Association of superior oblique muscle volumes with the presence or absence of the trochlear nerve on high-resolution MR imaging in congenital superior oblique palsy

Abstract

Background and purpose: Congenital superior oblique palsy is known to relate to trochlear nerve absence and a variable degree of superior oblique muscle hypoplasia. The purpose of this study was to determine whether superior oblique muscle volume predicts trochlear nerve absence in congenital superior oblique palsy.

Materials and methods: A retrospective study of high-resolution MR imaging to evaluate the presence of the trochlear nerve and to measure superior oblique muscle areas and volumes with the image analysis tools of a PACS was performed in 128 consecutive patients with unilateral congenital superior oblique palsy and 34 age-matched healthy controls.

Results: Of the 128 patients with congenital superior oblique palsy, 88 had an ipsilateral trochlear nerve absence (absent group) and 40 had both trochlear nerves (present group). In patients with congenital superior oblique palsy, the paretic side superior oblique muscle volume was significantly smaller compared with the normal side only in the absent group (P < .001). The left and right side superior oblique muscle volumes were not significantly different in controls (P = .750), and the paretic and normal side superior oblique muscle volumes were not significantly different in the present group (P = .536). The cutoff value of the paretic/normal side superior oblique muscle volume ratio for diagnosing trochlear nerve absence was ≤0.75 (sensitivity 98.9%, specificity 95.0%) in patients with congenital superior oblique palsy.

Conclusions: The ratio of paretic/normal side superior oblique muscle area and volume has an excellent predictability in diagnosing trochlear nerve absence in congenital superior oblique palsy.

Fig 1.

MR imaging in a patient with right superior oblique muscle palsy. A, The optic nerve–globe junction was defined as the standard plane (plane “0”), and SO areas were measured in the coronal sections of 5 contiguous planes, including the standard plane and planes that were 2 and 4 mm anterior or posterior to the standard plane. B, T2-weighted coronal image of the orbit and SO. The right SO is hypoplastic compared with the left. The area surrounded by the curvilinear line was measured in 5 different planes, by using DTU-710 (Wacom) and PACS software, which provides automatic measurements for area. The volume of the SO was defined as the sum of SO areas at the 5 planes multiplied by 2 mm.

Fig 2.

Boxplots of the superior oblique muscle volume ratio of the paretic-to-normal side in patients with congenital superior oblique palsy without (absent group) and with (present group) an ipsilateral trochlear nerve compared with controls. The SO volume ratio was significantly smaller in the absent group (P < .001) compared with controls. There was no significant difference of the SO volume ratio between controls and the present group.

Fig 3.

Receiver operating characteristic curves of paretic side SO areas, SO volume, paretic/normal side ratios of the SO area and SO volume for predicting trochlear nerve absence. The area under the receiver operating characteristic curves was largest for the paretic/normal side ratios of SO volume and SO area at the optic nerve–globe junction, with an AUC value of >0.950. AUCs of the paretic side SO areas were much smaller at the anterior planes (+4, +2) compared with the standard plane (0) or posterior planes (−2, −4).