Interpeduncular Fossa

Definition and Location

• Gray's Anatomy for Students notes: "Along the midline of the anterior surface [of the midbrain], there is a deep depression, the interpeduncular fossa. On either side of the interpeduncular fossa are the crus cerebri."

Boundaries

Contents

1. CN III - Oculomotor nerve: This is the key content. The third nerve fascicle traverses the midbrain (passing through or near the red nucleus and near the cerebral peduncles) before emerging as rootlets in the lateral interpeduncular fossa. These rootlets converge into a single third nerve trunk that then travels anteriorly through the subarachnoid space toward the cavernous sinus, passing between the superior cerebellar artery (SCA) and the posterior cerebral artery (PCA).
2. Posterior perforated substance (floor): Pierced by central perforating branches (thalamoperforating arteries) arising from the P1 segments of the PCAs and the top of the basilar artery, supplying the medial midbrain and thalamus.
3. Basilar artery bifurcation: The basilar artery divides into the two PCAs within/at the upper limit of the fossa. Paramedian branches from the top of the basilar artery enter the fossa to supply the medial midbrain and thalamus (including the arteries of Percheron).
4. Mammillary bodies: Located at the anterior margin, marking the posterior end of the hypothalamus.
5. Ventral tegmental area (VTA): Lies just medial and dorsal to the substantia nigra, at the base of the interpeduncular fossa; gives rise to the dopaminergic mesolimbic pathway.

Relations as a Cistern

• Ambient cistern - lateral to the midbrain
• Quadrigeminal cistern - posterior to the midbrain
• Interpeduncular cistern - ventral, between the peduncles
• Prepontine cistern - ventral to the pons (contains basilar artery + CN VI)

Clinical Significance

1. Oculomotor Nerve Palsy

• PCOM aneurysm (most common compressive cause): Pupillary fibers lie superomedially near the nerve surface and are preferentially compressed. Any pupil-involving or incomplete oculomotor palsy requires urgent imaging to exclude a PCOM aneurysm (risk of SAH and death if untreated).
• Herniation: Transtentorial herniation compresses CN III in this space.
• Inflammatory/neoplastic meningitis: Can involve the nerve here, either in isolation or with other cranial nerve signs.

2. Subarachnoid Hemorrhage (SAH)

• Rupture of a basilar tip aneurysm floods blood into the interpeduncular fossa; imaging shows hyperdensity here on CT, with generally poor prognosis.
• Perimesencephalic (non-aneurysmal) SAH: A small amount of blood localised to the interpeduncular cistern is characteristic of non-aneurysmal perimesencephalic SAH, which has a much more benign prognosis.
• Traumatic SAH can also pool in this location.

3. Midbrain Vascular Syndromes

4. Brainstem Fascicular CN III Syndromes (adjacent structures)

• Weber syndrome: CN III palsy + contralateral hemiparesis (cerebral peduncle)
• Claude syndrome: CN III palsy + contralateral hemiataxia (red nucleus/SCP)
• Benedikt syndrome: CN III palsy + contralateral chorea/tremor (red nucleus)
• Nothnagel syndrome: CN III palsy + ipsilateral hemiataxia (SCP)

5. Leptomeningeal Spread

Summary Diagram of Key Relations

          Mammillary bodies (anterior)
                    |
Crus cerebri -- [INTERPEDUNCULAR FOSSA] -- Crus cerebri
                    |
         Posterior perforated substance
         (perforated by thalamoperforating aa.)
                    |
              Pontine tegmentum