posterior commissure

**Anatomy:** The posterior commissure is a small, rounded band of myelinated fibers crossing the midline at the dorsal midbrain-diencephalic junction. It lies immediately dorsal to the cerebral aqueduct's opening into the third ventricle and attaches to the inferior (caudal) pineal stalk. The habenular commissure lies just superior to it at the pineal base, and the pineal recess lies between the two.

**Fiber content:** - Pretectal nucleus fibers that decussate to reach the contralateral Edinger-Westphal nucleus. This forms the consensual limb of the pupillary light reflex. - Fibers from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal, which mediate vertical gaze. - Connections between the nuclei of the posterior commissure (of Darkschewitsch) and other brainstem and diencephalic nuclei.

**Pupillary light reflex pathway:** Retina → optic nerve (CN II) → optic chiasm → optic tract → pretectal nucleus (bilateral) → via the posterior commissure and directly → Edinger-Westphal nuclei → CN III (preganglionic parasympathetic) → ciliary ganglion → sphincter pupillae → miosis.

**Clinical significance: Parinaud (dorsal midbrain) syndrome** - Cause: lesions compressing or damaging the dorsal midbrain and posterior commissure. Key causes are pineal tumors (germinoma, pineocytoma), hydrocephalus (aqueductal stenosis, dilated suprapineal recess), midbrain stroke, and MS. - Features: - Vertical gaze palsy, especially upgaze paralysis - Light-near dissociation (pupils constrict with accommodation but not to light), as in Argyll Robertson pupils - Convergence-retraction nystagmus - Eyelid retraction (Collier sign) - Possible papilledema from obstructive hydrocephalus - Mnemonic: "Parinaud = Pineal + upgaze palsy." - Why light-near dissociation occurs: the pretectal and posterior commissure pathway for light is interrupted. The near-response pathway, which is more ventral, is spared.

**High-yield contrasts:** - Anterior commissure: connects the temporal lobes and olfactory structures, and is part of the paleocortex. It is not involved in Parinaud syndrome. - Corpus callosum: the major interhemispheric neocortical commissure. - Argyll Robertson pupil (neurosyphilis): light-near dissociation without upgaze palsy. The lesion is thought to be in the pretectal area. - Compression of the superior colliculi or pretectal area from a pineal mass is the classic Step 1 vignette: a young male with a pineal germinoma, upgaze palsy, and hydrocephalus.

**Management:** Treat the underlying cause (CSF diversion such as endoscopic third ventriculostomy or shunt for hydrocephalus, and tumor-directed therapy: germinoma is highly radiosensitive). Check tumor markers (β-hCG, AFP).