Localize binocular diplopia, pupil and lid findings, pontine gaze circuits, and visual field loss with patient-sided H-test anatomy and clinical cases.
A single symptom can name several pathways. Predict what else each proposed lesion must change, then compare it with the patient’s actual examination.
First decide what failed
Cover either eye when someone reports two images. Binocular diplopia disappears with either eye covered because the two eyes are misaligned. If doubling persists while viewing through just the affected eye, consider ocular optics or retina before a gaze pathway. A missing portion of the visual world is not double vision. Test each eye and formal visual fields rather than labeling every complaint an eye-muscle palsy. Ask whether separation is horizontal, vertical, or tilted, whether distance or near is worse, and which gaze increases it. Pain, new headache, acuity loss, pupillary change, proptosis, and other neurologic findings change the urgency and localization. [1]
Predict. If only the patient left eye fails to abduct on left gaze while the right eye adducts, has the shared leftward command failed? No. The right adduction proves it reached the partner eye. This is a peripheral left abduction problem until other signs refine it. If both eyes fail leftward, inspect the pontine gaze circuit instead. This comparison works better than memorizing a photograph of a deviated eye. [1]
In down-right gaze the patient right eye is abducted and tested through inferior rectus, while the patient left eye is adducted and tested through superior oblique. Compare the labeled pathways and resulting deficit. [1]
The patient is facing you in the diagram. Patient right is at the left of an examiner looking at the face, but every direction named here belongs to the patient. To look patient right, the right eye abducts and the left adducts. Down-right tests the right inferior rectus in abduction and left superior oblique in adduction. The diagram describes a diagnostic position, not an assertion that one muscle alone causes the entire version. [1]
How muscle vectors become examination findings
From straight-ahead primary position, medial rectus adducts and lateral rectus abducts. Superior rectus chiefly elevates, inferior rectus chiefly depresses, superior oblique chiefly intorts, and inferior oblique chiefly extorts. Recti also contribute torsion and adduction; obliques also contribute vertical movement and abduction. Because their pulls are oblique to the globe, horizontal positioning isolates different vertical actions in the H test. In abduction, superior and inferior recti are useful elevators and depressors. In adduction, inferior oblique elevates and superior oblique depresses. Do not call downward depression the primary action of superior oblique in straight-ahead gaze. [1]
LR6, SO4, all others III. Lateral rectus receives abducens nerve VI; superior oblique receives trochlear IV. Oculomotor III supplies medial, superior and inferior recti, inferior oblique, the main upper-lid elevator, and parasympathetic pupil constriction. This is innervation, not a license to diagnose from one cardinal position alone. Repeat versions, compare each eye, inspect pupils and lids, and ask whether passive movement seems restricted. A fibrotic inferior rectus in thyroid eye disease can resist attempted elevation even though its nerve is not weak. Fatigable changing ptosis or ophthalmoparesis with spared pupils suggests a neuromuscular-junction mimic such as myasthenia. [1]
Trochlear injury weakens intorsion and depression in adduction. The affected eye may sit high and produce vertical or torsional diplopia during reading or descending stairs. A patient with a left superior oblique palsy often tilts the head right to reduce misalignment; observe rather than assume every head tilt has that cause. Its dorsal exit and course near the tentorium make IV vulnerable in trauma, but congenital and other causes exist. [1]
Try a transfer. On patient left-down gaze, which eye is adducted? The right. Failure of that right eye to descend there suggests right superior oblique dysfunction; the left inferior rectus is the abducted partner. This is a localizing prediction, not yet an etiology. [1]
Nerve, pupil, lid and pressure
An isolated peripheral VI palsy weakens ipsilateral lateral rectus. The affected eye may rest inward, abduction fails, and horizontal binocular diplopia often worsens at distance and toward the affected side. A VI nucleus lesion instead interrupts the ipsilateral lateral rectus command and interneurons to contralateral medial rectus. Raised intracranial pressure can stretch VI along its long clival course, producing an abduction deficit that does not identify the pressure source in the pons. Papilledema and headache change the interpretation. [1][3]
A substantial III palsy can combine poor adduction, elevation and depression with prominent ptosis and a down-and-out resting position. Superficial parasympathetic fibers make a larger poorly reactive pupil especially concerning for compression, classically a posterior communicating artery aneurysm. A complete isolated pupil-sparing palsy in a patient with vascular risk factors may be microvascular, yet a normal pupil does not exclude aneurysm, especially in partial or evolving palsy. Acute suspected III palsy warrants urgent expert evaluation and vascular imaging as appropriate; do not simply send it home because the pupil reacts. [2]
Horner syndrome differs. Sympathetic interruption weakens the small superior tarsal lid elevator, making typically mild ptosis with ipsilateral miosis, not a III-pattern external ophthalmoplegia. Anisocoria larger in darkness identifies the smaller pupil as the abnormal one; anisocoria larger in bright light makes the larger poorly constricting pupil more concerning. Light and dark comparisons are aids, not substitutes for acute assessment. Painful Horner with neck symptoms can indicate carotid pathology. [1]
Wernicke encephalopathy is a different urgent metabolic context. In a person at risk for thiamine deficiency, ophthalmoplegia or nystagmus with ataxia and encephalopathy is characteristic, but the complete triad is often absent. A lone abduction deficit is not proof of Wernicke. Compare systemic and neurologic findings before reducing every VI-looking deficit to one disease. [1][3]
Follow the command across the pons
For patient leftward horizontal gaze, the right frontal eye field drives the opposite left pontine gaze network. The left frontal eye field instead drives rightward gaze. In the pons, a left abducens nucleus activates the left lateral rectus through VI and sends interneurons across to the right medial rectus subnucleus through the right medial longitudinal fasciculus, or MLF. The PPRF supplies burst activity for fast ipsilateral saccades. Therefore a PPRF lesion can spare pursuit or vestibular movements that an abducens nuclear lesion does not. [1][3]
The abducens nucleus coordinates both eyes; the peripheral VI nerve supplies only ipsilateral lateral rectus. A PPRF lesion chiefly impairs rapid ipsilateral saccades. Compare the labeled pathways and resulting deficit. [3]
A right INO means the right eye adducts slowly or incompletely on left gaze, often with nystagmus in the abducting left eye. The lesion is in the right MLF. Convergence can remain effective because near vergence reaches the medial rectus by a different route, but it is not invariably intact. Myasthenia may imitate INO; consistency and associated signs matter. A left pontine VI nucleus or PPRF lesion plus left MLF lesion gives a leftward gaze palsy and failure of left adduction on right gaze. Only right abduction remains horizontally, the one-and-a-half pattern. Facial fascicles loop around VI nucleus, so ipsilateral lower-motor-neuron facial weakness can accompany it. [1][3]
A destructive left frontal eye field lesion, such as a stroke, often leaves the eyes turned left, toward the injured cortex, because rightward drive is lost. Irritation of the left field during a seizure can drive both eyes right, away from that focus. This is a conjugate cortical preference, not a fixed weakness of one lateral rectus. Test the whole neurologic examination rather than reversing lesion side from a single gaze snapshot. [1]
Afferent vision is a different map
Visual information begins in the retina. Nasal retinal fibers cross at the optic chiasm; temporal retinal fibers stay on their side. Because the image is optically reversed, a right visual-world defect in both eyes localizes to the left postchiasmal tract, radiations, or cortex. A lesion of one retina or optic nerve affects one eye; central chiasm compression preferentially removes the temporal fields in both. Formal perimetry distinguishes a real field defect from inability to aim a gaze at a target. [4]
One-eye loss is prechiasmal, bitemporal loss suggests chiasm, and a matching side of the world in both eyes points posterior to it. Compare the labeled pathways and resulting deficit. [4]
The optic tract carries contralateral field information toward the lateral geniculate nucleus. From there temporal lobe fibers sweep forward in Meyer loop carrying the contralateral upper visual quadrant. Parietal radiations carry the contralateral lower quadrant. Occipital lesions commonly produce a contralateral homonymous defect, sometimes with central vision spared; macular sparing is possible, not a required stamp of every occipital lesion. [4]
For a left-sided lesion, right superior quadrantanopia points to temporal Meyer loop; right inferior quadrantanopia points to parietal radiations. Compare the labeled pathways and resulting deficit. [4]
Decision tool. First cover an eye. If true doubling disappears with either cover, test alignment and each eye's movements. If vision itself is absent, test fields in each eye, find monocular versus bitemporal versus homonymous, then map the quadrant. For misalignment, compare one-eye with both-eye failure; then examine pupil, lid, convergence, nystagmus, gaze speed, and other neurologic findings. An acute III pattern gets urgent evaluation whatever the pupil; a VI pattern plus pressure signs should not be labeled a focal pontine lesion by default. [1][2][3][4]
Practice with clinical cases
Choose one best answer, then compare every explanation with the observed findings.
Case 1
Show answer and explanations for case 1
A. Left oculomotor nerve (Why this does not fit)
III can impair several vertical muscles and the lid. Isolated left hypertropia in right-down gaze with compensatory right tilt fits IV better.
B. Right oculomotor nerve (Why this does not fit)
III can cause complex diplopia. A right III lesion would primarily weaken the right eye, not selectively elevate the left adducted eye.
C. Left trochlear nerve (Best answer)
Left superior oblique depression in adduction is weak. The left eye is adducted on right gaze and rises when asked to descend.
D. Left abducens nerve (Why this does not fit)
A left VI lesion impairs left abduction. It would produce mainly horizontal separation on left gaze, not this adducted-eye hypertropia.
E. Right trochlear nerve (Why this does not fit)
A right IV palsy could produce tilted vertical images. It would raise the right rather than left eye in the diagnostic position.
Takeaway: Use the affected eye and its position in the H test before naming IV.
A. Visual-field testing alone (Why this does not fit)
Fields can detect afferent pathway loss. They do not address acute pupillary and motor III dysfunction with headache.
B. Ice-pack test alone (Why this does not fit)
Variable myasthenic ptosis may improve with cooling. It does not explain this poorly reactive enlarged pupil or remove aneurysm concern.
C. Outpatient glucose measurement alone (Why this does not fit)
Microvascular III palsy is possible in some patients. The severe headache and large sluggish pupil require urgent exclusion of compression.
D. Observe for several weeks without imaging (Why this does not fit)
Some ischemic III palsies recover. Waiting here could miss a symptomatic aneurysm.
E. Urgent intracranial vascular imaging and expert evaluation (Best answer)
The acute left III pattern with pupillary involvement raises aneurysm concern. A negative noncontrast CT does not examine the arterial cause adequately.
Takeaway: An acute III pattern requires urgent assessment; noncontrast CT alone is not vascular exclusion.
A right PPRF lesion impairs rapid rightward gaze. The concurrent left frontal lesion and right arm weakness make left cortical destruction the coherent site.
B. Left MLF infarction (Why this does not fit)
Left INO affects left adduction on right gaze. It does not cause a conjugate leftward resting preference.
C. Left peripheral VI palsy (Why this does not fit)
Left VI weakness impairs left abduction alone. Both eyes share a leftward preference with preserved vestibular movement.
D. Loss of left FEF drive to rightward gaze (Best answer)
A destructive left frontal eye field lesion weakens contralateral gaze drive. The intact opposite drive biases both eyes toward the left lesion.
E. Irritation of left FEF by seizure (Why this does not fit)
Left FEF excitation typically drives eyes right. The observed leftward preference is the destructive rather than irritative direction.
Takeaway: Destructive FEF lesions often point eyes toward the cortical lesion.
PPRF loss mainly affects rapid left saccades. Reflexive head rotation can bypass it, unlike this finding.
B. Right MLF alone (Why this does not fit)
Right MLF injury interrupts right adduction on left gaze. It would spare left abduction.
C. Left peripheral VI nerve (Why this does not fit)
Left VI palsy stops only left abduction. Right medial rectus adduction should still occur on left gaze.
D. Left frontal eye field alone (Why this does not fit)
FEF loss affects rightward voluntary gaze. The deficits are leftward and persist with reflexive stimulation.
E. Left VI nucleus at the facial colliculus (Best answer)
A nuclear lesion interrupts both leftward partners and can affect looping ipsilateral facial fibers. Reflex gaze failure strengthens this localization.
Takeaway: A VI nucleus lesion impairs both leftward eyes and may include VII.
