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AN31.1-5 | Orbit — Gate Quiz
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Which of the following extraocular muscles is supplied by the trochlear nerve (CN IV)?
Correct! The superior oblique is the only muscle supplied by the trochlear nerve (CN IV). Remember: LR6 SO4 rest 3.
Nerve supply of extraocular muscles: CN IV → superior oblique; CN VI → lateral rectus; CN III → all remaining four muscles (superior rectus, inferior rectus, medial rectus, inferior oblique) plus levator palpebrae superioris.
Incorrect. The trochlear nerve (CN IV) exclusively supplies the superior oblique. Use the mnemonic LR6 SO4 rest 3 — Lateral Rectus→CN6, Superior Oblique→CN4, all rest→CN3.
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The trochlea acts as a pulley for the superior oblique, changing the direction of its pull. As a result, when the eye is adducted, the superior oblique primarily:
Correct! After passing through the trochlea, the superior oblique tendon turns 45° and inserts posterior to the equator. In the adducted position it acts as a depressor and intorter. This is why CN IV palsy causes diplopia on looking down (e.g., descending stairs).
Superior oblique primary action (adducted eye): depression + intorsion. Secondary actions: abduction + intorsion (in primary position). The trochlea changes the muscle's mechanical axis, making CN IV palsy manifest as diplopia on downward gaze.
Incorrect. Due to the trochlear pulley redirecting its pull, the superior oblique depresses and intorts the adducted eye — this is its primary clinical action tested in CN IV palsy.
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The ophthalmic artery, the principal arterial supply to the orbit, is the first branch of:
Correct! The ophthalmic artery is the first branch of the internal carotid artery after it leaves the cavernous sinus. It enters the orbit through the optic canal, below the optic nerve.
Ophthalmic artery = first branch of ICA (after cavernous sinus). Enters optic canal below CN II. Key branch: central artery of retina — only supply to inner retina; occlusion causes sudden painless monocular blindness. ICA territory, not ECA.
Incorrect. The ophthalmic artery arises from the internal carotid artery — NOT the external carotid. This is clinically important: ICA occlusion can cause amaurosis fugax (transient monocular blindness).
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A 65-year-old male smoker presents with right ptosis (partial), right miosis, and anhidrosis of the right face and neck. Chest X-ray reveals an opacity at the right lung apex. The lesion has interrupted the:
Correct! A Pancoast tumour at the lung apex interrupts the preganglionic (second order) sympathetic neuron as it loops over the apex of the lung. Anhidrosis of the face and neck helps localise the lesion to the second order neuron (above the superior cervical ganglion).
Horner's syndrome localisation: Anhidrosis of face + neck = 2nd order lesion (preganglionic, above superior cervical ganglion). Anhidrosis of face only = 2nd order at ganglion level. No anhidrosis = 3rd order (postganglionic). Pancoast tumour at lung apex = classic 2nd order cause.
Incorrect. A lung apex (Pancoast) tumour affects the second order (preganglionic) neuron, which loops over the apex of the lung and runs along the carotid sheath. Anhidrosis of face + neck indicates the lesion is at or above the superior cervical ganglion level.
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The nasolacrimal duct drains tears from the lacrimal sac into which part of the nasal cavity?
Correct! The nasolacrimal duct opens into the inferior meatus of the nasal cavity, beneath the inferior turbinate. A small mucosal fold (Hasner's valve) guards this opening.
Nasolacrimal duct → inferior meatus. The inferior meatus also receives the inferior turbinate's lateral wall. Clinical: congenital obstruction at Hasner's valve causes epiphora in neonates — treated by digital massage or probing.
Incorrect. The nasolacrimal duct opens into the inferior meatus. Remember: the middle meatus receives the major paranasal sinuses (frontal, maxillary, anterior ethmoid); the inferior meatus receives the nasolacrimal duct.
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Secretomotor innervation of the lacrimal gland is relayed through which ganglion?
Correct! Preganglionic parasympathetic fibres from CN VII travel via the greater petrosal nerve → nerve of the pterygoid canal → pterygopalatine (sphenopalatine) ganglion → relay → postganglionic fibres join the zygomatic nerve → lacrimal nerve → lacrimal gland.
Lacrimal gland secretomotor: CN VII → greater petrosal nerve → pterygopalatine ganglion → postganglionic → zygomatic nerve → lacrimal nerve → gland. CN VII lesion proximal to the greater petrosal nerve causes dry eye (xerophthalmia) in addition to facial palsy.
Incorrect. Secretomotor fibres for the lacrimal gland relay in the pterygopalatine ganglion (CN VII pathway). The ciliary ganglion relays parasympathetic fibres for the pupil and ciliary muscle (CN III pathway).
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A 55-year-old hypertensive, diabetic patient presents with sudden-onset diplopia and right-sided ptosis. Examination shows the right eye deviating down and out, but the pupil is equal and reactive bilaterally. The most likely diagnosis is:
Correct! Pupil-sparing CN III palsy in a diabetic patient strongly suggests microvascular (diabetic) ischaemia. The ischaemia affects the core motor fibres while the superficial parasympathetic pupillary fibres are relatively spared.
Medical (pupil-spared) vs Surgical (pupil-involved) CN III palsy: Aneurysm/herniation compress the surface parasympathetic fibres first → dilated pupil. Microvascular ischaemia (diabetes, hypertension) affects core motor fibres → motor palsy with pupil sparing. Always an urgent MRI/CTA if the pupil is involved.
Incorrect. Pupil-sparing CN III palsy in a diabetic patient = microvascular ischaemic palsy (medical CN III). Pupil-involving CN III palsy = compressive cause (aneurysm, herniation) until proved otherwise.
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Paralysis of the abducent nerve (CN VI) results in which eye position at rest?
Correct! CN VI palsy causes paralysis of the lateral rectus. Without lateral pull, the medial rectus acts unopposed, pulling the eye medially — producing esotropia (convergent squint). The patient cannot abduct the affected eye.
CN VI (abducent) palsy: lateral rectus paralysed → esotropia (eye deviates medially). Patient cannot abduct the eye. Diplopia on lateral gaze to the affected side. CN VI has the longest intracranial course, making it vulnerable in raised ICP (false localising sign).
Incorrect. CN VI palsy → lateral rectus paralysis → medial rectus unopposed → eye pulled medially → esotropia (medial deviation). Diplopia is maximal on lateral gaze toward the affected side.
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The superior ophthalmic vein drains into the cavernous sinus without valves. This anatomical feature explains why facial infections in the "danger triangle" can cause:
Correct! The valveless superior ophthalmic vein provides a direct communication between facial veins (angular vein at the medial canthus) and the cavernous sinus. Infected thrombus from facial boils (especially in the danger triangle — nose, upper lip) can propagate retrogradely into the cavernous sinus.
Danger triangle of face (nose + upper lip): facial veins → angular vein → superior ophthalmic vein → cavernous sinus (valveless communication). Septic thrombophlebitis can spread intracranially. Signs of cavernous sinus thrombosis: proptosis, ophthalmoplegia, papilloedema, fever, meningism.
Incorrect. Valveless superior ophthalmic vein → allows retrograde propagation of infected facial thrombus → cavernous sinus thrombosis. This explains why squeezing boils in the nose/upper lip area is dangerous.
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A child with chronic suppurative otitis media develops ipsilateral lateral gaze palsy and severe facial pain. This triad (otorrhoea, CN VI palsy, facial pain) is characteristic of:
Correct! Gradenigo's syndrome is a complication of petrous apicitis (infection spreading from the middle ear to the apex of the petrous temporal bone). The petrous apex is adjacent to CN VI and the trigeminal ganglion, explaining the lateral gaze palsy and severe facial pain.
Gradenigo's syndrome (petrous apicitis): CSOM → infection extends to petrous apex → compresses CN VI (lateral rectus palsy, esotropia) and Gasserian ganglion (CN V — facial pain). Triad: ear discharge + lateral gaze palsy + facial pain. Treatment: IV antibiotics ± surgical drainage.
Incorrect. Gradenigo's syndrome = petrous apicitis from CSOM, causing CN VI palsy (lateral gaze) + facial pain (CN V) + otorrhoea (the primary ear infection). CN VI's long course over the petrous apex makes it especially vulnerable.
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