Eleven members of pedigrees FY and JH consented to participate. Of these, eight participants had DRS and harbored the DRS-associated allele (subjects 1–4 and 8–11). These eight subjects include five males and three females with an average age of 39 ± 6 (mean ± SE; range 2–76) years. The remaining three participating family members were clinically unaffected. Subject 5 was a 6-year-old boy who harbored the DRS-associated allele and was an obligate carrier of the DURS2 mutation. Subject 7 was a 50-year-old woman who harbored a portion of the DRS-associated allele, and it is unknown whether she carried the DURS2 mutation. Subject 6 was a 23-year-old woman who did not carry the DRS-associated allele but was enrolled in the clinical portion of the study because she traveled to UCLA with her family. All 11 participating family members underwent clinical examinations. General characteristics of these subjects are summarized in .
Characteristics of Study Participants
Six strabismic subjects without DRS of average age 46 ± 8 (SEM) years underwent MRI of the orbits before and after surgery for horizontal strabismus. This control group was included to determine whether strabismus surgery alters EOM volumes significantly. In this group, mean (±SEM) corrected visual acuity was 0.01 ± 0.04 logMAR in the right eye, and −0.01 ± 0.02 logMAR in the left eye. All subjects in this group underwent surgery on at least two horizontal rectus muscles, and one subject also underwent bilateral IO recession.
Detailed orbital MRI was performed in subjects 1 to 4, and 9 to 11, and MRI of cranial nerves in the skull base was performed in subjects 4, 9, 10, and 11. Subject 7 consented to and attempted orbital MRI, but the study was limited due to claustrophobia, and images were inadequate to trace intraorbital innervation. Subjects 5 and 8 were too young to participate in the MRI portion of the study, which has a minimum age for participation of 10 years. Subject 6 did not undergo MRI, because she did not have DRS and did not harbor the disease-associated haplotype.
Thirteen normal volunteers underwent MRI of the cranial nerves in the skull base. Normal control subjects were of average age 22 ± 4 (mean ± SD, range 17–26) years. All control subjects had normal ocular and lid motility and visual acuity in each eye correctable to 0 logarithm of the minimum angle resolvable in arc minutes (logMAR, 20/20) or better.
Clinical Findings in DURS2
Mean corrected letter visual acuity in the group was identical in the left and right eyes of affected subjects (), and averaged 0.04 logMAR (20/20− Snellen). The maximum interocular acuity difference observed was 0.2 logMAR, found in subjects 2 and 4, indicating minimal to no amblyopia in affected subjects. Quantitative acuity could not be obtained in subject 8, age 1 year, who could not identify optotypes.
With the exception of subject 1, who exhibited unilateral motility abnormalities in the right eye only, affected subjects exhibited bilateral, albeit often asymmetrical, manifestations of DRS. Posterior displacement of the globe, termed retraction, was evident on attempted adduction of all affected eyes except for the right eye of subject 4. Although globe retraction was associated with narrowing of the palpebral fissure on attempted adduction, blepharoptosis in central gaze was present only in subject 4, in whom it was bilateral and had required surgical correction before the study. Five of the affected subjects had undergone two or three surgeries each for strabismus correction before the study. Affected subjects 2, 8, and 10 had not undergone prior ocular surgery.
The common clinical classification by Huber of DRS consists of three groups: type 1, with limitation of abduction only; type 2, with limitation of adduction only; and type 3, with limitation of both ab- and adduction.2,25
As noted in , right and 2 left eyes were classified as DRS type 1, and 10 eyes exhibited DRS type 3. Four affected participants had bilateral type 3, three had unilateral type 3 and unilateral type 1, and one was unilaterally affected with type 3. No eye exhibited the limitation of adduction only characteristic of type 2.
As indicated in , three affected subjects exhibited esotropia in central gaze, and the other five exhibited exotropia. The strabismus was unaltered (concomitant) during vertical gaze changes in subjects 5 and 8 only, but varied with vertical gaze in the other subjects. Subjects 1, 2, 4, and 10 had incomitant horizontal strabismus evocative of the letter A or Greek letter λ because the eyes were in a more divergent position in down gaze than in up gaze. In subjects 1 and 10, esotropia was reduced in down gaze, whereas in subjects 2 and 4, exotropia was increased in down gaze. In several of these subjects, this A or λ pattern was highly suggestive of aberrant innervation of the lateral rectus (LR) muscle during infraduction, and LR inhibition during attempted supraduction (). Subject 3 exhibited limited infraduction of the left eye and so could not be evaluated for vertical incomitance of his horizontal strabismus.
Figure 1 A-pattern in subject 4, showing esotropia in attempted upward gaze, and exotropia in central and downward gaze. Note limitation of vertical gaze, and left hypotropia. The upper eyelid configuration was created at surgery for blepharoptosis. There was (more ...)
Orbital Imaging Findings in DURS2-Linked DRS
Despite prior strabismus surgery in several cases, orbital MRI was thought to provide a reasonable reflection of the sizes and positions of the EOM bellies because surgery is largely confined to the region of the insertional tendons. This impression was confirmed in the strabismic control group without DURS2 by demonstration that EOM volumes did not change significantly after conventional surgery for horizontal strabismus (). Mean volumes of each of the four rectus EOMs in the six contiguous image planes including and posterior to the junction of the globe and ON did not significantly differ between control subjects, and subjects with DURS2 (P > 0.05). The volume measurement did not, however, incorporate rectus EOMs in their most apical portions.
Muscle Volumes in Subjects with DURS2-Linked DRS
Structural abnormalities of the LR were common among subjects with DURS2. All had structural abnormalities of the left LR, including posterior hypoplasia in subject 1, vertical splitting in subjects 2 to 4 and 9 and 11, and internal bright signal in subject 10. Structural abnormalities of the right LR were common but not universal. Right LR abnormalities included posterior hypoplasia in subject 1, posterior disorganization in subjects 3 and 4, and internal bright signal in subject 10. The anterior portion of the LR was typically normal, but the deep portion was split into superior and inferior portions (, ), one or both of which were hypoplastic (). It should be noted that LR volume measurements in do not include the deep portion, where structural abnormalities were most severe in pedigree FY. Quasi-sagittal MRI distinguished a lengthy separation between the superior and inferior parts of the LR along its length ().
Figure 2 Quasicoronal MRI planes 2 mm thick of left orbit of subject 4 from pedigree SB, demonstrating a small, dysplastic SO, and dysplasia of the LPS, forming an abnormal slip extending toward the SO. The LR muscle was split into superior and inferior portions. (more ...)
Figure 3 Quasicoronal MRI planes 2 mm thick of the right orbit of subject 4 from pedigree FY, demonstrating marked dysplasia of the LR, with continuity and apparent innervation of the LR by a branch of the inferior division of the oculomotor nerve (N3). The deep (more ...)
Figure 4 Quasisagittal, T1-weighted MRI of subject 4 showing longitudinal splitting of the left LR into superior and inferior portions, with intercalated bright signal consistent with orbital fat: 312-μm resolution in 2-mm image plane. LG, orbital lobe (more ...)
Although structural EOM abnormalities were confined to the LR in pedigree JH, other EOMs were commonly abnormal in pedigree FY. Subjects in pedigree FY exhibited hypoplasia of at least one superior oblique (SO) muscle; this was bilateral in subjects 1 to 3 and in the left eye only of subject 4 (). The left SO of subject 4 also exhibited dysplasia with vertical elongation in the region of an abnormal slip of the dysplastic levator palpebrae superioris (LPS) muscle (). Subjects 1 and 4 of pedigree FY also had unilateral hypoplasia of the superior rectus (SR) muscle (). The normal configuration of the SO and LPS EOMs may be seen for comparison in , illustrating the right orbit of subject 4.
Oblique EOM Size
The size of the IO was not systematically determined because quasisagittal imaging was performed only in three EOMs of two subjects (subjects 2 and 4), in whom mean (± SEM) IO volume was significantly subnormal at 161 ± 2.2 microliters (P < 0.0001). Control IO volume averaged 301 ± 11 mL (n = 55).
For comparability to the published literature, SO size was assessed by maximum cross section in quasicoronal image planes. Averaging over both eyes of 10 normal subjects, mean maximum SO cross section was 18.8 ± 0.7 mm2 (SEM). In DURS2, the mean maximum SO cross section was significantly smaller than normal at 14.2 < 1.9 mm2 (P < 0.025).
Rectus Muscle Paths
Paths of the rectus EOMs were determined from area centroids in multiple contiguous image planes. The EOMs pass through their connective tissue pulleys, so that the anterior locations of these paths indicate the respective pulley locations in the coronal plane.20
Because subjects with DURS2 were typically unable to achieve eccentric gaze positions, no inflections in rectus EOM paths were present to identify the anteroposterior coordinates of the rectus pulleys as is possible in normal subjects. It therefore was assumed that the anteroposterior coordinates of the rectus pulleys are the same as those known for normal subjects.20
This was considered reasonable, since variations in anteroposterior coordinates would on geometric grounds be expected to have only a small effect on horizontal and vertical pulley coordinates in central gaze. After 3-D averaging of the paths of the IR, MR, and LR in subjects with DURS2, the horizontal coordinates were determined at the anteroposterior locations of normal rectus pulleys. This analysis indicated that the 3-D coordinates of all rectus pulleys in DURS2 do not differ significantly from normal.
Imaging of Intraorbital Motor Nerves
Since the posterior orbit is less susceptible to motion artifacts from eye movement than is the anterior orbit, it was possible to examine in the deep orbit the motor nerves to the EOMs in image planes of 1.5- to 2-mm thickness, and field of view 6 to 8 cm. The superior division of CN3 was difficult to follow and was not analyzed systematically. The inferior division of CN3 and its individual branches are normally prominent in the orbit.11,12
Most posteriorly, it may be seen on MRI to divide into an inferior trunk, one branch of which bifurcates repeatedly as it travels anteriorly on the global surface of the IR that enters the EOM. The inferior trunk has a lateral branch that courses anteriorly along the lateral border of the IR and enters the IO at the point where it crosses the IR. The medial trunk of the inferior division on CN3 normally crosses inferior to the ON to make a prominent entry into the global surface of the MR. The normal CN6 is smaller than the branches of CN3, but typically may be seen on MRI to bifurcate repeatedly to form a manifold on the global surface of the LR as it courses anteriorly to innervate the LR. Since normal intraorbital motor nerves to individual EOMs are represented by one or at most a few pixels in the coronal image planes used in this study, the images were regarded as insufficiently precise for quantitative analysis of motor nerve size. However, qualitative impressions were consistently obtained and are illustrated herein Assessments were confirmed by evaluation of multiple contiguous MRI planes to trace the paths of presumed nerves to their target EOMs.
The CN6 was absent or below detection in the right orbits of subjects 1, 4, and 11, and appeared smaller than normal in subjects 9 and 10. All the foregoing exhibited DRS type 3 on the right. The CN6 was absent or below detection in the left orbit of subjects 10 and 11, both of whom had DRS type 1 in the affected orbit. In subject 1, who exhibited no clinical evidence of DRS on the left, the left CN6 was identified to be present in the orbit despite LR hypoplasia. All remaining subjects imaged had type 3 DRS on the left, and had identifiable CN6 in the orbit. When present, CN6 innervated the superior belly of the split LR. In the right orbit of subjects 3, 4, 6, 7, and 8 and in the left orbit of subjects 1, 4, 9, and 10, a branch of CN3 was in close contact with the inferior belly of the LR. As illustrated in , the intimate contact of the inferior division of CN3 with the LR suggested that the CN3 branch entered the EOM, although the limited resolution of MRI precludes confirmation of actual innervation at the level of the EOM fibers. The MR and IR were innervated normally by branches of CN3 in all orbits imaged.
Imaging of Intracranial Motor Nerves
-weighted imaging of the skull base region was conducted in 1-mm-thick slices at 390-μ
m resolution in the plane of the optic chiasm and major cranial nerves to the orbit. This technique has just sufficient resolution to demonstrate the normal CN6s coursing anteriorly from the pons (), whereas it easily and consistently demonstrates the larger course of the CN3s of normal subjects ().11
Figure 5 Oblique, axial, heavily T2-weighted MR images showing at left the normal course of the abducens nerve (CN6 dark) from the pons highlighted against the bright signal of the surrounding cerebrospinal fluid. In contrast, the CN6s of subject 4 with DURS2 (more ...)
Figure 6 Heavily T2-weighted axial MR images of 1-mm thickness at the level of the midbrain obtained in the plane of the optic chiasm. In all normal subjects, the oculomotor nerve (CN3) was prominent in multiple contiguous image planes. In subject 6 with DURS2, (more ...)
The heavily T2-weighted imaging technique demonstrated the CN6 in all normal subjects (, left). In adjacent sections, CN6 could be traced from the pons across the cerebrospinal fluid space to the clivus in every normal subject. The CN6 was not demonstrable bilaterally using the identical technique in subjects 4, 10, and 11. In subject 9, the right CN6 was hypoplastic, whereas the left CN6 was not definable within a dysplastic region spanning the subarachnoid space. The remaining affected subjects did not undergo this imaging.
The heavily T2-weighted imaging technique readily demonstrated the CN3 in multiple contiguous image planes in all 13 normal subjects (, left). Imaging capable of demonstrating CN3 at the brain stem was performed in subjects 4, 9, 10, and 11. The right CN3 was unilaterally hypoplastic in affected subjects 9 and 10, and appeared qualitatively normal in subjects 4 and 11. Averaging bilaterally, mean ± SEM. CN3 width was 1.55 ± 0.18 mm in affected subjects, significantly smaller than the width of 2.10 ± 0.07 mm in normal subjects (P < 0.005).
Despite the normal ophthalmoscopic appearance of the ON in all affected subjects, the coronal plane MRI was notable for the appearance of subnormal ON size in several subjects (). Because the ON cross section normally decreases from anterior to posterior in the orbit due to the reduction of connective tissues surrounding the axon bundles,24
ON cross sections were analyzed at the 2-mm-thick image plane thickness closest to the globe-ON junction. Mean (± SEM) cross section of the ON in 14 orbits with DURS2 was 6.85 ± 0.36 mm2
, significantly smaller than the mean cross section of 18 normal control orbits of 9.19 ± 0.46 mm2
Functional Evidence for Misinnervation
Abnormal patterns of EOM contraction provided evidence of misinnervation in several illustrative cases. Subject 1 had the clinical phenotype of DRS type 3 on the right, with limited abduction, adduction, and supraduction, downshoot on adduction, and A-pattern esotropia (). Coronal plane imaging was performed during target fixation by the left eye to control innervational effort, and was repeated in attempted abduction, central gaze, and attempted adduction (). Although there was some abduction of the right eye, the right LR exhibited modest contractile thickening mainly in its more anterior portion, with little contractile change in the deep orbit (). Although right MR exhibited robust contractile thickening in adduction (), adduction was slightly limited. However, in adduction, the right eye exhibited a downshoot that was associated with an increase in IR cross section suggestive of active contraction (). This inference of anomalous IR contraction on adduction is supported by the absence of an increase in SO cross section, the other EOM that would normally mediate infraduction. Anterior views near the level of the rectus pulleys showed no evidence of horizontal rectus EOM sideslip, indicating that the downshoot was not due to a “bridle effect” during horizontal rectus co-contraction.
Figure 7 Coronal MRI of the right orbit of subject 1 with the clinical phenotype of unilateral DRS type 3 on the right. Image planes 2-mm thick spaced by 2 mm, resolution 312 μm in planes arranged from posterior at top to anterior at bottom. Fixation was (more ...)
Ductions of the left eye of subject 1 appeared clinically normal. However, MRI of the left orbit at 234-μm resolution indicated the presence of CN6 but hypoplasia of the deep LR belly (). This observation suggests that the DRS endophenotype (internal phenotype) of LR hypoplasia was bilateral in subject 1. Apparently the presence of CN6 innervation to the hypoplastic left LR in subject 1 was sufficient to maintain an apparently normal clinical motility phenotype on the left. The absence of the right CN6 was associated with the right DRS type 3 in subject 1.
Figure 8 Coronal MRI of left orbit in planes 2-mm thick spaced by 2 mm, resolution 234 μm in plane. Top row: subject 1 with clinical phenotype of DRS unilateral type 3 on the right only. Note hypoplasia of the deep portion of the LR, without a recognizable (more ...)