Overview
The fourth cranial nerve (CNIV) is known as the trochlear nerve. It innervates the superior oblique muscle which is necessary to depress the eyeball and move it inwards.
The trochlear nerve is unique as it decussates within the brainstem before emerging. Therefore, a fourth nerve palsy affecting it after it emerges from the brainstem causes an ipsilateral problem. A central lesion affecting the trochlear nucleus (pre-decussation) causes a contralateral lesion.
Features
- Vertical diplopia – patients often notice this when going downstairs or reading a book
- Patients may have a head tilt to compensate – they may not be aware of this
- The eye is “up and outwards”
Causes
- Orbital trauma
- Diabetic neuropathy
- Hypertension
