Clinical characteristics and surgical outcomes in patients with superior oblique muscle palsy: a retrospective study on 1057 patients.

BACKGROUND: To evaluate the clinical findings of patients with SOP who underwent surgery. METHODS: This historical cohort study was performed on 1057 SOP patients managed with surgery in Farabi Hospital, Iran, from 2011 to 2022. RESULTS: There were 990 (93.7%) patients with unilateral SOP with the mean age of 21.8 ± 14.8 years. Of these, 715 patients (72.2%) were diagnosed with congenital SOP, and 275 patients (27.8%) had acquired SOP (P < 0.001). In contrast, 67 (6.3%) patients were diagnosed with bilateral SOP, with the mean age of 19.4 ± 15.6 years. Among these, 18 cases exhibited the masked type. The mean angle of vertical deviation in primary position at far in unilateral and bilateral cases was 15.6 ± 8.3 and 13.3 ± 9.1 △, respectively (P < 0.001). In unilateral cases, abnormal head posture (AHP) was detected in 847 (85.5%) patients and 12 (1.2%) had paradoxical AHP. Amblyopia was found in 89 (9.9%) unilateral and 7 (10.3%) bilateral cases. Solitary inferior oblique myectomy, was the most common surgery in both unilateral (n = 756, 77.1%) and bilateral (n = 35, 52.2%) patients. The second surgery was performed for 84 (8.6%) unilateral and 33 (49.3%) bilateral cases (P < 0.001). The prevalence of amblyopia and the mean angle of horizontal deviation were significantly higher in patients who needed more than one surgery (all P < 0.05). CONCLUSION: Congenital SOP was more than twice as frequent as acquired SOP and about 90% of unilateral and 50% of bilateral cases were managed with one surgery. Amblyopia and significant horizontal deviation were the most important factors for reoperation. TRIAL REGISTRATION: The Institutional Review Board approval was obtained from the Tehran University of Medical Sciences (IR.TUMS.FNM.REC.1400.012) and this study adhered to the tenets of the Declaration of Helsinki and HIPAA.

Postoperative outcomes for unilateral congenital trochlear nerve palsy: A retrospective cohort study.

PURPOSE: Congenital trochlear nerve palsy is the most common cause of vertical strabismus. The goal of this study was to investigate surgical outcomes after superior oblique tendon plication with or without inferior oblique recession in children and adults with unilateral congenital trochlear nerve palsy. METHODS: Data and outcomes were collected in patients with a diagnosis of unilateral congenital superior oblique palsy during a retrospective single-center study conducted at the University Hospital of Tours. A reproducible, standard ophthalmological and oculomotor examination was performed pre- and postoperatively at 1 year, including presence or absence of diplopia, vertical and horizontal deviations, and compensatory head posture. Surgical success, defined as an endpoint including absence of diplopia in primary position, absence of head tilt, and vertical deviation at distance fixation<5 prism diopters (PD), was analyzed. RESULTS: A total of fifty-seven patients (median [IQR] age of 11 years [5-42]) were analyzed. Patients experienced a significant reduction in vertical distance and near deviations (p<0.001), compensatory head tilt (p < 0.001), and diplopia after surgery (p < 0.001). Surgical success was higher in adults (17/24, 70.8%) than in children (15/33, 45.5%), although this did not reach statistical significance (p=0.0657). CONCLUSION: This study suggests that plication of the superior oblique muscle tendon, with or without recession of the inferior oblique muscle, can be effective in treating unilateral congenital trochlear nerve palsy. Further studies are necessary to compare surgical procedures and investigate their efficacy in adults compared to children in the short and long term.

Effect of inferior oblique myectomy on ocular torsion according to the absence of the trochlear nerve in unilateral congenital superior oblique palsy.

OBJECTIVES: To evaluate the effect of inferior oblique (IO) myectomy on ocular torsion according to the absence of the trochlear nerve in unilateral congenital superior oblique palsy (UCSOP). METHODS: We retrospectively reviewed the clinical data of patients who had been diagnosed with UCSOP and underwent ipsilateral IO myectomy (n = 43). Patients were classified into the present and absent groups according to the absence of the trochlear nerve and superior oblique hypoplasia on magnetic resonance imaging (MRI). For quantitative analysis of ocular torsion, disc-fovea angles (DFA) were collected in both eyes using fundus photographs taken within three months before surgery and one month after surgery. RESULTS: DFA of the paretic eye did not differ according to the absence of the trochlear nerve (9.4±5.6° in the present group vs. 11.0±5.4° in the absent group, p = 0.508). However, the present group had a larger DFA in the non-paretic eye than the absent group (14.1±6.7° in the present group vs. 8.0±5.0° in the absent group, p = 0.003). The change of ocular torsion after IO myectomy in the paretic eye was -5.3±3.7° in the present group and -4.8±3.5° in the absent group, respectively (p = 0.801). In the non-paretic eye, the change in DFA was -1.5±3.0° in the present group, which was larger than that in the absent group (0.7±2.6°, p = 0.047). In the multivariate analysis, the change in DFA was correlated with only the preoperative DFA (standardized ß = -0.617, p<0.001 in the paretic eye, and standardized ß = -0.517, p<0.001 in the non-paretic eye). CONCLUSIONS: In the paretic eye, there was no significant difference in the change of ocular torsion between both groups, whereas in the non-paretic eye, the present group had a larger change in DFA after IO myectomy than the absent group. However, in the multivariable analysis, the change in ocular torsion was significantly correlated with preoperative excyclotorsion but not with the presence of the trochlear nerve itself.

Comparison of ocular torsion between congenital and acquired unilateral superior oblique palsy.

BACKGROUND/OBJECTIVES: To compare ocular torsion between congenital and acquired unilateral superior oblique palsy (USOP). SUBJECTS/METHODS: Retrospective review was performed on medical records of 163 USOP patients. Qualitative fundus torsional states in both eyes were determined based on locations of optic disc and fovea. Disc-fovea angles (DFA) were collected in both eyes for quantitative analysis. DFAs and the proportions of qualitative fundus torsional states in paretic and non-paretic eyes were compared between USOP patients and normal controls, and between congenital and acquired USOP patients. RESULTS: This study included 90 patients with congenital USOP, 73 patients with acquired USOP, and 66 normal controls. Most control subjects showed no torsion in both eyes (93.9%), whereas 61.1% of congenital and 46.5% of acquired USOP patients showed extorsion in either eye. More patients with congenital USOP showed fundus extorsion in the non-paretic eye (24.4% versus 12.3%) or both eyes (20.0% versus 6.8%), compared with patients with acquired USOP (P = 0.007). DFAs of paretic and non-paretic eyes were larger in USOP patients than in normal controls (P < 0.001, for both congenital USOP versus control and acquired USOP versus control). DFAs of non-paretic eyes were larger in congenital USOP patients than in acquired USOP patients (10.3° versus 8.5°, P = 0.018). CONCLUSIONS: Congenital USOP showed greater fundus extorsion in the non-paretic eye, compared with acquired USOP. Fundus photographs of both eyes are necessary to understand the ocular torsion in USOP patients and the variations in fundus torsion with varying USOP aetiology.

Trochlear-oculomotor synkinesis: another congenital cranial dysinnervation disorder? / Sincinesia troclear-oculomotora: outro distúrbio congênito de denervação craniana?

ABSTRACT This report documents an unusual phenomenon. A 6-year-old girl with trochlear-oculomotor synkinesis presented with superior oblique and palpebral levator co-contraction. The literature was reviewed and the possibility of classifying this entity as a congenital cranial dysinnervation disorder was speculated.

RESUMO Este relato descreve um fenômeno incomum. Uma menina de 6 anos com sincinesia troclear-oculomotora apresentou co-contração do oblíquo superior e do levantador da pálpebra. A literatura foi revisada e especulou-se a possibilidade de classificar essa desordem como um distúrbio da congenital cranial dysinnervation disorder.

Trochlear-oculomotor synkinesis: another congenital cranial dysinnervation disorder?

This report documents an unusual phenomenon. A 6-year-old girl with trochlear-oculomotor synkinesis presented with superior oblique and palpebral levator co-contraction. The literature was reviewed and the possibility of classifying this entity as a congenital cranial dysinnervation disorder was speculated.

Predictive factors for corrective effect of inferior rectus recession for congenital superior oblique palsy.

PURPOSE: To identify preoperative factors associated with the surgical corrective effect of contralateral inferior rectus recession (IRR) for vertical deviation in patients with congenital superior oblique palsy (SOP). METHODS: This retrospective study included 20 treatment-naïve patients with unilateral congenital SOP (age range, 6-79 years) who underwent contralateral IRR according to our basic policy to select IRR for paretic eye fixation. The corrective effect (°/mm) of IRR was defined as the difference in the vertical deviation at the primary gaze position between before and 6-18 months after surgery per distance of recession. We also measured the preoperative vertical deviation at primary and secondary gaze positions, and vertical deviation with head-tilting, and calculated the difference in vertical deviation between these positions. We analyzed the correlation between the corrective effect of IRR and these study parameters. RESULTS: The mean corrective effect of IRR was 2.4 ± 1.6°/mm, which had a significant correlation with preoperative differences in vertical deviation between the primary gaze position and the downward (P = 0.004, r = -0.61) and contralateral gaze positions (P = 0.03, r = -0.48); and the presence of preoperative stereopsis (P = 0.02, r = -0.51). After excluding a statistical outlier, the correlation between the corrective effect and the difference between the primary and contralateral gaze positions was no longer significant (P = 0.07), while the other two relationships remained significant. CONCLUSIONS: Our findings suggest that preoperative differences in vertical deviation between the primary and downward gaze positions and the presence of preoperative stereopsis are important considerations prior to performing IRR for congenital SOP, particularly with paretic eye fixation.

Ocular Torsion According to Trochlear Nerve Absence in Unilateral Superior Oblique Palsy.

Purpose: To investigate the relationship between objective ocular torsion and the presence or absence of the trochlear nerve in subjects with unilateral superior oblique palsy (SOP). Methods: A total of 159 subjects with congenital and acquired unilateral SOP were reviewed. Eighty-four subjects who had a normal trochlear nerve (present group) and 75 subjects without a trochlear nerve (absent group) were included. Cyclovertical motility parameters and objective ocular torsion were compared between groups, and factors related to ocular torsion were evaluated. Results: The degree of "net" excyclotorsion in the paretic eye was larger in the absent group compared to the present group (P = 0.002). The proportion of net excyclotorsion in the paretic eye was greater in the absent group (11% vs. 37%), while net incyclotorsion was greater in the present group (41% vs. 23%) (P < 0.001). Net excyclotorsion of the paretic eye was associated with absence of the trochlear nerve (P < 0.001) and smaller size of the paretic SO (P < 0.001). Net incyclotorsion of the paretic eye was related with a normal trochlear nerve (P = 0.005), larger size of the paretic SO (P = 0.002), and greater hypertropia during ipsilateral gaze (P = 0.024). Conclusions: The status of the trochlear nerve, paretic SO size, and hypertropia during ipsilateral gaze which reflects the tensile strength of the ipsilateral superior rectus, significantly contribute to ocular torsion in unilateral SOP.

Surgical treatment of superior oblique palsy: Predictors of outcome.

PURPOSE: The purpose of this study was to evaluate the incidence and outcome of surgically treated superior oblique palsy (SOP) and the factors involved in its resolution. METHODS: We performed a retrospective study of 76 patients who underwent surgery for SOP. We recorded data from the physical examination and the number and type of procedures performed. Favorable outcome was defined as resolution of or improvement in torticollis (≤5°) and diplopia in primary position (PP) and downgaze or as vertical deviation (VD) <5 prism diopters (pd) in PP and 10 pd in the oblique diagnostic position. RESULTS: Mean age was 33.12 years. Congenital SOP was the most frequent type (65.8%). Mean preoperative VD was 15.89 ± 9.94 pd, decreasing to 3.07 ± 4.36 pd after surgery. Associated horizontal deviation was recorded in 51.32% of cases. The mean number of procedures was 1.37 ± 0.62 (range 1-4), with 69.7% of patients requiring only one procedure. The mean number of muscles operated on was 1.96 ± 1.01 (inferior oblique being the most frequent). A greater reduction in VD after surgery was observed in patients with congenital SOP (P = 0.04). Although none of the factors evaluated influenced surgical outcome, amblyopic patients had a greater risk of reoperation (P = 0.04). A favorable outcome was achieved in 75% of cases. Mean follow-up was 37.08 months. CONCLUSION: Congenital SOP was twice as frequent as acquired SOP and although surgery was successful in most cases, a greater reduction in VD was obtained in congenital cases. Amblyopia was identified as a risk factor for reoperation.

The influence of ocular sighting dominance on Fundus torsion in patients with unilateral congenital superior oblique palsy.

PURPOSE: The purpose of our study was to determine whether ocular sighting dominance may influence the ocular torsion in patients with unilateral congenital superior oblique palsy (UCSOP). METHODS: This retrospective study included 22 UCSOP patients with radiologic evidence of unilateral superior oblique muscle hypoplasia on orbital magnetic resonance imaging and 66 healthy individuals with normal ocular motility as controls. Ocular torsion was assessed both quantitatively and qualitatively using digital fundus photography. The disc-fovea angle (DFA) was measured quantitatively using image software on a computer screen. All fundus photographs were qualitatively graded as normal torsion, extorsion, or intorsion in all subjects, based on the location of the optic disc relative to the fovea, according to the Bixenman and von Noorden's criteria. Ocular sighting dominance was assessed by the hole-in-the-card test and the pointing test. The Mann-Whitney U test and Fisher's exact test were used to determine the association between the ocular sighting dominance and the ocular torsion. RESULTS: The median DFA was significantly larger in the eyes of patients with UCSOP (9.1° in the paretic eyes and 9.3° in the non-paretic eyes) than the eyes of the control group (4.3°, p < 0.001 for both). Ocular dominance tests displayed that, among 22 patients, 11 were paretic eye dominant and the other 11 were non-paretic eye dominant. The sighting-dominant eyes demonstrated significantly smaller median DFA than the non-dominant eyes (8.3° and 10.7°, respectively, p = 0.033), regardless of which eyes were paretic. Ten eyes of ten patients had extorsion, none had intorsion, and all the eyes of remaining 12 patients had no abnormal torsion, qualitatively. All the eyes showing extorsion in fundus photography were non-dominant eyes, regardless of whether the eyes were paretic or non-paretic. CONCLUSIONS: Our findings illuminate the importance of considering ocular sighting dominance for properly assessing ocular torsion in patients with UCSOP. Ocular sighting dominance may have an influence on objective ocular torsion in a way that decreases the torsion in the dominant eye, thereby hindering the abnormal ocular torsion from appearing in that eye.

Anterior and nasal transposition of inferior oblique muscle in cases of superior oblique palsy.

PURPOSE: To report long-term outcome of inferior oblique anterior and nasal transposition in superior oblique palsy. METHODS: The medical records of patients with superior oblique palsy who underwent inferior oblique anterior nasal transposition were reviewed retrospectively. A comprehensive ophthalmic evaluation, including prism bar cover test and measurement of torsion, was performed for all cases. One-year postoperative results were evaluated for alignment in primary gaze, contralateral gaze, and upgaze; reduction in inferior oblique overaction and changes in fundus torsion to assess long-term outcome of the procedure. RESULTS: A total of 12 patients were included. Three cases also underwent horizontal muscle surgery. Mean age at the time of surgery was 20.6 years. The median preoperative hypertropia was 21.5Δ (range, 12Δ-36Δ), corrected to 4.5Δ (range, 2Δ-10Δ) at 12 months postoperatively (P = 0.002). Median inferior oblique overaction decreased from +3 (range, +1 to +4) to 0 (range, -1 to +1). Preoperative fundus extorsion was 19.2° ± 6.7°; postoperative, 0.58° ± 1.8° (P < 0.001). No consecutive hypotropia or underaction in elevation was observed in 10patients; 1 patient complained of torsional diplopia in upgaze. Extorsion was eliminated and head tilt improved in all patients. CONCLUSIONS: Inferior oblique anterior and nasal transposition resulted in good long-term outcomes in our patients with superior oblique palsy presenting with hypertropia, inferior oblique overaction, and extorsion in primary gaze.

Comparing Hypertropia in Upgaze and Downgaze Distinguishes Congenital From Acquired Fourth Nerve Palsies.

BACKGROUND: Isolated fourth nerve palsies are commonly caused by decompensation of a congenitally dysfunctional superior oblique muscle ("decompensated congenital palsies"). Distinguishing such palsies at initial presentation from palsies caused by presumed microvascular ischemia ("ischemic palsies") has value for patient reassurance and in forestalling ancillary testing. Abnormally large vertical fusional amplitudes traditionally have been used to identify decompensated congenital palsies, but that may not be a reliable distinguishing feature. This study was undertaken to determine if the amount of hypertropia in upgaze and downgaze might be a more efficient separator. We also studied traumatic and tumorous fourth nerve palsies to see if they could be distinguished from decompensated congenital palsies by using this hypertropia comparison. METHODS: Retrospective review of case records of patients diagnosed with isolated fourth nerve palsies at the University of Michigan Neuro-Ophthalmology Clinics over the past 15 years. We recorded the age, gender, vascular risk factors, duration of follow-up, cause, side of palsy, and alignment measurements in all patients. RESULTS: Inclusion criteria were met by 118 patients. Hypertropia was equal or greater in upgaze than downgaze in 50 of the 58 decompensated congenital palsies (86%) in whom those data were recorded. Hypertropia was never greatest in upgaze in the 15 patients with traumatic palsies. Vertical fusional amplitudes were increased in only 15 of 27 patients (56%) with decompensated palsies in whom those data were recorded. Torsional misalignment on double Maddox rod testing was present in 16 (94%), 13 (87%), and 3 (100%) patients with ischemic, traumatic, and tumorous palsies, but also in 19 patients (54%) with decompensated congenital palsies in whom those data were recorded. CONCLUSIONS: Hypertropia greater in upgaze than downgaze or equal in upgaze and downgaze was an efficient separator of congenital from ischemic and tumorous fourth nerve palsies, being characteristic of patients with decompensated congenital palsies and never present in patients with ischemic, traumatic, or tumorous palsies. Vertical fusional amplitudes and torsional misalignment did not effectively differentiate between the patient groups. Comparing the hypertropia in upgaze and downgaze improved differential diagnosis and reduces the potential for unnecessary ancillary tests.

Evaluation of Surgical Strategy Based on the Intraoperative Superior Oblique Tendon Traction Test.

PURPOSE: To clarify the efficacy of a surgical strategy based on the superior oblique tendon traction test. METHODS: A retrospective chart review was performed between January 2002 and June 2015. During that period, a single inferior oblique muscle (IO) myectomy and a combined IO myectomy and superior oblique muscle (SO) tuck procedure were performed based on SO tendon looseness as revealed by a traction test. The surgical effects of both procedures and the number of operations were analyzed. RESULTS: Sixty-five cases were retrieved. Seventy-four surgeries were required. The IO myectomy and simultaneous groups included 48 and 17 cases, respectively. Pre-operative vertical deviation was significantly lower in the IO myectomy (11.8 prism diopters) than in the simultaneous (27.2 prism diopters; Mann-Whitney U-test, P < 0.001) group. The mean induced changes were 9.4 prism diopters and 21.6 prism diopters in the IO myectomy and simultaneous groups, respectively, and the postoperative vertical deviation was not significantly different. On average, 1.13 and 1.18 surgeries per patient were performed in the IO myectomy and simultaneous groups, respectively. CONCLUSION: The simultaneous surgery of inferior oblique myectomy and superior oblique tuck is safe and effective for treating large angle of congenital/idiopathic superior oblique palsy with a lax superior oblique tendon, as determined by the traction test.

Imaging demonstration of trochlear nerve agenesis in superior oblique palsy emerging during the later life.

BACKGROUND: Congenital trochlear palsy may manifest with sudden vertical diplopia due to decompensation during the later life, which may bring a diagnostic challenge. CASE PRESENTATION: Two men with vertical diplopia for several years after age of 50 were referred with persisting or suddenly aggravating diplopia. Findings were consistent with unilateral superior oblique palsy (SOP) in both patients with a contraversive head tilt. Facial asymmetry was suggestive of a congenital cause in a patient. High resolution magnetic resonance image (MRI)s disclosed atrophic superior oblique and absent trochlear nerve in the side of SOP in both patients. CONCLUSION: Imaging demonstration of superior oblique atrophy and absent trochlear nerve may aid in diagnosis of congenital SOP presenting sudden vertical diplopia during the later life due to delayed decompensation.

Mechanisms of Vertical Fusional Vergence in Patients With "Congenital Superior Oblique Paresis" Investigated With an Eye-Tracking Haploscope.

PURPOSE: To determine the mechanisms of vertical fusional vergence in patients with "congenital unilateral superior oblique paresis" (SOP) and to discuss the implications of these mechanisms. METHODS: Eleven patients were examined with our eye-tracking haploscope. RESULTS: Three different fusion mechanisms were found, producing significantly different cyclovergence to vertical vergence ratios (P < 0.05): primary use of the vertical rectus muscles in seven patients (ratio: 0.36 ± 1.6), primary use of the oblique muscles in one patient (0.04), and use of the superior oblique muscle in the higher eye and the superior rectus muscle in the lower eye in three patients (1.15 ± 0.32). Lancaster red-green testing showed alignment differences among these groups, primarily differences in amount of subjective extorsion between the two eyes in straight-ahead gaze: The patient with oblique-muscle-mediated fusion showed essentially no subjective extorsion (0.5°), the patients with vertical-rectus-muscle-mediated vertical fusion showed a mean ± SD subjective extorsion of 3.6° ± 1.4°, and the patients with the mixed (oblique/rectus) fusion mechanism showed 7.0° ± 1.7° (P < 0.05). CONCLUSIONS: The choice of fusion mechanism may be a function of how much intorting effect is needed. Use of the oblique muscles bilaterally causes the least intorting effect, use of the vertical rectus muscles bilaterally adds more intorting effect, and activation of the "paretic" superior oblique muscle in the higher eye and the superior rectus muscle in the lower eye provides the greatest intorting effect. Subclassifying "congenital SOP" in this way (in which the "paretic" muscle may remain functional in many cases) may help guide its optimal surgical correction.

Quantitative Intraoperative Torsional Forced Duction Test.

PURPOSE: We developed a method for quantifying intraoperative torsional forced ductions and validated the new test by comparing patients with oblique dysfunction and controls. DESIGN: Comparative case series. SUBJECTS: We studied 33 eyes with oblique dysfunction (9 with presumed congenital superior oblique palsy [SOP], 13 with acquired SOP, 7 with Brown syndrome, and 4 with inverted Brown syndrome) and 31 controls. We also studied 6 eyes after superior oblique (SO) disinsertion and 2 eyes after inferior oblique (IO) disinsertion. METHODS: Under deep general anesthesia, the 12 and 6 o'clock positions at the limbus were marked and the globe was maximally excyclorotated and incyclorotated without retroplacement until the first resistance was felt, and the angle of rotation (in degrees) was read on a Mendez ring by the surgeon. A photograph was taken in each position to be read by a masked observer. MAIN OUTCOME MEASURES: Maximal excyclorotation and maximal incyclorotation in each oblique dysfunction and in controls by both surgeon's report and photographic assessment. We duplicated the photographs to evaluate test-retest reliability and to evaluate agreement between the surgeon's assessments and the photographic assessment. RESULTS: Surgeon's assessment revealed greater maximal excyclorotation in eyes with presumed congenital SOP than in controls (median, 40 vs. 30 degrees). Maximal excyclorotation in eyes with acquired SOP was similar to that in controls (30 degrees in both). Eyes with Brown syndrome and inverted Brown syndrome had lower maximal excyclorotation than in controls (10 and 20 vs. 30 degrees, respectively). Maximal incyclorotation in eyes with inverted Brown syndrome was lower than in controls (12.5 vs. 30 degrees), whereas it was similar to that of controls in eyes with presumed congenital SOP, acquired SOP, and Brown syndrome (30 degrees in each condition). Median maximal excyclorotation after SO disinsertion was 62.5 degrees, and maximal incyclorotation after IO disinsertion was 60 degrees. Photographic assessment yielded findings essentially identical to the surgeon's report. Test-retest reliability of the photographic reading was excellent, and agreement between the surgeon's report and the photographic reading was also excellent (95% limits of agreement, 4.4 and 11.6; intraclass correlation coefficient, 0.97 and 0.82, respectively). CONCLUSIONS: The new torsional forced duction test enables quantitative assessment of SO and IO tightness and laxity.

Morphometry of the trochlear nerve and superior oblique muscle volume in congenital superior oblique palsy.

PURPOSE: To infer the pathogenic mechanism of congenital superior oblique palsy (SOP) by evaluating trochlear nerve diameter (CN4D) and superior oblique muscle (SO) volume in patients with congenital SOP. METHODS: The medical records of 125 patients diagnosed with unilateral congenital SOP and 34 age-matched healthy controls were reviewed retrospectively. Using thin-section high-resolution magnetic resonance imaging, we evaluated the presence of the trochlear nerve, CN4D, SO volume, and their relationship. RESULTS: Of the 125 patients with congenital SOP, 87 showed absence of the trochlear nerve (n = 87, 70%, absent group) and 38 showed bilateral presence of the trochlear nerve (n = 38, 30%, present group). The nonparetic side CN4D was smaller than controls in the absent group (P = 0.001), and larger than controls in the present group (P = 0.001). Trochlear nerve diameter positively correlated with SO volume in controls (P = 0.014, R(2) = 0.174) and in the nonparetic sides of congenital SOP (present group P = 0.023, R(2) = 0.135; absent group, P = 0.008, R(2) = 0.079). The paretic side SO volume did not show a linear correlation between CN4D and SO volume in the present group (P = 0.243). CONCLUSIONS: In congenital SOP patients without a trochlear nerve, the nonparetic side CN4D was also reduced in contrast to those with a trochlear nerve. The relatively weaker relationship of CN4D and nonparetic side SO volume in the absent group than that of the present group suggests different pathogenic mechanisms underlying these two entities of congenital SOP.