Trochlear Nerve Palsy (Fourth Cranial Nerve Palsy) manifests as an isolated vertical, diagonal, or cyclo-torsional diplopia (double vision). It is worse on looking down and to the side opposite the lesion. In addition to the defect in eye movement, the affected eye produces characteristic head tilt away from the affected side. Fourth cranial nerve palsies are relatively uncommon. Isolated fourth cranial nerve palsy is the most common cause producing vertical diplopia. Trauma is the usual trigger of trochlear nerve palsy. In adults, isolated fourth cranial nerve palsy may occur due to infarction of peripheral trochlear nerve or because of monocular visual deprivation due to decompensation of asymptomatic congenital nerve palsy.
Trochlear nerve supplies superior oblique muscles of both eyes, and it produces depression, incycloduction, and abduction. The extraocular muscles rotate eyeball around vertical, antero-posterior, and horizontal axis.
Trochlear nerve may be affected in its intracranial course in diseases such as
Closed head injury is the most common cause of acquired trochlear nerve palsy.
Involvement of trochlear nerve is less common than third and sixth cranial nerves in intracranial mass lesions.
Bilateral trochlear nerve palsy causes inability to depress either eye fully in adduction.
Knapp P. Classification and treatment of superior oblique palsy. Am Orthopt J. 1974. 24:18-22.
Mitchell PR, Parks MM. Surgery of bilateral superior oblique palsy. Ophthalmology. 1982. 89(5):484-8.
Patients with isolated trochlear nerve palsy may be asymptomatic.
The Causes of trochlear nerve palsy may be
Virtually all microvascular cases resolve even without treatment.
Diagnosis depends upon the clinical features and the clinical tests.
Trochlear nerve palsy typically produces diplopia, which is worse in downgaze. Because of diplopia in downgaze, patients tend to close one eye while reading.
Some cases reveal limited downgaze in adducted position. However, ocular movements are grossly normal in most of the cases.
Ipsilateral head tilting increases vertical strabismus, and thus, diplopia. Therefore, patients tilt their head on the opposite side to avoid diplopia.
Step 1: Find the side of hypertropia in the primary gaze. For example, if the right eye is higher, the weakness lies either in the muscles depressing right eye (right superior oblique muscle and right inferior rectus muscle) or in the elevators of left eye (left superior rectus muscle and left inferior oblique muscle).
Step 2: Determine which gaze, left or right, the hypertropia is more. If the right eye deviates most when the head is turned to the right (both eyes turn to the left), then only right superior oblique muscle or left superior rectus muscle is affected.
Step 3: Determine which side the head tilt, left or right, the hypertropia is more. This test relies on the torsional balancing reflexes provoked by head tilt. The higher eye extorts (because of inferior oblique muscle), while the lower eye intorts (because of superior oblique muscle).
Since superior oblique muscle is by far most commonly responsible for vertical diplopia, a head tilt to the same side as the involved muscle exacerbates it. The eye that is highest in adduction looks at the affected muscle.
Bilateral trochlear nerve palsy should be considered as well, whenever unilateral palsy is diagnosed, especially following head injury.
Bilateral trochlear nerve palsy presents with
In addition, a fourth step, is also useful. The fourth step determines whether the vertical separation is greater in downgaze or upgaze, and also check for relative cyclotropia.
Trochlear nerve palsy may be differentiated from conditions like
Knapp classified superior oblique palsy cases by noting field of gaze in which deviation was greatest. Based on this, he recommended surgery on muscles that acted in the direction of gaze.
For cases with inferior oblique over-action, it may be weakened by tenotomy, recession, or disinsertion.
In cases with ipsilateral superior rectus restriction, a superior rectus recession is done.
Superior oblique tendon tuck is done, if there is significant laxity of the tendon, as is described in congenital cases.
Cases with no superior rectus restriction or superior oblique tendon laxity, with greatest deviation in downgaze, may undergo contralateral inferior rectus recession.
Patients with bilateral superior oblique palsy, especially with large ex-cyclotorsion, may undergo surgery for both superior oblique muscles.
Prognosis of Trochlear Nerve palsy depends upon the underlying aetiology.
Congenital nerve palsies often remain static for long. Idiopathic, traumatic or ischaemic palsies usually resolve with time.