Block (IV) nerve lesion (n. trochlearis)

Symptoms of trochlear nerve damage

Acute onset of vertical diplopia without ptosis, combined with characteristic head posture, is typical of trochlear nerve injury. The manifestations of nuclear, fascicular, and peripheral trochlear nerve injury are clinically identical except that nuclear injury causes weakness of the contralateral superior oblique muscle. Left trochlear nerve injury is illustrated.

• Limited eye depression on the left during adduction due to weakness of the superior oblique muscle.
• Excyclotorsion.
• Vertical torsional diplopia, increasing when looking down.
• Hyperdeviation of the left eye ("left over right") in the primary position when fixating the intact right eye, due to weakness of the left superior oblique muscle.
• Hypertropia of the left eye is increased with rightward gaze due to hyperactivity of the left inferior oblique muscle and is minimal or absent with leftward gaze.

A forced head position is adopted to eliminate diplopia.

• When the eye is rotated (mitigating excyclotorsion), there is a contralateral head tilt.
• If the eye is unable to be lowered during adduction, the face is turned to the right and the chin is lowered.

It is impossible to look down and to the right or to rotate the left eye. This is compensated by head movements.

Bilateral lesions of the trochlear nerve are characterized by:

• Hypertropia of the right eye when looking to the left, of the left when looking to the right.
• Cyclodeviation greater than 10 in a double Maddox test.
• V-pattern ezzotropy.
• Bilateral positive Bielschowsky test.

Causes of isolated trochlear nerve damage

1. Congenital lesions are common, but symptoms may not develop until adulthood. Reviewing old photographs for abnormal head posture may be helpful, as may the fusion range increased by a vertical prism.
2. Trauma often results in bilateral involvement of the fourth cranial nerve. The long, thin nerves are vulnerable to physical impact at the tentorial margin in the superior medullary velum where they cross.
3. Vascular lesions are common, but aneurysms and tumors are rare.

Patients with damage to the trochlear nerve complain of vertical double vision, which is most pronounced when looking down and in the opposite direction. This picture is caused by unilateral paralysis of the superior oblique muscle of the eye (m. obliquus superior), which turns the eyeball outward and downward. Patients with such paralysis usually tilt their head to the side opposite the paretic muscle to reduce the sensation of double vision (less often the head is tilted to the side of the paralysis, which presumably allows the patient to more clearly distinguish the visual image on the retina of one eye and ignore it in the other). It is necessary to remember that paralysis of the superior oblique muscle can be accompanied by signs of hyperactivity and even contracture of the inferior oblique muscle. Damage to the trochlear nerve is recognized less often than damage to the III or VI nerves.

Trochlear nerve palsy can be unilateral or bilateral.

Topical diagnostics of trochlear nerve damage is possible at the following four levels:

• I. The level of the nucleus or root of the trochlear nerve (or both) in the brainstem.
• P. Level of the nerve in the subarachnoid space.
• III. The level of the trochlear nerve in the cavernous sinus.
• IV. Level of the nerve in the orbit.

I. Damage to the trochlear nerve at the level of its nucleus or root (or both) in the brainstem. In this case, paralysis of the superior oblique muscle develops contralateral to the damage.

Depending on which adjacent structures of the brainstem are involved in the pathological process, the following clinical picture may be observed:

Involvement of only one nucleus or root of the IV nerve (rare) is accompanied only by the picture of isolated damage to the trochlear nerve.

Lesions of the pretectal region result in vertical gaze palsy (dorsal midbrain syndrome). Lesions of the superior cerebellar peduncle are accompanied by dysmetria on the affected side.

Involvement of the descending sympathetic fibers is manifested by Horner's syndrome on the side of the lesion. Involvement of the posterior (medial) longitudinal fasciculus is manifested by ipsilateral paresis of the adductor muscle with nystagmus in the contralateral eyeball during its abduction.

Damage to the superior colliculus leads to a contralateral syndrome of the so-called relative afferent pupillary defect (Marcus-Gun pupil or asymmetry of the pupillary response to light; a normal direct reaction of both pupils to light is observed; with rapid alternation of illumination of one and the other eye, dilation of the pupil on the side of the brain lesion is observed when the light source moves from the healthy side to the affected one) without visual disturbances.

Damage to the anterior medullary velum is accompanied by bilateral damage to the trochlear nerve.

II. Damage to the trochlear nerve in the subarachnoid space leads to ipsilateral paralysis of the superior oblique muscle unless the mesencephalon is compressed.

Damage to only one IV nerve is accompanied only by the picture of isolated damage to the trochlear nerve.

Lesions of the superior cerebellar peduncle are accompanied by ipsilateral dysmetria.

Damage to the cerebral peduncle is accompanied by contralateral hemiparesis.

III. Lesion of the trochlear nerve in the cavernous sinus and/or superior orbital fissure

The defeat of only one IV nerve is accompanied only by the picture of isolated defeat of the trochlear nerve (rare). Involvement of III, VI cranial nerves and sympathetic fibers leads to ophthalmoplegia; the pupil can be small, wide or intact; ptosis is observed. Involvement of the V cranial nerve (first branch) is accompanied by facial or retroorbital pain, impaired sensitivity in the zone of the first branch of the trigeminal nerve. Increased venous pressure will manifest itself as proptosis (exophthalmos) and chemosis.

IV. Traumatic nerve injuries in the orbit

Damage to the trochlear nerve, superior oblique muscle or its tendon is manifested by paralysis of the superior oblique muscle.

Mechanical restriction of the superior oblique tendon results in Strongrown syndrome: a form of strabismus in which there is fibrosis and shortening of the superior oblique muscle of the eye, resulting in a characteristic limitation of eyeball movement.

Involvement of other motor nerves of the eyeballor external eye muscles leads to ophthalmoplegia, ptosis, limitation of eyeball movements. Involvement of the optic nerve is manifested by decreased vision, edema or atrophy of the optic disc. The presence of a mass effect will manifest itself as exophthalmos (sometimes enophthalmos), chemosis, and swelling of the eyelids.

The main causes of unilateral or bilateral trochlear nerve lesions are: trauma (including neurosurgical and spinal anesthesia), aplasia of the nerve nucleus, mesencephalic ischemic or hemorrhagic stroke, tumor, arteriovenous malformation, demyelination, subdural hematoma with brainstem compression, ischemic neuropathy of the fourth nerve in diabetes mellitus or other vasculopathies, Guillain-Barré syndrome (other cranial nerves are also involved), ophthalmic herpes zoster (rare), neonatal hypoxia, encephalitis, complications of cardiac surgery, space-occupying and infiltrative processes in the orbit. Rare causes of isolated paralysis of the superior oblique muscle of the eye include myasthenia gravis or dysthyroid orbitopathy.

Most injuries to the trochlear nucleus involve adjacent structures. Ipsilateral cerebellar signs are common. Involvement of the trochlear nucleus or its root causes contralateral superior oblique palsy. Unilateral injury to the trochlear nucleus or root before its decussation in the anterior medullary velum with sympathetic fibers may cause ipsilateral Horner's syndrome and contralateral superior oblique palsy. Unilateral mesencephalic injury to the trochlear nucleus (or its fibers before the decussation) and the medial longitudinal fasciculus may cause ipsilateral internuclear ophthalmoplegia and contralateral superior oblique palsy. Injury involving the superior colliculus and the pretrochlear nucleus or root may cause a contralateral relative afferent pupillary defect without visual disturbances and contralateral superior oblique palsy. Bilateral paresis of the superior oblique muscle with symptoms of spinothalamic tract involvement on one side has been described with a small spontaneous hemorrhage in the mesencephalic tegmentum area.

Isolated myokymia obliquus superior usually has a benign course (but has also been described as a sign of damage to the midbrain tegmentum) and is not accompanied by symptoms of paralysis of this muscle.

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