Ocular Motility Examination — Incomitant/Vertical Strabismus Tests

Parks-Bielschowsky Three-Step Test

When a patient reports vertical diplopia and cover testing confirms a hypertropia, the Parks-Bielschowsky three-step test can isolate the single paretic cyclovertical muscle responsible. It works through three sequential questions.

• Step 1: which eye is hyper in primary gaze? This narrows the culprit to four possible muscles: the two depressors of the hypertropic eye (superior oblique, inferior rectus) or the two elevators of the hypotropic eye (superior rectus, inferior oblique).
• Step 2: is the deviation worse in right gaze or left gaze? This narrows the four candidates from Step 1 to two.
• Step 3 (the Bielschowsky head tilt test): is the deviation worse with the head tilted right or left? This final step isolates the single paretic muscle responsible.

Superior Oblique (CN IV) Palsy

Superior oblique palsy is the diagnosis the three-step test is built around, and it's the most common cause of an isolated cyclovertical muscle palsy. In right superior oblique palsy, cover testing in primary position shows a right hypertropia. The superior oblique depresses and intorts the adducted eye, so weakness there lets the affected eye drift upward. The hypertropia worsens in contralateral gaze (left gaze, for a right SO palsy) and with ipsilateral head tilt.

The Bielschowsky head tilt test is this third step performed in isolation. Tilting the head toward the affected side removes the compensatory intorsion the now-weak superior oblique normally supplies, unmasking or worsening the hypertropia through the unopposed action of the superior rectus. A vertical deviation that worsens with ipsilateral head tilt is classically diagnostic of superior oblique palsy.

The three-step test assumes the vertical deviation comes from a weak muscle, but a mechanically restricted one can mimic the same pattern. Forced duction testing tells the two apart. After topical anesthesia, the examiner grasps the conjunctiva near the muscle insertion with forceps and manually rotates the globe. Free rotation means the muscle moves normally once passive resistance is removed, pointing to a neurogenic palsy. Resistance to rotation means something is physically tethering the muscle, as with an inferior rectus trapped in an orbital floor fracture, and that changes management toward surgical release rather than watching for nerve recovery. Active force generation testing adds a second check: the patient tries to move the eye into the suspected muscle's field against the examiner's resisting forceps, and a muscle too weak to generate normal force confirms paresis even when the duction itself was free.

Maddox Rod, Hess Screen, and Worth 4-Dot Testing

The Maddox rod uses a grooved glass rod to turn a point light source into a perceived red streak in one eye. The brain cannot fuse that line percept with the point of light the other eye sees, so vision dissociates completely between the two eyes. The patient reports where the streak falls relative to the dot, and the displacement gives the direction and magnitude of the deviation. It measures ocular muscle balance (heterophoria), not visual acuity or color vision.

The Hess screen test plots ocular alignment across the nine diagnostic positions of gaze using red and green dissociating goggles on a tangent screen. It produces two separate charts, one per eye. The smaller chart marks the eye with the paretic or restricted muscle, since its plotted points cluster closer together toward the direction that weak muscle should be pulling. Because the two eyes' charts can be compared directly from one visit to the next, the test is as useful for following a known palsy over time as it is for the initial diagnosis, whether the underlying process is a cranial nerve palsy or a restrictive one like orbital blowout fracture or thyroid eye disease.

The Worth 4-dot test assesses sensory fusion and suppression rather than motor alignment, using red-green glasses and a target of four lights: one red, two green, one white. With a red filter over one eye and a green filter over the other, a patient with normal binocular fusion sees all four lights (the red light, both green lights, and the white light, which takes on the color of either filter). Seeing only 2 lights means the eye behind the green filter is suppressed. Seeing 3 lights means the eye behind the red filter is suppressed. Seeing 5 lights, the white light split into separate red and green images, means diplopia with no fusion at all.

A and V Pattern Strabismus

Pattern strabismus is a horizontal deviation whose magnitude changes between upgaze and downgaze. In V-pattern esotropia (more esodeviation in downgaze than upgaze), the usual cause is bilateral inferior oblique overaction. The inferior oblique both abducts and elevates the eye, so its increased pull in upgaze produces an exotropic shift that partially cancels the esodeviation there, leaving the deviation worse in downgaze, where that effect is absent. Bilateral superior oblique overaction produces the opposite pattern: an A-pattern esotropia.

INO, Duane Syndrome, and Ocular Tilt Reaction

Internuclear ophthalmoplegia (INO) results from a lesion of the medial longitudinal fasciculus (MLF), the tract connecting the CN VI nucleus (which drives ipsilateral abduction) to the contralateral CN III nucleus (which drives adduction). It's diagnosed clinically by impaired adduction of one eye on lateral gaze, paired with dissociated (abducting) nystagmus in the contralateral, abducting eye. The adduction deficit localizes the MLF lesion to the side with the weak adduction.

Duane retraction syndrome type 1, the most common subtype, results from an absent CN VI nucleus with aberrant CN III innervation of the lateral rectus. That aberrant innervation causes co-contraction of the medial and lateral rectus on attempted adduction, producing globe retraction and lid-fissure narrowing. Abduction is limited because the lateral rectus lacks proper CN VI innervation, while adduction is relatively preserved, just accompanied by the retraction.

The ocular tilt reaction (OTR) is a triad of head tilt, skew deviation (vertical misalignment), and bilateral cyclorotation. It's caused by a lesion in the otolith-ocular pathway: asymmetric input from the utricle travels via cranial nerve VIII to the brainstem vestibular nuclei, then through the MLF to the trochlear and oculomotor nuclei that control cyclorotation. OTR can mimic an isolated superior oblique palsy on cursory exam, so the full triad needs recognizing, not just the vertical misalignment, or a brainstem or cerebellar problem gets misattributed to an isolated cranial nerve palsy.

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