Correct! This is an important neuro-ophthalmic landmark! Lying at the junction of the thalamus and midbrain, it mediates upward gaze by connecting the rostral interstitial nuclei of the medial longitudinal fasciculus (RIMLF) and interstitial nucleus of Cajal (INC) on both sides. A lesion here SELECTIVELY interferes with upward gaze, sparing downward gaze, which is mediated more ventrally. When this commissure is damaged by an intrinsic lesion of the caudal thalamus or pretectal or tectal midbrain regions, or by an extrinsic lesion like a pinealoma, expect elements of a “dorsal midbrain syndrome” (also called “pretectal syndrome,” “Sylvian aqueduct syndrome,” or “Parinaud syndrome” if you are French, “Koerber-Salus-Elschnig syndrome” if you are German). The clinical features of this syndrome include lid retraction (“Collier’s sign”), dilated pupils with light-near dissociation, and sometimes esotropia, exotropia, or skew deviation. Why does a lesion here cause such a diverse set of clinical phenomena? Disruption of upgaze produces a “spill-over” activation of all extraocular muscles, pulling the eyes backwards into the orbit. Because the medial recti are the most powerful muscles, the eyes tend to converge. Dilated pupils with light-near dissociation (“tectal pupils”) are attributed to interruption of the pupillary light reflex pathway in the dorsal midbrain. Esotropia and exotropia result from disruption of the vergence pathway in that region, and skew deviation results from interruption of the vestibulo-ocular pathway as it reaches the midbrain. This patient had a pinealoma.

Early diagnosis favors a better outcome of treatment. By the way, even with early diagnosis of pineal tumor, some features of a dorsal midbrain syndrome—especially upgaze deficiency and convergence-retraction--often linger. Why is this fact important? Because if upgaze deficiency and convergence-retraction are detected long after treatment, they should not be taken as signs of tumor recurrence or of a complication of radiotherapy, even if not previously documented!