Fourth Nerve Palsies

Common cause of vertical strabismus

Anatomy nuggets

Nucleus lies just beneath the IIIn nucleus in the midbrain at the level of the inferior colliculus
Decussates and exits the midbrain posteriorly then curves around to travel anteriorly
Travels in the lateral wall of the cavernous sinus inferolateral to the IIIn
Enters the orbit through the SOF but outside of the annulus

Aetiology

1/3rd are congenital but may manifest in adulthood
Acquired:
- Traumatic (especially bilateral): fourth nerve palsy is the most common nerve palsy following closed head injury
- Microvascular
- Idiopathic
- Iatrogenic (eg ENT or neurosurgery)
- Tumour eg pinealoma
- Demyelination
- Vasculitis
- Meningitis
- Cavernous sinus lesion
- Tolosa-Hunt syndrome
- HZO

Features of congenital fourth

Large vertical fusion range (10-15 PD)
High concomitance (may also be present in longstanding acquired lesions)
Head tilt on old photos
Facial asymmetry

Clinical features

Vertical and torsional diplopia
Head tilt away from the side of the lesion
Aesthenopia (eye strain)
Ipsilateral hypertropia/phoria exacerbated on downgaze or head tilttowardsthe side of the lesion
Reduced depression especially on adduction
Extorsion: can be seen on fundoscopy compared to fellow eye (eg. consider fundus photos)
Parks-Bielschowsky 3-step test
- 1. Cover test: identify higher eye
- 2. Cover test with gaze to the right, then left: identify where deviation/diplopia is greatest
  - Ie. if greatest in adduction, then obliques involved; if greatest in abduction, then vertical recti involved
  - Step 2: tells you whether superior or inferior muscles
- 3. Cover test with head tilt to the right, then left: identify where deviation is greatest
  - Superior muscles cause intorsion, while inferior muscles extort
  - Step 3: tells you which eye

Hot Topic

Bilateral fourth nerve palsies (eg. traumatic or congenital): reversing hypertropia

R/L on left gaze and L/R on right gaze
V patternesotropia
Chin down head posture
>10 degrees of excyclotorsion
Failure of adduction on depression bilaterally

Tests

Check old photos for abnormal head posture
Vascular risk factors (including arteritis)
Consider neuroimaging if unclear cause or no recovery after 3 months
Orthoptists monitoring including Hess charts
Double maddox rod: measure cyclotorsion

Hot Topic

Vertical fusion amplitudes: in congenital fourth, the misalignment from birth means patients are often able to fuse vertical deviations much larger than the normal 2-3 dioptres. Measuring these can be helpful in identifying a longstanding congenital fourth that may have recently decompensated.

Management

Conservative: management of symptomatic diplopia with prisms, occlusion, CL
Botox
Muscle surgery:
- Harada-Ito for excyclotorsion
- Inferior oblique weakening (eg. myectomy, recession) for IOOA