The midbrain is the smallest and most rostral part of the brainstem, but it packs an extraordinary density of structures into a small space. The oculomotor and trochlear nerve nuclei, the cerebral peduncles carrying corticospinal and corticobulbar fibers, the red nucleus, the substantia nigra, the medial longitudinal fasciculus, the periaqueductal gray, and the reticular activating system all lie within or pass through the midbrain. Midbrain lesions therefore produce a wide variety of focal syndromes — many of which carry eponyms recalling their first descriptions in the late 19th century. This page covers the anatomy of the midbrain, the localizing syndromes, and the differential diagnosis of midbrain lesions.

Anatomy of the Midbrain

The midbrain is divided in cross-section into three zones:

• Tectum (dorsal): the roof, containing the superior colliculi (visual reflex, including saccade initiation) and inferior colliculi (auditory relay). The pretectal area lies just rostral to the superior colliculi and contains structures for pupillary light reflex.
• Tegmentum (middle): contains the oculomotor and trochlear nuclei, the medial longitudinal fasciculus, the red nucleus, the periaqueductal gray, the medial lemniscus, the spinothalamic tract, the reticular activating system, and several cranial nerve nuclei.
• Cerebral peduncles (ventral): contain the corticospinal and corticobulbar tracts plus corticopontine fibers. The substantia nigra lies between the tegmentum and peduncle (sometimes considered part of the tegmentum).

Levels of the Midbrain

• Superior colliculus level: CN III nucleus and fascicle, red nucleus, MLF, medial lemniscus, spinothalamic tract, substantia nigra, cerebral peduncle.
• Inferior colliculus level: CN IV nucleus (and decussation), cerebral peduncle, tegmentum, substantia nigra. The CN IV is the only cranial nerve that decussates BEFORE exit AND exits dorsally.

Blood Supply

• Paramedian midbrain: from PCA P1 perforators (thalamoperforator arteries). Includes the medial midbrain syndromes; bilaterally in the artery of Percheron variant.
• Lateral midbrain: from PCA short circumferential branches and SCA branches.
• Dorsal midbrain: from PCA quadrigeminal arteries and posterior choroidal artery.

Eponymous Midbrain Syndromes

Weber Syndrome

Lesion of the medial midbrain at the level of CN III fascicle and crus cerebri. Features:

• Ipsilateral CN III palsy (oculomotor — pupil-involving usually): eye is “down and out,” dilated pupil, ptosis.
• Contralateral hemiparesis (cerebral peduncle, corticospinal tract): face, arm, leg.

Anatomic substrate: the corticospinal tract has not yet decussated, so a midbrain lesion produces contralateral hemiparesis. The CN III fascicle is descending through the same area and is interrupted on the ipsilateral side. The intersection: ventral midbrain on one side gives the “Weber” combination.

Causes: PCA territory stroke (paramedian perforator), tumor, demyelination, hemorrhage.

Benedikt Syndrome

Lesion of the midbrain tegmentum including red nucleus and CN III fascicle. Features:

• Ipsilateral CN III palsy.
• Contralateral tremor / chorea / athetosis / ataxia (red nucleus involvement).

Causes: PCA territory stroke involving tegmentum, tumor, demyelination.

Claude Syndrome

Lesion of the dorsomedial midbrain tegmentum affecting CN III fascicle, red nucleus, and brachium conjunctivum (superior cerebellar peduncle). Features:

• Ipsilateral CN III palsy.
• Contralateral cerebellar ataxia.

Anatomically, Claude shares features with Benedikt but adds the cerebellar component more prominently.

Parinaud Syndrome (Dorsal Midbrain Syndrome)

Lesion of the dorsal midbrain (pretectal area, around the rostral interstitial nucleus of the MLF). Features:

• Upgaze palsy: failure of upward saccades and pursuit.
• Convergence-retraction nystagmus: on attempted upward saccade, the eyes converge and retract into the orbit.
• Light-near dissociation of pupils: pupils don’t react to light but do constrict for near targets.
• Lid retraction (Collier sign).
• Sometimes downgaze palsy added (depending on extent).

Causes:

• Pineal gland tumors (germinoma, pineoblastoma, pineocytoma) — classical in young patients.
• Hydrocephalus with aqueductal stenosis (compression from above).
• Midbrain stroke (less common).
• Demyelinating disease.
• Tumor (glioma) of dorsal midbrain.

Artery of Percheron Infarct

The artery of Percheron is a single trunk arising from one PCA P1 that supplies the bilateral paramedian thalami AND the rostral midbrain. Occlusion produces:

• Bilateral thalamic infarction with severely altered consciousness.
• Vertical gaze palsy (upgaze, or both).
• Memory impairment.
• Hypersomnolence, abulia.

Often misdiagnosed initially because the patient appears “comatose” but with no clear focal motor signs. MRI shows bilateral paramedian thalamic infarcts — characteristic finding.

Top of the Basilar Syndrome

Occlusion of the distal basilar artery → infarction of the rostral midbrain, bilateral thalami, occipital cortex, and sometimes cerebellum. Features:

• Altered consciousness.
• Visual abnormalities — hallucinations, hemianopia.
• Vertical gaze palsy.
• Pupillary abnormalities.
• Behavioral changes.
• Often with limb findings.

Locked-In Syndrome from Midbrain

Less commonly a midbrain lesion can compromise corticospinal and corticobulbar pathways while sparing the dorsal tegmentum (ARAS), producing a locked-in picture. The patient is conscious but unable to move, except sometimes vertical eye movements. More commonly locked-in is a ventral pontine syndrome (basilar artery occlusion at pontine level).

Specific Anatomic Lesions and Their Findings

Causes of Midbrain Lesions

Vascular

• PCA paramedian infarct: medial midbrain syndromes (Weber, Benedikt).
• Artery of Percheron infarct: bilateral paramedian thalami + midbrain.
• Top of the basilar syndrome.
• Midbrain hemorrhage (often hypertensive, paramedian).
• Cavernous malformation.

Demyelinating

• MS plaques in midbrain — INO, vertical gaze palsy, CN III findings.
• NMO, MOGAD: can involve midbrain (especially area postrema/dorsal pons/midbrain).

Neoplastic

• Brainstem glioma (especially in children — pontine more common, but midbrain also).
• Metastasis.
• Pineal tumors compressing dorsal midbrain → Parinaud syndrome.
• Lymphoma.

Inflammatory / Infectious

• Rhomboencephalitis (Listeria, herpes simplex, Lyme, EBV).
• Sarcoidosis, neurobehçet, vasculitis.
• Progressive multifocal leukoencephalopathy.

Compressive

• Mass effect from supratentorial lesion → uncal herniation, with midbrain compression.
• Pineal mass, dorsal midbrain compression.
• Hydrocephalus with aqueductal stenosis.

Neurodegenerative

• Progressive supranuclear palsy (PSP): degeneration of midbrain (especially riMLF and substantia nigra) — vertical gaze palsy, axial rigidity, falls, frontal cognitive features.
• Parkinson disease and related syndromes: substantia nigra degeneration.

Imaging the Midbrain

• MRI is the modality of choice (CT often misses midbrain pathology).
• Coronal and axial T2 sequences best visualize midbrain.
• Specific midbrain atrophy patterns (PSP — “hummingbird” or “Mickey Mouse” appearance) on sagittal MRI.
• Diffusion-weighted imaging for acute infarct.
• Susceptibility weighted imaging for microbleeds or cavernous malformations.
• Gadolinium for inflammatory, infectious, neoplastic lesions.

The Approach to a Suspected Midbrain Lesion

1. Cranial nerve examination: especially CN III and IV; check pupils carefully.
2. Examine eye movements: full versions, saccades, pursuit, convergence.
3. Vertical gaze: look for upgaze or downgaze paresis.
4. Look for INO.
5. Pupillary light-near dissociation.
6. Long tract findings: corticospinal (often contralateral hemiparesis), sensory.
7. Cerebellar findings (contralateral if midbrain).
8. Movement disorders: tremor, parkinsonism.
9. Mental status: altered consciousness, hypersomnolence.
10. MRI brain with attention to midbrain.

🔍 Did You Know?

The artery of Percheron is an anatomical variant where a single dominant artery — instead of paired thalamoperforator arteries on each side — arises from one PCA P1 segment and supplies both paramedian thalami plus the rostral midbrain. The variant is present in maybe 4-11% of people. When this single trunk artery is occluded by a small embolus, the patient experiences a remarkable clinical picture: sudden onset of profoundly altered consciousness — sometimes coma — with bilateral vertical gaze palsy (often more affecting upgaze) and memory impairment persisting after the patient awakens. The picture is often initially misdiagnosed as “metabolic encephalopathy” or “psychiatric” because there are no clear focal motor signs. The MRI is diagnostic — bilateral paramedian thalamic infarcts appear as symmetric T2 hyperintensities, with rostral midbrain involvement in some cases. Recognition matters because thrombectomy may be feasible if recognized early. The mortality is significant, and many survivors have persistent memory and cognitive deficits even after recovering motor function. Albert Percheron, the French neurologist who described the variant in 1973, deserves credit for one of the most clinically consequential anatomical observations in stroke neurology.

Pitfalls and Pearls

• Weber syndrome: ipsilateral CN III + contralateral hemiparesis = ventral midbrain.
• Benedikt syndrome: ipsilateral CN III + contralateral tremor/ataxia = tegmental midbrain.
• Claude syndrome: ipsilateral CN III + contralateral cerebellar ataxia.
• Parinaud syndrome: upgaze palsy + convergence-retraction nystagmus + light-near dissociation + lid retraction = dorsal midbrain.
• Pineal tumor in young patient with Parinaud syndrome — image immediately.
• Hydrocephalus with aqueductal stenosis can cause dorsal midbrain compression — Parinaud syndrome.
• Artery of Percheron infarct: bilateral paramedian thalamic infarcts + vertical gaze palsy + altered consciousness. Often mistaken for metabolic encephalopathy.
• Top of the basilar syndrome: altered consciousness + vertical gaze palsy + visual hallucinations + memory disturbance.
• PSP: vertical gaze palsy (downgaze first) + axial rigidity + falls + frontal cognitive findings; “hummingbird sign” on MRI.
• INO: localizes to MLF in dorsal midbrain (between CN III and CN VI nuclei).
• Bilateral ptosis from a single midline CN III nucleus lesion — unusual.
• CN IV decussates before exit — a nuclear lesion produces contralateral CN IV palsy.
• Light-near dissociation: pupils constrict for near targets but not light. Argyll Robertson (syphilis), dorsal midbrain (Parinaud), diabetes.
• Midbrain lesions can compress the aqueduct → obstructive hydrocephalus.

References

1. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.
2. Caplan LR. Caplan’s Stroke: A Clinical Approach. 5th ed. Cambridge University Press; 2016.
3. Lazzaro NA, Wright B, Castillo M, et al. Artery of percheron infarction: imaging patterns and clinical spectrum. AJNR Am J Neuroradiol. 2010;31(7):1283-1289.
4. Leigh RJ, Zee DS. The Neurology of Eye Movements. 5th ed. Oxford University Press; 2015.
5. Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 11th ed. McGraw-Hill; 2019.