Cranial Nerve Palsies That Cause Diplopia
Diplopia (double vision) is most commonly caused by cranial nerve palsies affecting the 3rd (oculomotor), 4th (trochlear), and 6th (abducens) cranial nerves, which control eye movement and alignment. 1
Primary Causes of Diplopia by Cranial Nerve
3rd Cranial Nerve (Oculomotor) Palsy
- Second most common cranial neuropathy causing diplopia 1
- Presents with:
- Horizontal and vertical misalignment components
- Ptosis (drooping eyelid) which may partially mask diplopia
- Eye positioned in "down and out" position (abducted and infraducted)
- Possible pupil involvement (pupil-sparing vs. pupil-involving)
- Accommodative deficiency causing reading difficulties 1
4th Cranial Nerve (Trochlear) Palsy
- Controls the superior oblique muscle
- Most commonly caused by trauma 1
- Results in vertical diplopia that worsens with downward gaze
- Patients often tilt their head away from the affected side to compensate
6th Cranial Nerve (Abducens) Palsy
- Controls the lateral rectus muscle
- Causes horizontal diplopia that worsens with gaze toward the affected side
- May be caused by lesions in prepontine cistern, skull base, cavernous sinus, or sella 1
- Can occur with increased intracranial pressure without direct nerve compression 1
Clinical Presentation and Localization
The pattern of ocular misalignment helps localize the affected nerve:
- 3rd nerve palsy: Eye appears "down and out" due to unopposed action of the lateral rectus and superior oblique muscles
- 4th nerve palsy: Vertical diplopia, often with compensatory head tilt
- 6th nerve palsy: Inability to abduct the affected eye, causing horizontal diplopia
Diagnostic Considerations
Important Clinical Features
- Pupil involvement in 3rd nerve palsy suggests compression (often aneurysm) vs. vasculopathic etiology when pupil-sparing 1
- Multiple ipsilateral cranial nerve palsies (3rd, 4th, and 6th) suggest cavernous sinus or orbital apex lesion 1
- Internuclear ophthalmoplegia indicates a brainstem lesion affecting the medial longitudinal fasciculus 1
Imaging Recommendations
- MRI of brain/orbits with contrast is preferred for suspected orbital or brainstem pathology 1
- Vascular imaging (CTA or MRA) is indicated when compression from aneurysm is suspected, especially with pupil-involving 3rd nerve palsy 1
Management Approach
Treatment goals include:
- Identifying and treating the underlying cause
- Eliminating diplopia in primary position
- Creating a reasonable field of single binocular vision 1
Treatment Options
- Address underlying cause (vascular, inflammatory, compressive)
- Temporary measures: patching, prism glasses
- Botulinum toxin injections for temporary relief
- Strabismus surgery for persistent cases
Important Caveats
- Patients should be advised that even with treatment, diplopia may persist in extreme lateral gaze positions 1
- Acute onset diplopia may indicate posterior circulation stroke and requires urgent evaluation 1
- Neuroimaging is recommended for:
- All young patients
- Patients with other neurological symptoms
- Cases without clear vasculopathic risk factors
- Elderly patients with persistent symptoms 1
Remember that accurate diagnosis of which cranial nerve is affected is the critical first step in evaluation and management of diplopia, as it guides both workup and treatment decisions.