Ocular movements are controlled by cranial nerves. Lesions of these nerves result in characteristic clinical presentations which a commonly tested in exams.
CN3 Lesions
CN3 lesions are categorised as medical or surgical. Medical lesions characteristically spare the pupil (i.e pupil reflexes are intact and there is no anisometropia).
Pathology
Medical
- Much more common than surgical type
- Typically caused by microvascular complications of diabetes and hypertension
- The pupil will react to light
Surgical
- Characteristic surgical causes include posterior communicating artery aneurysm and uncal hernia
- The pupil will not react to light
The pupil is affected in surgical causes because they violently compress the pupillomotor fibres which run within CN3. Microvascular problems are unlikely to affect these superficial fibres because of the rich blood supply via pial vessels. Hence the pupil reflexes are spared in medical CN3 lesions.
Diagnostics
Presentation
- Ptosis
- Ophthalmoplegia with only abduction preserved
- Down and out eye
- Dilated pupil (in surgical type)
Investigation
- Urgent CT angiography in acute surgical third nerve palsies to rule out posterior communicating artery aneurysm
- Medical cases should still get neuroimaging and workup for vascular risk factors, but this is far less serious in comparison.
Weber's stroke syndrome is: CN3 palsy + contralateral hemiparesis
CN4 Lesions
CN4 lesions are typically either congenital, traumatic or microvascular (most common)
Diagnostics
Presentation
- Patients present with vertical diplopia and hypertropia (superior displacement of the eye)
Parks Test
Park-Bielschowsky 3 step test is used to identify CN4 palsy
- Find the hypertropic eye in the primary position
- Evaluate how the diplopia changes with horizontal gaze direction
- Image separation is worse in contralateral gaze direction
- I.e if the left eye is affected, diplopia is worse on right gaze
- Evaluate how diplopia changes with head tilt
- Image separation is worse on ipsilateral head tilt
- I.e if the left eye is affected, diplopia is worse on left head tilt.
CN6 Lesions
The most common cause is microvascular disease resulting in nerve ischemia.
Diagnostics
Presentation
- Presents with horizontal diplopia and esotropia (the affected eye is deviated inwards).
- Abduction is severely limited
Gradenigo Syndrome
- A classic syndrome of otitis + CN6 palsy
- CN6 travels very close to the auditory structures so an infection in the ear can result in CN6 palsy.
- Characteristically caused by pathology at the petrous apex of the temporal bone
