AN59.1-4 | Pons — Self-Directed Learning

CLINICAL SCENARIO

A 50-year-old man with hypertension collapses in a Chennai hospital canteen. On examination: he is conscious, aware of surroundings, can blink in response to commands, but cannot speak, cannot move his arms or legs, cannot move his eyes horizontally. He can only move his eyes vertically and blink. All four limbs are completely paralysed (quadriplegia). He appears terrified but fully conscious.

MRI: massive haemorrhage in the pons (pontine haemorrhage).

This patient is completely aware but cannot communicate except through vertical eye movements — "locked-in syndrome." What structures in the pons are destroyed, and why are vertical eye movements spared?

WHY THIS MATTERS

Pontine anatomy is clinically important:

• Locked-in syndrome — rare but devastating; full consciousness with complete quadriplegia; vertical eye movements are the only communication channel
• Pontine haemorrhage — hypertensive bleed into the pons; high mortality; the "pin-point pupils" sign is pathognomonic
• Millard-Gubler and Foville syndromes — classic crossed brainstem syndromes tested in every Indian university examination
• Acoustic neuroma (vestibular schwannoma) — arises at the cerebellopontine angle; involves CN VII and VIII; most common intracranial tumour to affect young adults in India
• Internuclear ophthalmoplegia (INO) — MLF lesion in the pons; a hallmark of multiple sclerosis in young women

RECALL

Before we begin, recall:

• The pons lies between the medulla (below) and the midbrain (above)
• It forms the upper part of the floor of the 4th ventricle
• Cranial nerves V (trigeminal), VI (abducens), VII (facial), VIII (vestibulocochlear) have nuclei in or near the pons
• The pons connects to the cerebellum via the MIDDLE cerebellar peduncles (the largest)

Cranial Nerve Nuclei in the Pons

Cranial Nerve Nuclei in the Pons

Pontine Clinical Syndromes

Figure: CN VIII — Vestibulocochlear

Pontine Clinical Syndromes

SELF-CHECK — : Pons

The facial colliculus on the floor of the 4th ventricle is formed by the facial nerve fibres looping over which nucleus?

A. CN VII motor nucleus

B. CN VI (abducens) nucleus

C. CN VIII cochlear nucleus

D. Superior olivary nucleus

Pin-point pupils in pontine haemorrhage result from:

A. Direct compression of CN III

B. Cerebellar herniation

C. Bilateral destruction of descending sympathetic fibres in the pontine tegmentum

D. Activation of Edinger-Westphal nucleus

The mesencephalic nucleus of CN V is unique among cranial nerve nuclei because:

A. Its first-order neurons are located within the CNS (not in a peripheral ganglion)

B. It is found only in the medulla

C. It provides taste sensation from the anterior tongue

D. It contains parasympathetic preganglionic neurons

CLINICAL PEARL

Internuclear Ophthalmoplegia (INO) — MLF Lesion

The medial longitudinal fasciculus (MLF) connects the CN VI nucleus on one side to the CN III nucleus on the other side, coordinating horizontal conjugate gaze.

Figure: Internuclear Ophthalmoplegia (INO) — MLF Lesion

INO: Lesion of the MLF in the pons → on attempted horizontal gaze toward the side of the LESION, the IPSILATERAL eye (adducting eye) fails to adduct fully, while the contralateral eye shows horizontal nystagmus.

• Cause in young adults: Multiple sclerosis (bilateral INO = virtually diagnostic of MS)
• Cause in elderly: Brainstem stroke (usually unilateral INO)
• Test: Observe each eye during horizontal gaze — the adducting eye lags

Acoustic neuroma (vestibular schwannoma):
• Tumour of CN VIII Schwann cells at the cerebellopontine angle (CPA)
• Presents with: unilateral sensorineural deafness, tinnitus, and vestibular dysfunction
• Later: involves CN VII (facial weakness) and the pons (brainstem compression)
• Treated with microsurgery or stereotactic radiosurgery (Gamma Knife)

REFLECT

Return to the hook case — the locked-in patient:

1. Explain why he cannot move his limbs (all four are paralysed). Which tracts in the pons are destroyed?
2. Why can he move his eyes VERTICALLY but not HORIZONTALLY? Where is vertical gaze controlled?
3. Why is he fully conscious despite a massive pontine haemorrhage? What part of the pons is relatively spared?
4. If another patient has unilateral pontine lesion with ipsilateral CN VII palsy + ipsilateral lateral rectus palsy + contralateral hemiplegia, which syndrome is this?

Discussion: (1) Bilateral corticospinal tracts in basis pontis destroyed → quadriplegia. (2) Horizontal gaze = PPRF + CN VI nuclei in lower pons (destroyed). Vertical gaze = midbrain (rostral interstitial nucleus of MLF, riMLF — intact). (3) Reticular activating system is in the TEGMENTUM (dorsal pons); basis pontis lesion spares tegmentum → consciousness preserved. (4) Millard-Gubler syndrome (CN VI + CN VII ipsilateral + contralateral hemiplegia).

KEY TAKEAWAYS

Key Takeaways — Pons

• External features: Basilar groove (basilar artery), transverse fibres → middle cerebellar peduncles, facial colliculus (4th ventricle floor)
• Lower pons section: Basis pontis (corticospinal, pontine nuclei, transverse fibres) + Tegmentum (CN VI, CN VII genu, MLF, medial lemniscus)
• Upper pons section: CN V motor + principal sensory + mesencephalic nuclei; middle cerebellar peduncles dominant
• CN VII nuclei: Motor (SVE), Superior salivatory (GVE-parasympathetic), NTS (taste SVA), Spinal CN V (GSA)
• Syndromes: Locked-in (bilateral basis pontis) = quadriplegia + preserved vertical gaze + consciousness; Millard-Gubler (CN VI + VII ipsilateral + contralateral hemiplegia); Raymond (CN VI + contralateral hemiplegia); Foville (gaze palsy + CN VII + contralateral)
• Pin-point pupils = pontine haemorrhage (sympathetics destroyed bilaterally)