In a 29-year-old patient with vertical diplopia that worsens when reading and descending stairs, and a right eye hypertropia that increases on left gaze and with right head tilt, what is the most likely diagnosis?

Right CN IV Palsy (Answer: B)

This patient has a right superior oblique palsy (CN IV palsy), confirmed by the classic Parks-Bielschowsky three-step test: right hypertropia that worsens with left gaze and right head tilt 1.

Clinical Reasoning

The presentation is pathognomonic for right fourth nerve palsy:

• Vertical diplopia worsening with reading and descending stairs is the hallmark complaint of superior oblique dysfunction, as the superior oblique is critical for depression in adduction (looking down and in) 1

• The three-step test confirms the diagnosis algorithmically:

  1. Right hypertropia in primary position
  2. Hypertropia increases in left gaze (opposite side) - the superior oblique depresses the eye maximally when the eye is adducted
  3. Hypertropia increases with right head tilt (same side) - due to loss of intorsion from the paretic superior oblique 1

Why Not the Other Options?

• CN III palsy (A): Would cause hypotropia (not hypertropia) of the affected eye, with associated ptosis, pupil involvement (if complete), and limitation of adduction, elevation, and depression 1

• CN VI palsy (C or D): Causes horizontal diplopia with esotropia and abduction deficit, not vertical diplopia 2

Diagnostic Considerations

At age 29, determine if this is congenital (decompensated) versus acquired 1:

• Review old photographs for compensatory head tilt (suggests long-standing congenital palsy)
• Measure torsion with double Maddox rods - subjective torsion often less than objective in congenital cases 1
• Neuroimaging with MRI/MRA is indicated for acquired cases without trauma, as this can herald trochlear schwannoma, demyelinating disease, or other compressive lesions 1, 3

Common pitfall: Occult thyroid eye disease can mimic superior oblique palsy with a positive three-step test 1. However, TED typically shows restriction on forced ductions and affects older patients.

Management Approach

• Observation if symptoms are mild and not bothersome 1
• Prism correction for small deviations, though lateral incomitance often limits success 1
• Surgery for larger/symptomatic deviations after 6-12 months of stability: inferior oblique weakening (ipsilateral) is most common for deviations ≤15 PD 1, 4