Diplopia with Horizontal vs Vertical Image Separation: Cranial Nerve Localization
Horizontal diplopia (side-by-side image separation) indicates involvement of cranial nerves III (oculomotor), IV (trochlear), or VI (abducens), while vertical diplopia (up-and-down image separation) primarily suggests CN III or CN IV pathology, or skew deviation from brainstem/vestibular lesions. 1
Horizontal Diplopia: CN VI (Abducens) Most Common
CN VI palsy is the most frequent cause of isolated horizontal diplopia in adults, presenting with:
- Side-by-side image separation worse at distance than near 1
- Worsening when looking toward the affected side 1
- Incomitant esotropia (eye turned inward) due to unopposed medial rectus action 1, 2
- No ptosis, no pupillary abnormalities, no vertical eye movement deficits 2
The majority of acute CN VI palsies are vasculopathic (diabetes, hypertension), with most resolving within 6 months 1. Other causes include trauma, neoplasm, increased intracranial pressure, and demyelinating disease 1.
Vertical Diplopia: CN III or CN IV Involvement
CN III (oculomotor) palsy causes:
- Limited adduction, elevation, and depression of the eye 2
- Ptosis (drooping eyelid) - a distinguishing feature from CN VI palsy 2
- "Down and out" eye position due to unopposed CN VI and CN IV action 2
- Possible pupillary dilation if parasympathetic fibers involved 1
CN IV (trochlear) palsy produces:
Skew Deviation: Brainstem/Vestibular Pathology
Skew deviation presents with vertical misalignment but is not a cranial nerve palsy - it results from disruption of supranuclear pathways in the brainstem or vestibular system 1. Key features include:
- Vertical diplopia with head tilt 1
- Rostral brainstem/midbrain lesions cause contralateral hypotropia 1
- Vestibular/medullary lesions cause ipsilateral hypotropia 1
Critical Diagnostic Algorithm
Step 1: Determine direction of diplopia
- Horizontal → Consider CN VI primarily, also CN III 1
- Vertical → Consider CN III, CN IV, or skew deviation 1
Step 2: Check for ptosis
- Ptosis present → CN III palsy 2
- No ptosis with horizontal diplopia → CN VI palsy 2
- No ptosis with vertical diplopia → CN IV or skew deviation 1
Step 3: Assess pupil involvement
- Pupil-involving CN III palsy → Suspect aneurysmal compression, requires urgent vascular imaging (CTA/MRA) 1
- Pupil-sparing CN III palsy → Vasculopathic etiology more likely 1
Step 4: Evaluate for associated neurologic signs
- Facial weakness with CN VI palsy → Pontine lesion (CN VII curves over CN VI nucleus) 1
- Multiple cranial nerve involvement → Cavernous sinus or skull base pathology 1
- Contralateral hemiparesis with CN VI → Pontine lesion affecting corticospinal tracts 1
Imaging Recommendations
For horizontal diplopia (suspected CN VI palsy):
- MRI head with contrast including high-resolution T2 sequences of cranial nerves from brainstem nuclei through cavernous sinus 1
- Evaluate entire nerve pathway: nucleus (pons) → cisternal segment → cavernous sinus → orbit 1
For vertical diplopia with pupil involvement (suspected CN III with compression):
- CTA or MRA to exclude aneurysm as primary consideration 1
- MRI head with contrast as complementary anatomic assessment 1
Common Pitfalls to Avoid
- Do not assume all horizontal diplopia is CN VI palsy - CN III palsy also causes horizontal misalignment due to impaired adduction 2
- Always check for ptosis - its presence definitively indicates CN III involvement rather than CN VI 2
- Do not miss giant cell arteritis in elderly patients with scalp tenderness or jaw claudication presenting with CN VI palsy, as this requires urgent treatment to prevent permanent vision loss 1
- Bilateral CN VI palsy suggests increased intracranial pressure, clival tumor, or meningeal process rather than isolated vasculopathic etiology 1
- If no recovery by 6 months, approximately 40% have serious underlying pathology requiring further workup 1