilateral recession and resection in Duane syndrome.

PURPOSE: To assess the efficacy of lateral rectus resection with medial rectus recession in the affected eye of patients with Duane retraction syndrome (DRS) with esotropia and limited abduction, compared with bilateral medial rectus recessions. METHODS: The charts of 9 patients with DRS who underwent a recession-resection procedure and 10 patients with DRS who underwent bilateral medial rectus recessions were reviewed. Ocular ductions (graded from 0 = full duction to -4 = total deficit), severity of retraction, alignment, head position, and binocular single vision field (for study group only) were recorded before and after surgery. RESULTS: Before surgery, the study and control groups did not differ in mean primary position esotropia (16.9 and 18.8 PD, respectively), face turn (16.5 degrees and 15.0 degrees, respectively), average limitation of abduction (-3.9 and -3.7, respectively), or adduction (-0.1 and -0.3, respectively). After surgery, both groups had similar mean face turns (3.9 degrees and 1.0 degrees ), esotropia (3.3 PD and 1.0 PD), and abduction limitation in the affected eye (-2.4 and -2.6). However, mean adduction was significantly worse in the control group than in the study group (-1.5 vs -0.6, P = .02). Globe retraction improved in all control subjects. It worsened in 5 study subjects and did not improve in the other 4. In the study group, 1 patient required reoperation for undercorrection and another was overcorrected. CONCLUSION: Seven of 9 patients with DRS, selected on the basis of esotropia, limited abduction, and mild retraction, benefited from a recession-resection procedure. Abduction improved to the same degree as seen after bilateral medial rectus recessions, with less tendency to limit adduction.

The double-bellied inferior oblique muscle: clinical correlates.

PURPOSE: We previously reported an 8% incidence of double-bellied inferior oblique (IO) muscles at the surgical capture site (10-12 mm from insertion) in cadaveric specimens. This companion study sought to determine how often this anomaly is encountered at surgery for clinically overacting IO muscles and whether clinical findings or surgical outcomes in cases with double-bellied muscles differ from those with single-bellied muscles. METHODS: For 7 years we collected preoperative, intraoperative, and postoperative data on all patients for whom one surgeon performed primary IO weakening operations for overactions. We compared eyes with double-bellied IO muscles to those with single-bellied muscles on 4 variables--gradings of preoperative IO and superior oblique (SO) actions, presence of fundus excyclotropia, differences between horizontal deviations in upgaze and downgaze, and presence and sizes of primary position hypertropias--to determine whether one or more of them could predict the presence of a double-bellied muscle. Finally, we assessed postoperative IO actions to determine whether the presence of a double-bellied muscle influenced the effectiveness of IO weakening surgery in reducing overaction. RESULTS: Among 162 patients (247 eyes) who underwent this surgery, 77 (77 eyes) had unilateral surgery and 85 (170 eyes) bilateral. Twenty-seven (10.9%) of the 247 muscles had double bellies. Among all variables compared, only the incidence of fundus excyclotropia differed significantly between groups, occurring more often in eyes with double-bellied IO muscles (48% vs 27%; P =.041). The efficacy of weakening surgery in reducing overactions was similar in both groups. CONCLUSION: The finding that eyes with double-bellied IO muscles showed a higher incidence of fundus excyclotropia suggests that the presence of a second belly may alter the physiologic action of the IO muscle.

Outcome of inferior oblique recession with or without vertical rectus recession for unilateral superior oblique paresis.

PURPOSE: To determine the effectiveness of a standard fixed 10 mm inferior oblique (IO) recession with or without vertical rectus recession in visually mature patients with unilateral superior oblique paresis (SOP) and mild to moderate IO overaction. METHODS: The records of 24 patients over 12 years of age who had 10 mm IO recession for SOP, for IO overaction of +1 to +3 (out of maximum +4), with 6+ months of followup were reviewed. Criteria required for a "successful" outcome included: 1. hyperdeviation of 5delta or less in primary position; 2. elimination of any compensatory abnormal head posture; and 3. elimination of diplopia in the central 30 degrees of the binocular visual field. RESULTS: In 16 cases of IO recession alone, 88% were "successful" and in 8 cases who had in addition either contralateral inferior rectus recession or ipsilateral superior rectus recession, 75% were "successful". IO 10 mm recession alone led to an average reduction of 9.1 PD of hypertropia in primary position. CONCLUSION: A standard ungraded 10 mm recession of the IO alone or in combination with vertical rectus muscle recession is an effective weakening procedure with a high success rate for patients with unilateral SOP with mild to moderate IO overaction. In occasional cases of undercorrection, a subsequent IO myectomy is very feasible and effective.

A prospective evaluation of anterior transposition of the inferior oblique muscle, with and without resection, in the treatment of dissociated vertical deviation.

BACKGROUND: Anterior transposition of the inferior oblique muscle (ATIO) has become a popular surgical treatment for dissociated vertical deviation (DVD), particularly in patients with coexisting inferior oblique muscle overaction (IOOA). We wanted to assess whether adding a resection improves the outcome compared with standard anteriorization. METHODS: We undertook a prospective, randomized evaluation of ATIO, with and without a 7-mm resection, in patients with DVD of at least 5 PD in one eye. We included 51 eyes of 30 patients, 26 eyes treated with the standard ATIO and 25 treated with a 7-mm resection added. We recorded the size of the preoperative and final DVD, grade of the preoperative and final IOOA, rates of reoperation, and complications. Mean follow-up was 15.4 months in the standard group and 25.0 months in the resection group, with a minimum of 4 months for all cases. RESULTS: The median preoperative and postoperative DVD was 12 PD and 4 PD in the standard group, respectively. This compared with 14 PD and 4 PD, respectively, in the resection group, representing no statistically significant difference in outcome. The presence or absence of IOOA did not influence the result of ATIO for either group. No significant complications of surgery occurred in either group. CONCLUSIONS: ATIO is an effective treatment for DVD and can be used to treat DVD in patients with or without IOOA, with few adverse effects. Our study revealed no advantage to adding a 7-mm resection to the standard procedure.

Strabismus examination by telemedicine.

OBJECTIVE: To evaluate the reliability of strabismus assessment using telemedicine (TM) technology. DESIGN: Two prospective interobserver agreement studies. One study compared the agreement between a standard and a TM examination, whereas the other assessed agreement between two independent standard examinations. PARTICIPANTS: Strabismus patients over 4 years of age examined at a remote community clinic and patients assessed in a strabismologist's urban practice. METHODS: Forty-two patients were examined in person by a pediatric ophthalmologist at the remote community and independently by a pediatric ophthalmology fellow by means of TM (TM-standard study). The TM examination was performed with the help of a qualified ophthalmic assistant at the remote telecommunication center using a Power Cam 100 camera, a Picture Tel Concorde 4500 teleconferencing system, and a 224 kilobyte bandwidth. For comparison, independent in person examinations were performed on 43 patients by both examiners (standard-standard study). Agreement was measured using unweighted kappa (k) for categorical data, the intraclass correlation coefficient (ICC) for continuous data, and percent agreement. The odds of disagreement with TM (comparing the TM-standard versus standard-standard studies) was assessed with logistic regression analysis. MAIN OUTCOME MEASURES: Three parameters were assessed: (1) category of strabismus, determined by observation without cover test; (2) angle of deviation at 0.33 and 6.0 m; and (3) ocular muscle action. RESULTS: Agreement on the category of strabismus was good (k > 0.61) other than for vertical deviations. However, there was good to excellent agreement between TM and standard examinations on the vertical (ICC = 0.78) and horizontal (ICC = 0.79) angles of deviation with 6-m fixation with the cover test. Muscle ratings agreed within one point for the lateral, superior, and inferior rectus muscle actions in more than 90% of the eyes examined. Although good agreement was observed in the TM-standard study, it was inferior to the agreement in the standard-standard study. Examination by TM increased the odds of disagreement compared with examination in person by twofold to threefold. CONCLUSIONS: Strabismus examination can be performed with a good level of reliability with the use of medium bandwidth video teleconferencing equipment. However, reduced reliability has been noted in the detection of small vertical deviations by inspection and in evaluating oblique muscle actions.

Bilateral idiopathic Brown's syndrome with delayed onset in the second eye.

PURPOSE: We describe 6 cases of a previously unreported variation of bilateral Brown's syndrome that presented in congenital form in one eye and developed later in the fellow eye with no underlying cause. METHODS: We reviewed the clinical records of 6 patients from 6 separate practices to determine whether there were any common clinical features on presentation or in their clinical courses. RESULTS: All 6 patients were diagnosed with unilateral congenital Brown's syndrome at the first ophthalmologic assessment but showed no evidence of the syndrome in the fellow eye. In 5 cases the contralateral syndrome developed in the second eye after surgery was performed on the first eye, and in 1 case it developed before any surgery was done. The ages at onset of the syndrome in the second eye ranged from 2 to 8 years. None of the children had any evidence of systemic illness or local orbital disease to explain an acquired Brown's syndrome. CONCLUSION: To our knowledge, this is the first reported series of cases of bilateral Brown's syndrome that manifested sequentially in the eyes with no known causes for an acquired syndrome in the second eye. This finding supports the premise that congenital and acquired Brown's syndrome are on a continuum with a common pathophysiology of restriction of free movement of the superior oblique tendon in the trochlea.

Anatomic variations of the inferior oblique muscle: a potential cause of failed inferior oblique weakening surgery.

PURPOSE: To document the variations in normal anatomy that occur at the insertion of the inferior oblique muscle and in the vicinity of its surgical capture site (10 to 12 mm from the insertion). METHODS: One hundred intact cadaver orbits with no history of eye muscle or orbital disorders during life were carefully dissected to expose the entire length of the inferior oblique muscle. The number of divisions of muscle at the insertion, total width of the muscle belly, and variations in anatomy 10 and 12 mm from the insertion were recorded. RESULTS: Seventeen (17%) of the 100 inferior oblique muscles had multiple divisions at the insertion. Eight muscles (8%) had two bellies at 10 or 12 mm from the insertion. Among these eight, four had two distinct (bifid) bellies extending to the insertion, and four had dehiscences in the muscle. The mean muscle width among these eight specimens was 0.5 and 0.7 mm larger than the mean width of the other 92 specimens at the 10 mm and 12 mm positions, respectively. Neither difference was significant at the .05 level. CONCLUSIONS: Multiple insertions were found in 17% of inferior oblique muscles examined; duplications of the inferior oblique muscle at the surgical capture site were found in 8%. These duplications may account for some cases of recurrence or persistence of inferior oblique overaction after weakening surgery, owing to inadvertent incomplete capture of the muscle during surgery.

Monocular horizontal OKN in observers with early- and late-onset strabismus.

Several reports on monocular optokinetic nystagmus (OKN) in observers with strabismus have found that asymmetry of OKN tends to occur in both eyes of observers with an early onset of strabismus but only in the deviating eye of those with a later onset of strabismus. Our objective was to quantify and compare the magnitude of the OKN asymmetry in each eye as a function of observer's age at onset of strabismus. We studied monocular OKN in ten observers with early-onset (up to 24 months of age), seven observers with late-onset (after 24 months of age) unilateral strabismus, and 12 normally sighted control observers. In the deviating eye, observers with early-onset strabismus showed large OKN asymmetries in favour of nasalward motion while observers with late-onset strabismus showed smaller OKN asymmetries in that eye. The majority of early- and late-onset observers showed near normal OKN in the non-deviating eye although the early-onset observers showed bilateral asymmetries more often. These findings may be due to both age at onset of strabismus and chronological age and are discussed in terms of the issue of plasticity or recovery of function.

Electrophysiological evidence of cortical fusion in children with early-onset esotropia.

PURPOSE: To investigate sensory fusion responses in infants and children with early-onset esotropia to gain insights into the sequence of events that leads to strabismus. METHODS: Sensory fusion was tested by measuring visual evoked potential (VEP) responses to dynamic random dot correlograms (DRDCs) in a group of children (n = 23) with early-onset esotropia. Thirteen children were tested before surgical alignment, and 13 children were tested after surgical alignment (three children were tested before and after surgery). If the angle of strabismus was larger than 5 prism diopters, it was corrected with Fresnel prisms (Fresnel Prism and Lens, Scottsdale, AZ). RESULTS: Five (38%) of the 13 children who were tested before surgery showed detectable VEP responses to correlogram stimuli compared with 11 (85%) of the 13 children who were tested after surgical alignment. There were no significant statistical differences between VEP responses to DRDCs from the postsurgery group and VEP responses from an age-matched control group with normal binocular vision. CONCLUSIONS: The presence of cortical sensory fusion in children with early-onset esotropia suggests that a congenital defect of sensory fusion cannot be the root cause of esotropia in most children. The data suggest that sensory fusion, when measured by VEP responses to DRDCs, is more robust than stereopsis to abnormal binocular experience and support the notion that pathways processing correlated/anticorrelated stimuli may not completely overlap with pathways processing disparity information.

Outcome criteria in strabismus surgery.

Criteria for success in strabismus surgery should include more than just the alignment result in primary position. It is important to consider as well the extent of the field of BSV gained: restoring a useful range of BSV will maximize the patient's ability to carry on daily activities. Psychometric tests can also be used to provide objective measures of improvement following strabismus surgery. Irrespective of the patient's age, the realigning of an eye is reconstructive, not cosmetic, surgery. Surgery in children and adults can restore fusion and normalize the field of BSV in most cases. These gains are possible even in the presence of amblyopia. It could be argued that realigning a blind eye yields no functional improvement and should not necessarily be covered by insurance plans. These cases are not common. In all other cases the functional benefits of adult strabismus surgery justify maintaining insurance coverage for this surgery in jurisdictions throughout North America.

Cortical binocularity and monocular optokinetic asymmetry in early-onset esotropia.

PURPOSE: To investigate the correlation between directional asymmetry in ocular responses to monocularly viewed optokinetic stimuli (monocular optokinetic nystagmus, MOKN) and sensory fusion in infants and toddlers with early-onset esotropia. METHODS: Subjects were 14 infants and toddlers with early-onset esotropia (7-26 months old; median, 10 months), and 16 with no esotropia (6-22 months; median, 11 months) who provided control data. Monocular optokinetic nystagmus in response to a 30 degrees/sec square-wave grating (0.25 cycles/degree) was measured by electro-oculogram. Sensory fusion was assessed with visual evoked potentials (VEPs) to random-dot correlograms after correction of the strabismus angle with Fresnel prisms. RESULTS: All subjects with early-onset esotropia had MOKN with a faster slow-phase component for temporal-to-nasalward (TN) than nasal-to-temporalward (NT) motion. Ninety-three percent of subjects had MOKN asymmetry higher than the 95th percentile of the control group. Of subjects who cooperated with VEP fusion testing, 5 subjects with early-onset esotropia (45%) and 11 control subjects (92%) showed evidence of sensory fusion. CONCLUSIONS: Symmetrical MOKN did not develop in infants and toddlers with early-onset esotropia. This deficit existed in most infants who showed sensory- cortical fusion. These results are consistent with the belief that optokinetic nystagmus asymmetry may not be associated with a deficit in the cortical fusion facility, but rather with deficits in binocular pathways projecting to MOKN control centers. These deficits may be associated with abnormal processing subsequent to sensory fusion or with abnormal processing in motion pathways, which run parallel to sensory fusion pathways.

Registered eye position: short- and long-term effects of botulinum toxin injected into eye muscle.

Botulinum toxin is sometimes injected into human eye muscles as an alternative to surgery in the correction of strabismus. Within minutes of botulinum injections into ungulate eye muscles, proprioceptive discharge from muscle spindles decreases dramatically. It is only over 7-48 h, however, that surgically treated strabismus patients usually show an altered proprioceptive signal about eye position, presumably from the palisade endings attached to the global multiply innervated fibers. How quickly will botulinum toxin alter proprioceptive registration of eye position in humans? First, to examine the short-term effects, we measured open-loop pointing responses (which requires knowledge of eye position) in six strabismus patients preinjection and then over a 45 min postinjection period, and in six normal controls over the same time period. Second, to examine the long-term effects, 13 strabismus patients were tested preinjection and then daily over the next 3 weeks, and three normal controls over the same time period. We compared their open-loop pointing responses with the injected eye fixating the target to the photographically determined position of the occluded other eye (a measure of where the patient would point if eye position were determined by efference, not proprioception). There were three groups of patients: esotropes with no previous injection, exotropes with no previous injection, and exotropes with previous injection. First, all patients showed significant correction of their tropias. Second, over the short-term, there was no difference in pointing responses found between the patients and the controls (t(18) = -1.427, P = 0.1706). Third, over the long-term, however, the difference between the pointing responses and eye position information was compared among the four groups across four posttests and a significant difference found (F3,12 = 58.988, P < 0.00001). Only in patients with no previous injections was there altered proprioceptive feedback about eye position. Also, injections into the medial rectus induced a significantly greater proprioceptive response than those injected into the lateral rectus. In humans, botulinum toxin alters proprioception from eye muscles only over the long-term. We suggest that the toxin temporarily affects proprioceptive feedback from palisade endings.

Screening for childhood strabismus by primary care physicians.

OBJECTIVE: To review the clinical classification of strabismus, to describe the timing and method of strabismus screening examinations, and to discuss the principles of treatment. QUALITY OF EVIDENCE: Current literature (1983 to 1995) was searched via MEDLINE using the MeSH headings strabismus, ocular motility disorders, and amblyopia. Articles were selected based on their date of publication, clinical relevance, and availability. Preference was given to more recent articles, articles with large numbers of subjects, and well-designed cohort studies. Official recommendations from academic groups were analyzed. Descriptions of clinical tests and their illustrations are based on classic texts. MAIN FINDINGS: Primary care physicians should screen all low-risk children. High-risk children (low birth weight, family history of strabismus, congenital ocular abnormality, or systemic conditions with vision-threatening ocular manifestations) should be referred to an ophthalmologist for screening. Screening should be performed in the neonatal period, at 6 months, and at 3 years (Grade A recommendation), as well as at 5 to 6 years (Grade B recommendation). Screening examination includes inspection, examining visual acuity, determining pupillary reactions, checking ocular alignment, testing eye movements, and ophthalmoscopy. CONCLUSIONS: Primary care physicians are essential to early detection of strabismus and amblyopia. Early detection can help minimize visual dysfunction, allow for normal development of binocular vision and depth perception, and prevent psychosocial dysfunction.

Vertical latent nystagmus component and vertical saccadic asymmetries in subjects with dissociated vertical deviation.

PURPOSE: This study was conducted to quantify the vertical component of a latent nystagmus observed in subjects with dissociated vertical deviation (DVD), as well as to provide further evidence for vertical saccadic asymmetries in these individuals. METHODS: Binocular eye movements of subjects with DVD were recorded in two dimensions using a noninvasive video-based eye tracker while cover/uncover tests, alternate cover tests, and vertical saccades were performed. RESULTS: A small amplitude (1.5 degrees or less) vertical component of latent nystagmus can be observed in some subjects with DVD and is larger in the deviating eye than in the viewing eye. The frequency of the vertical nystagmus component is the same in each eye for any given fixation condition but may change depending on which eye is fixating. DVD in the presence of a vertical component of latent nystagmus can be adequately modeled by the algebraic sum of an exponentially decreasing velocity DVD and a nystagmus with an exponentially decreasing slow phase velocity. In general, the occluded eyes of DVD subjects make smaller downward saccades than the viewing eyes. CONCLUSIONS: It is possible but not obligatory that DVD subjects will have a vertical component of latent nystagmus. Algebraic summation of an exponentially decreasing velocity DVD and a vertical component of latent nystagmus provides a more parsimonious explanation of the observed saccadic eye movements than modeling the DVD itself as a combination of vergence and saccadic movements. Subjects with DVD show a range of saccadic yoking from nearly complete saccadic conjugacy to nearly complete dissociation.

Postoperative drifts after adjustable-suture strabismus surgery.

OBJECTIVE: To determine the drift patterns and success rates of adjustable-suture horizontal strabismus surgery when patients are aligned to predetermined target angles. DESIGN: Case series. SETTING: University-affiliated tertiary care hospital in Toronto; clinic and office care. PATIENTS: A total of 109 patients aged 15 to 72 years who underwent adjustablesuture strabismus surgery (primary procedure or reoperation) for esotropia or exotropia performed by one surgeon between 1990 and 1994 who were followed for at least 6 months. Their final postoperative angles were in the target ranges of under 4 prism dioptres (PD) for esotropia surgery and 3 PD to 7 PD of esotropia for exotropia surgery. OUTCOME MEASURES: Primary-position alignment before surgery, immediately after surgery or after adjustment of sutures, if needed (final alignment), and at 1 to 2 weeks, 6 to 8 weeks and 6 to 8 months after surgery; size and direction of drift from final alignment after surgery at 6 to 8 months; and rate at 6 to 8 months of reduction of strabismus angle to less than 10 PD. RESULTS: Of the 109 patients 44 had esotropia (17 primary repair [group 1] and 27 reoperation [group 2]) and 65 had exotropia (29 primary repair [group 3] and 36 reoperation [group 4]). Overall, 48 patients (44.0%) required postoperative suture adjustment. The mean drifts from the final postoperative alignments were 1 PD for groups 1 and 2, 7 PD for group 3 and 3 PD for group 4, all in the exotropic direction. The corresponding surgical success rates were 82.4%, 92.6%, 93.1% and 94.4%. Over 6 to 8 months both esotropia groups had approximately equal tendencies to drift in either direction. Both exotropia groups showed almost equal tendencies to drift in either direction away from the mean exotropic shift for the group. CONCLUSIONS: Because of our target alignments, our success rates at 6 to 8 months were high, for both primary surgery and reoperation. In all four patient groups the drift tendencies were symmetric about the mean.

Aplasia of the inferior rectus muscle. A case report and review of the literature.

PURPOSE: The authors describe the clinical findings, results of preoperative radiologic studies, and results of surgery in a boy with congenital unilateral inferior rectus aplasia. METHODS: A 2-year-old boy is described who presented with a significant anomalous head posture, a large incomitant vertical deviation with marked deficit of depression of the left eye, and positive head-tilt test results. Preoperative magnetic resonance imaging (MRI) defined an absent inferior rectus muscle; computed tomography showed no evidence of a craniofacial syndrome. Full tendon inferior transposition of the horizontal rectus muscles was carried out on the affected eye. RESULTS: The compensatory head position was eliminated. The infraduction improved. The patient had a small left hypotropia after the healing phase. CONCLUSIONS: To the authors' knowledge, this case is the first that used MRI to confirm preoperatively the isolated absence of an inferior rectus muscle. The MRI allowed effective surgical planning to correct the congenital abnormality.

Dissociated vertical deviation: head and body orientation affect the amplitude and velocity of the vertical drift.

PURPOSE: Five subjects with dissociated vertical deviation (DVD) were studied to determine if the amplitude or velocity of the vertical components of the DVD were affected by head/body orientation with respect to gravity. METHODS: Deviations were measured in head upright, head supine, and supine positions, with head hanging postures using a binocular CCD video-based infrared eye tracker. Subjects were required to fixate a target presented in the primary position during alternate or cover/uncover tests. RESULTS: Amplitude and velocity of DVD both in onset and recovery were affected by head/body orientation with respect to gravity. In four of five subjects, the amplitude of the DVD was asymmetric between the two eyes when the head was upright. When the head/body was moved from an upright to a supine with head hanging backward condition, the amplitude of the DVD in the two eyes inverted. The eye with the larger DVD in the upright position had a smaller DVD in the head-hanging orientation. A similar relationship existed between velocity and head/body orientation. We found that DVD velocity increased with amplitude. CONCLUSIONS: Passive effects of gravity on the eye-inorbit do not influence DVD magnitude or frequency of occurrence. The data suggest, however, that otolithic and possibly neck afferent inputs play a role in DVD magnitude and may be a part of the etiology of the condition.