Bilateral 4th Nerve Palsy (Trochlear Nerve Palsy)

Question 1

What cranial nerve has the longest intracranial course?

Answer 1

Smallest of the cranial nerves (in terms of number of axons) but with the longest intracranial course.

Question 2

Where does the trochlear nerve begin?

Answer 2

The trochlear nucleus from the posterior part of the midbrain

Question 3

How does the trochlear nucleus run?

Answer 3

It then runs anteriorly and inferiorly in subarachnoid space.

Pierces the dura mater within the sphenoid bone at the inferior margin of the tentorium cerebelli.

Runs along the lateral wall of the cavernous sinus alongside CN3, CN6, CN5 V1 and V2 and with the internal carotid artery.

It enters the eye via the superior orbital fissure external to the common tendinous ring (remember SNIA)

Question 4

How does the trochlear nerve attach?

Answer 4

Attaches to the SO tendon on the contralateral side of the associated nucleus. It ‘fans’ out making a curved insertion.

Question 5

How does the trochlear nerve insert?

Answer 5

The 2 insertions it has are in the anterior portion which is aligned transversely and gives the muscle its torsional action (to intort the globe) and the posterior portion which is parallel to the anteroposterior axis and gives the muscle its depressing and abducting action.

Question 6

Where does the trochlear nerve exit?

Answer 6

Dorsal exit from the brainstem

Question 7

Common aetiologies for bilateral 4th nerve palsies?

Answer 7

• Trauma
  Frontal head trauma including whiplash and concussions (Burger et al., 1970) found that frontolateral blows were associated with unilateral trochlear nerve palsy while mid-frontal blows were more likely to result in bilateral palsy). Damage to the cavernous sinus and SOF can caused 4th nerve palsies.
• Vascular Disease
  Diabetes or vascular disease can cause poor blood flow to the trochlear nerve bilaterally
• Aneurysms or Increased Intracranial Pressure or brain/eye tumours
  Bulging arteries can compress the nerve in the same way that increased pressure in the skull can also
• Stroke
• Multiple Sclerosis (MS)
• Congenital
• Idiopathic

Question 8

What are the muscle actions with CNIV?

Answer 8

The trochlear nerve has a purely a somatic motor function to the superior oblique muscle on the contralateral side of the associated nucleus. This allows for the eye to depress, intort and abduct.

Question 9

What are the symptoms of bilateral 4th nerve palsy?

Answer 9

• Double vision
• One iris higher than the other
• Tilting the head to compensate (more common in unilateral cases) and
• Pain above the eyebrow
• Hypertropia, Excyclotorsion and Esotropia

Question 10

When should Bilateral superior oblique palsy be suspected?

Answer 10

• Alternating hypertropia in gazes and tilts (characterized by an ipsilateral hypertropia that manifests on contralateral gaze)
• Absent or small hypertropia in primary gaze
• Positive Bielschowsky head tilt test to either shoulder
• Underaction of both superior obliques on duction testing
• Objective torsion more than 10 degrees (expected to increase in downgaze),
• V- pattern esotropia (greater than 25 prism dioptres)
• Bilateral fundus torsion,
• Chin-down head posture.
  (Mantopoulos, Hunter & Cestari, 2011)-

Question 11

What might a bilateral superior oblique paresis be misdiagnosed as?

Answer 11

A case of bilateral superior oblique paresis may be misdiagnosed as unilateral paresis (masked bilateral superior oblique paresis; Ellis, Stein & Guyton, 1998)

Question 12

What is AHP also known as?

Answer 12

Torticollis

Question 13

What AHP will someone with a bilateral 4th have?

Answer 13

To compensate for the diplopia, the patient may adopt a head tilt and face turn to the opposite side. Sometimes they turn to the same side (paradoxical AHP) if the diplopic image can be pushed further away to be out of their line of sight. This however is more common in unilateral cases of 4th nerve palsy whereas in bilateral it’s more common to see a chin-down position. This is because they often won’t have BSV in a tilted position where bilateral, however, if they have an asymmetrical presentation they may develop a tilt to the milder side of the oblique palsy.

Question 14

What AHP is usually seen in unilateral CNIV palsy compared to bilateral cases?

Answer 14

Patients with congenital CN IV palsies may compensate for diplopia with variable head positioning; chin-down head posture is seen in bilateral CN IV palsy and contralateral head tilt is typically seen in unilateral CN IV palsy.

Question 15

What will we see on cover test in bilateral 4th nerve palsy?

Answer 15

• Alternating hypertropia on horizontal gaze or tilt but small or absent in primary position
• Positive Bielschowsky head tilt test to either shoulder
• Large degree of excyclotorsion (when measuring will be >10 degrees)

Question 16

What is the Parks-Bielschowsky Three-Step Test used for?

Answer 16

• Step 1
  Which eye appears to be hypertropic in primary? If there is right hypertropia in primary position, then the depressors of the R eye (IR/SO) or the elevators of the L eye are weak (SR/IO).
• Step 2
  Does the hypertropia increase in right or left gaze? The recti muscles have the highest vertical action in abduction whereas the obliques are greatest in adduction
• Step 3
  Does the hypertropia increase in right or left head tilt. During right head tilt, the right eye intorts (SO/SR) and the left eye extorts (IO/IR).

Question 17

How is a bilateral CNIV palsy seen on a Parks-Bielschowsky 3-Step Test?

Answer 17

There are specific suggestive features of a bilateral fourth nerve palsy such as alternating hypertropia on horizontal gaze or tilt as well as positive bielschowsky head tilt test to either shoulder( i.e., a right hypertropia in left gaze and right tilt that changes to a left hypertropia in right gaze and left tilt). This is because the superior rectus muscle’s actions aren’t counteracted by the superior oblique muscles.

Question 18

What are the ocular movement findings in a bilateral CNIV palsy?

Answer 18

• Underaction of both superior obliques
• V-pattern ET >25PD
• Reversing hypertropia on left and right gaze
• Underaction of the superior oblique in depression while adducting
• Overaction of the inferior oblique in elevation while adducting

Question 19

What is the muscle sequelae pattern in a bilateral 4th nerve palsy?

Answer 19

• u/a RSO, o/a LIR, o/a RIO, u/a LSR
• u/a LSO, o/a RIR, o/a LIO, u/a RSR

Question 20

What do we see in terms of hyper deviations in bilateral CNIV palsy?

Hint: think symmetrical and asymmetrical

Answer 20

If symmetrical = don’t see hyper deviations as much in pp

If asymmetrical = can see hyper deviation in pp
can also see an alternating hyper deviation e.g. when looking to LHS = R eye is higher and looking to RHS = L eye is higher, because of the IO overactions

Question 21

What is the field of BSV in a bilateral CNIV palsy?

Answer 21

This means that a patient can have worsening diplopia when:

• Tilting their head either side (due to bilateral palsy hypertropia will reverse with either shoulder head tilt),
• In dextroelevation or laevoelevation
• In adduction (as bilateral this will be present in both dextroversion and laevoversion) with a height reversal
• In dextro and laevo depression
• We may be expecting a hypertropia in primary position, however, the two eyes counterbalance each other in a bilateral 4th nerve palsy as in LSO palsy we expect a LHT but in a RSO we expect a RHT so unless asymmetrical in nature we don’t expect a hypertropia in primary but expect complaints of torsional diplopia. They’ll choose to fix with either eye at any time so may see a small HT. If asymmetrical we will see a hyper deviation in pp.

Question 22

What do we see on a Hess Chart in cases of bilateral CNIV palsy?

Answer 22

Give very typical Hess fields where you can see the “tipping” of the fields from the two SO underactions.

Question 23

What is a masked bilateral CNIV palsy?

Answer 23

A masked bilateral fourth nerve palsy is when the palsy mimics a unilateral palsy, typically the bilateral nature is only revealed after surgery. This is in the case of asymmetrical 4th nerve palsies where there is a relatively large palsy one side and a milder palsy on the other.

Question 24

Why are masked bilateral CNIV palsies hard to identify?

Answer 24

Because they make seeing the reversal of height difficult to see in OM’s and to see the reversal of height in the head tilt test and so it’s often better to assume a bilateral 4th until otherwise proven, particularly in the cases of trauma.

Question 25

When should we suspect a bilateral CNIV palsy?

Answer 25

In masked bilateral SO palsy then we need to be suspicious if there is a NEAR reversal. This will be due to asymmetry so the more minimally involved side may be masked.