Trochlear Nerve Palsy
- Author: Zafar A Sheik, MD, MD; Chief Editor: Andrew G Lee, MD more...
Trochlear nerve palsy is mentioned in ophthalmology texts dating to the mid nineteenth century. However, it received little more than a brief mention and was no doubt an underrecognized entity. In 1935, Bielschowsky correctly noted that trochlear nerve palsy was the most common cause of vertical diplopia and introduced his classic head-tilt test. With greater clinical interest, the number of identified fourth nerve palsies has increased.
A fourth nerve palsy is a common cause of binocular vertical diplopia in isolation.
The fourth cranial nerve exits dorsally and has the longest intracranial course.
An isolated fourth cranial nerve palsy can be diagnosed using the three-step test.
The primary action of the superior oblique muscle is intorsion.
History of the Procedure
Surgical therapy for this condition has been refined over the last 30 years. The introduction of the Harada-Ito procedure in the 1960s and Knapp's surgical approach in the 1970s enhanced the ability to successfully treat this challenging clinical entity.
The fourth cranial nerve innervates superior oblique muscle, which intorts, depresses, and abducts the globe. Fourth nerve palsy can be congenital or acquired, unilateral or bilateral, each of which presents with a distinct clinical picture. Clinicians must carefully assess the patient to determine both etiology and extent of disease. Acquired weakness of this muscle usually leads to complaints of vertical diplopia, sometimes with a torsional component. Surgery may be required to treat these patients. Thorough assessment and careful preoperative planning maximize the chances of a successful surgical outcome.
Estimating the true frequency of congenital fourth nerve palsy is difficult. Many patients compensate with use of head-tilt or large fusional amplitudes; therefore, it may not present to an ophthalmologist until adulthood, when their fusional control begins to deteriorate.
Some of the best information regarding the incidence of acquired fourth nerve palsy can be found in the Mayo Clinic series. Several studies, performed over the last 4 decades, reported the incidence and etiology of acquired cranial nerve palsies in adult and pediatric patients. Trochlear nerve palsy was less common than abducens or oculomotor palsies. Of 4,373 acquired cases of extraocular muscle palsy in adults, there were only 657 cases of isolated fourth nerve disease. Fourth nerve palsy was also the least frequent in pediatric population. In a similar Mayo Clinic study of 160 children, 19 of them had isolated fourth nerve palsy.[5, 6]
The underlying etiology of congenital disease remains obscure; there is debate as to whether it is the result of dysgenesis of fourth nerve nucleus or from abnormal development of peripheral nerve or tendon.
The most common cause of acquired isolated fourth nerve palsy, after idiopathic, is head trauma.[7, 8, 9]
Generally, trauma must be severe with resultant loss of consciousness. However, in some cases, the trauma is less severe and unassociated with loss of consciousness.
One must consider the possibility of underlying structural abnormalities (eg, skull based tumor) if fourth nerve palsy results after only minor trauma.
There are rare reports of thyroid ophthalmopathy and myasthenia gravis mimicking an isolated fourth nerve palsy. These patients eventually develop other findings, unmasking the underlying diagnosis.
Fourth nerve palsy may become manifest after cataract surgery. Patients with underlying, well-controlled, and asymptomatic fourth nerve palsy may decompensate gradually as they lose binocular function resulting from cataract. Following restoration of good vision, these patients become aware of diplopia.
Congenital Trochlear Nerve Palsy
Whether congenital fourth nerve palsy is secondary to dysgenesis of fourth nerve nucleus or abnormalities of peripheral nerve is unclear. Patients with congenital disease are likely to have abnormal superior oblique muscle or tendon as well and a compensatory head tilt.
See the image below.
Helveston, in a series of 36 congenital superior oblique palsy patients, found 33 abnormal superior oblique tendons. The tendon may be abnormally lax, have an abnormal insertion, or be absent altogether.
Acquired Trochlear Nerve Palsy
The long course of the trochlear nerve makes it especially susceptible to injury in association with severe head trauma. Contrecoup forces can compress the nerve against the rigid tentorium, which lies adjacent to the nerve for much of its course. Injury to nerve can occur anywhere along its course from midbrain to orbit. Lesions at the nucleus cause contralateral superior oblique palsy, since the nerve decussates at anterior medullary velum, caudal to inferior colliculus. Midbrain trauma can produce bilateral superior oblique palsy by contusive injury of decussation of nerves. Compression or ischemia at this site also can produce bilateral palsy.
See the image below.
One should suspect a lesion to the trochlear nucleus or fascicle when palsy is associated with a contralateral Horner syndrome or an ipsilateral relative afferent pupillary defect (especially without concomitant visual loss). This is due to the close proximity of the sympathetic pathways in the dorsolateral tegmentum of the midbrain and the pretectal afferent pupillary fibers that run through the superior colliculus.
Tumors or aneurysms causing compressive injury in the subarachnoid space generally damage adjacent structures and produce associated neurologic signs. The same is true of lesions in area of cavernous sinus and orbital apex, which generally produce multiple cranial neuropathies. In rare cases, fourth nerve palsy may result from any cause of increased intracranial pressure such as pseudotumor cerebri or meningitis. Direct orbital injury can result in a clinical picture that resembles fourth nerve palsy, but superior oblique weakness in this setting most likely is due to direct damage to muscle or tendon.
The superior oblique muscle depresses, intorts, and abducts the globe.
In acquired lesions of fourth nerve, patients report vertical, torsional, or oblique diplopia. Diplopia is usually worse on downgaze and gaze away from side of affected muscle.
In case of trauma, patients usually report symptoms immediately after regaining consciousness.
Torsional diplopia and downgaze horizontal diplopia may be predominant complaints in bilateral palsies.
Patients often adopt a characteristic head tilt, away from affected side to reduce their diplopia. Interestingly, some patients develop head tilt toward side of lesion. This so-called paradoxic head tilt is used to create a wider separation of images, which allows the patient to suppress or ignore one image. Old photographs may provide clear documentation of a head tilt in congenital fourth nerve palsy.
Three-step test can be extremely useful in evaluation of vertical diplopia caused by a paretic cyclovertical muscle. However, results of this test can be misleading in the setting of restrictive ophthalmopathy. Each step reduces by half the number of possible affected muscles until only 1 remains, as follows:
First step is to identify the hypertropic eye in primary gaze. This implicates depressors of hypertropic eye or elevators of hypotropic eye.
Second step is to ascertain if hypertropia is worse on left gaze or right gaze. This will identify 4 muscles that act in that direction of gaze.
Third step is to determine if hypertropia is worse on right head tilt or left head tilt.
Bielschowsky head-tilt test stimulates intorsion of globe on the side to which head is tilted and extorsion of globe on the side away from which head is tilted. Intorters and extorters of each globe have opposite vertical functions, and, when there is a paretic muscle, unopposed vertical action of other muscle makes hyperdeviation more apparent in that field of action. Only the paretic muscle will have been implicated in each step of the test.
In case of bilateral fourth nerve palsy, interpretation of 3-step test may be confusing. Right hypertropia manifests on right head tilt, and left hypertropia manifests on left head tilt. Other findings, such as V-pattern esotropia and large amounts of excyclotorsion, also are suggestive of bilateral disease.
Excyclotorsion may be measured using double Maddox rod test.[7, 16] Details are as follows:
Patients are seated in a dark room to minimize their reliance on environmental cues.
Red Maddox rod is placed before each eye with axes oriented obliquely at about 5-10° from the vertical.
Patient is asked to rotate 1 frame until the 2 lines are parallel.
Patients with bilateral disease typically show more than 10° of excyclotorsion.
Congenital fourth nerve palsies may present with several unique findings, as follows:
Patients with long-standing head tilt present during early childhood may develop facial asymmetry. Characteristically, there is shallowing of face between lateral canthus and side of mouth on the side of head tilt. Any condition that leads to torticollis in early life may result in similar facial asymmetry.
Patients with congenital palsies also tend to develop large, vertical fusional amplitudes, and they may have lack of subjective torsion even when large amounts of fundus torsion are present.
For patients with decompensating congenital fourth nerve palsy, indications for intervention include cosmetically or functionally unacceptable head position, and onset of increasing frequency of diplopia.
Patients with acquired disease from tumors or compressive lesions are usually significantly disturbed by symptoms and are likely to require prism or, in some cases, surgical intervention.
The trochlear nucleus is located in tegmentum of midbrain, at the level of inferior colliculus.[7, 2] The trochlear nerves decussate at anterior medullary velum in the roof of aqueduct before exiting from dorsal aspect of midbrain. The fourth nerve courses between posterior cerebral and superior cerebellar arteries before entering the cavernous sinus. The fourth nerve then enters the orbit through superior orbital fissure, outside annulus of Zinn. From here, the nerve crosses medially over levator palpebrae superioris and superior rectus muscles before entering the belly of superior oblique muscle.
The superior oblique muscle originates from the orbital apex, above the annulus, and runs along superonasal aspect of orbit before becoming a tendinous cord. The superior oblique tendon then passes through trochlea and abruptly turns laterally and posteriorly to insert on the globe. The tendon is cordlike as it passes beneath nasal border of superior rectus but fans out to form a broad insertion.
When performing a superior oblique tenotomy, the superior rectus muscle insertion may be used as a landmark. The portion of tendon that is cut during the tenotomy may be isolated by dissecting to a point approximately 8-12 mm posterior to nasal aspect of superior rectus insertion. Broad superior oblique insertion, which is 10-18 mm in length, has great functional importance. Anterior fibers act mainly to intort the globe and do little to abduct or depress the eye. Conversely, more posterior fibers are responsible for abduction and depression but have little torsional action. Surgical procedures designed to alleviate torsional diplopia, such as the Harada-Ito procedure, consist of advancing only anterior fibers of tendon insertion.
Patients with microvascular disease should be counseled about the high likelihood of spontaneous resolution, and these patients should be observed. These patients may be advised to patch 1 eye or use monovision lenses to minimize their symptoms.
Similarly, patients who have traumatic fourth nerve palsy should be observed for 6 months prior to surgical intervention because of the possibility of spontaneous resolution. Some traumatic palsies may recover as late as 1 year after injury.
The prognosis of a fourth nerve palsy depends on the underlying etiology. Congenital palsies are long standing and often remain static. Acquired, demyelinating (rare), traumatic, ischemic (microvascular), and idiopathic palsies usually resolve over time. The prognosis of fourth nerve palsies due to a structural lesion depend on the treatment of the underlying lesion. Most patients with symptoms that do not recover spontaneously can improve with prism or surgery.
Patients should be advised on the etiology and prognosis of the fourth nerve palsy. Prism or surgical therapy can be considered in patients who have stable and unresolved ocular deviations.
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