Optic Nerve Sheath as a Novel Mechanical Load on the Globe in Ocular Duction

Abstract

Purpose: The optic nerve (ON) sheath's role in limiting duction has been previously unappreciated. This study employed magnetic resonance imaging (MRI) to demonstrate this constraint on adduction.

Methods: High-resolution, surface coil axial MRI was obtained in 11 normal adults, 14 subjects with esotropia (ET) having normal axial length (AL) < 25.8 mm, 13 myopic subjects with ET and mean AL 29.3 ± 3.3 (SD) mm, and 7 subjects with exotropia (XT). Gaze angles and ON lengths were measured for scans employing eccentric lateral fixation in which an ON became completely straightened.

Results: In all groups, ON straightening occurred only in the adducting, not abducting, eye. Adduction at ON straightening was 26.0 ± 8.8° in normal subjects, not significantly different from XT at 22.2 ± 11.8°. However, there was significant increase in comparable adduction in ET to 36.3 ± 9.3°, and in myopic ET to 33.6 ± 10.7° (P < 0.04). Optic nerve length at straightening was 27.6 ± 2.7 mm in normals, not significantly different from 28.2 ± 2.8 mm in ET and 27.8 ± 2.7 mm in XT. In myopic ET, ON length at straightening was significantly reduced to 24.0 ± 2.9 mm (P < 0.002) and was associated with globe retraction in adduction, suggesting ON tethering.

Conclusions: Large adduction may exhaust length redundancy in the normally sinuous ON and sheath, so that additional adduction must stretch the sheath and retract or deform the globe. These mechanical effects are most significant in ET with axial myopia, but may also exert traction on the posterior sclera absent strabismus or myopia. Tethering by the ON sheath in adduction is an important, novel mechanical load on the globe.

Figure 1

Quasi-coronal, T2-weighted MRI of left orbit of a representative normal subject in 2-mm-thick contiguous image planes. The middle column immediately posterior to the junction of the globe and optic nerve (ON) shows the maximum diameter of the ON sheath expansion, with a cuff of bright signal cerebrospinal fluid surrounding the ON within its sheath. This fluid space is reduced posteriorly (left column). The ON is centered within the sheath in central gaze (middle row) and abduction (bottom row), but shifts temporally in adduction (top row). IR, inferior rectus muscle; LR, lateral rectus muscle; MR, medial rectus muscle; SO, superior oblique muscle; SR, superior rectus muscle.

Figure 2

T2-weighted MRI of representative normal subject in contiguous, 2-mm-thick axial planes centered (middle column) on the optic nerve (ON) and including inferior and superior planes to illustrate the full diameter of the ON sheath. MRI was repeated in right (top row), center (middle row), and left (lower row) gazes. Note ON redundancy in central gaze and abduction, with the ON either sinuous within the same plane or between adjacent axial planes. However, in adduction of each eye, the ON is straightened and becomes apposed to the straight ON sheath surrounding it. ON length was measured from the posterior globe to the orbital apex as illustrated in the lower image. Anteroposterior (AP) orbit length was measured from the apex, the location of the common tendonous origin of the rectus muscles, to the anterior border of the orbital fat in the axial image plane closest to the center of the ON in central gaze.

Figure 3

Quasi-coronal, T2 MRI of normal left orbit demonstrating measurements of the immediately retrobulbar optic nerve diameter (ON diam.), ON sheath outer diameter, sheath thickness, and cerebrospinal fluid gap (subarachnoid space). Diameters of circular structures were inferred from cross-sectional areas to achieve subpixel resolution, and circular symmetry with constant sheath and gap thickness was assumed. IR, inferior rectus muscle; LPS, levator palpebrae superioris; LR, lateral rectus muscle; MR, medial rectus muscle; SO, superior oblique muscle; SOV, superior ophthalmic vein.

Figure 4

Axial MRI illustrating progressive straightening of the optic nerve (ON) from its sinuous course in abduction at left to progressively greater adduction in the right columns. (A) T1 sequence with gadodiamide contrast in orthotropic normal subject. The white horizontal line denotes the anterior corneal surface. Note that progressive adduction from the third image from left (ON length 28.0 mm, 11.5° adduction) is associated with slight globe surface retraction in large adduction (upper right, ON length 28.9 mm, 25.6° adduction). (B) T2 fast spin echo sequence in esotropic subject with axial high myopia demonstrating persistence of ON straightening during progressively increasing adduction at the lower center (ON length 24.6 mm, 19.0° adduction) and lower right (ON length 25.0 mm, 32.4° adduction). The myopic globe is nonspherical.

Figure 5

Axial MRI in right (top) and left (bottom) gazes showing optic nerve (ON) straightening in the adducting eye of moderately myopic subject who had 55Δ esotropia in central gaze. Ghost images of the sclera, cornea, and lens visible within the vitreous cavity indicate movement artifacts presumably due to involuntary eye movements induced by ON traction in the adducting eye.

Figure 6

Axial MRI in highly axially myopic, esotropic subject. Note that when the left optic nerve (ON) straightened in adduction during right gaze (top), the anterior surface of the left globe underwent approximately 4-mm retraction. When the right ON straightened in right eye adduction during left gaze (bottom), the right globe underwent slightly less retraction. Ghost images, particularly in the lower image, represent motion artifacts due to involuntary eye motion typically observed during globe retraction.

Figure 7

Axial MRI in mildly myopic, exotropic subject. The optic nerve (ON) straightened in adduction, while having a sinuous path in abduction for each eye. Note in lower left the temporal shift of the ON within the sheath in adduction as temporal redundancy in the sheath was lost during straightening.

Figure 8

Box and whiskers plot showing effect of horizontal duction on anteroposterior position of the equatorial globe centroid as measured in axial MRI. Retraction in nonmyopic subjects with esotropia (ET) was significant by 1-sample t-testing against zero. Anteroposterior centroid position was significantly more posterior in adduction than abduction in axially myopic subjects with ET (P = 0.022 by Kruskal-Wallis test). Rectangles represent interquartile range with median indicated by horizontal bar. Whiskers represent maxima and minima. Individual observations are indicated by filled circles.

Figure 9

Adduction angle at which optic nerve straightening was observed in control and strabismic subjects. While the angle did not differ significantly between control and exotropic subjects, adduction angle was significantly greater in esotropic subjects but significantly more so in those who were nonmyopic. Statistical comparisons are 2-tailed t-tests assuming unequal variance. SEM, standard error of mean.

Figure 10

Length of the optic nerve (ON) at which straightening was observed in adduction. While ON length did not differ significantly between control and exotropic subjects, the ON was significantly shorter in esotropic subjects. Statistical comparisons are 2-tailed t-tests assuming unequal variance. Control and exotropic subjects did not differ significantly. SEM, standard error of mean.

Figure 11

Relationship between anteroposterior length of the orbit and length of the optic nerve (ON) at straightening based upon pooled data for all subjects in all groups. While the linear correlation is significant (P = 0.0025), orbit length accounts for only 11% of the variance in ON length.