You are in: eMedicine Specialties > Ophthalmology > EXTRAOCULAR MUSCLES Trochlear Nerve PalsyArticle Last Updated: Nov 2, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Zafar A Sheik, MD, Director, Department of Ophthalmology, St Joseph's Medical Center Zafar A Sheik is a member of the following medical societies: American Academy of Ophthalmology and International Society of Refractive Surgery Coauthor(s): Kelly A Hutcheson, MD, Associate Professor, Department of Ophthalmology, George Washington University School of Medicine, Children's National Medical Center, Editors: Edsel Ing, MD, FRCSC, Assistant Professor, Department of Ophthalmology & Vision Sciences, University of Toronto: Consulting Staff, Toronto East General Hospital; Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles; Brian R Younge, MD, Professor of Ophthalmology, Mayo Clinic School of Medicine; Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri; Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences Author and Editor Disclosure Synonyms and related keywords: fourth nerve palsy, fourth cranial nerve palsy, trochlear palsy, superior oblique palsy, vertical diplopia, head-tilt test INTRODUCTIONSection 2 of 10
  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. History of the ProcedureSurgical 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. ProblemThe 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. FrequencyEstimating 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. EtiologyThe 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.
PathophysiologyCongenital 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. 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 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. One should suspect a lesion to the trochlear nucleus or fascicle when palsy is associated with a contralateral Horner or an ipsilateral relative afferent pupillary defect. This is due to the close proximity of the sympathetic pathways in the dorsolateral tegmentum of the midbrain and the pupillomotor 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. 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. Clinical
INDICATIONSSection 3 of 10
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 surgical intervention. RELEVANT ANATOMYSection 4 of 10
Trochlear nucleus is located in tegmentum of midbrain, at the level of inferior colliculus. Nerves decussate at anterior medullary velum in the roof of aqueduct before exiting from dorsal aspect of midbrain. Fourth nerve courses between posterior cerebral and superior cerebellar arteries before entering cavernous sinus. The nerve 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. Superior oblique muscle originates from orbital apex, above annulus, and runs along superonasal aspect of orbit before becoming a tendonous cord. Tendon passes through trochlea and abruptly turns laterally and posteriorly to insert on the globe. 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 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. CONTRAINDICATIONSSection 5 of 10
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. TREATMENTSection 6 of 10
Medical TherapyPrisms may be used for patients with small deviations and diplopia without torsional component. Incomitance of deviation often limits usefulness of this therapy. Botulinum toxin also has been studied in treatment of fourth nerve palsy. It is a neuromuscular agent that acts presynaptically to block neurotransmitter release and results in muscle weakening. Use of this agent as primary therapy for fourth nerve palsy has been discouraging. However, it may be used best to correct residual deviation after strabismus surgery to delay or avoid further surgery. BOTOX® is purified botulinum toxin A, derived from a culture of the Hall strain of Clostridium botulinum. It acts by binding to receptor sites on motor nerve terminals and inhibiting the release of acetylcholine. BOTOX® may be used for the treatment of strabismus and blepharospasm in patients 12 years and older. It is pregnancy category C. Side effects for use in strabismus include ptosis and vertical deviation by action at extraocular muscles close to the site of injection. Injection should be performed under direct visualization during a surgical procedure or with the aid of electromyography. Each vial of BOTOX® contains 100 units of botulinum toxin A in a vacuum-dried form. It needs to be reconstituted using preservative free 0.9% sodium chloride as the diluent. Doses used in strabismus range from 1.25-5 units, depending on the amount of deviation. Surgical TherapyIn 1970s, Knapp developed a surgical approach for superior oblique palsy. He classified superior oblique palsy by determining field of gaze in which deviation was greatest. Based on this classification, he recommended operation on the muscle or muscles that acted in this direction of gaze. Plager described a tailored treatment plan that evolved from Knapp's recommendations, with some additions based on more recent operative algorithms. For a deviation of less than 15 prism diopters, single muscle surgery may suffice. If there is any inferior oblique overaction, inferior oblique weakening by myectomy or recession is the procedure of choice. Without any evidence of inferior oblique overaction, another muscle may be chosen. In case of ipsilateral superior rectus restriction, a superior rectus recession would be indicated. Superior oblique tendon tuck is preferred if significant tendon laxity is present, as has been described in congenital cases. Contralateral inferior rectus recession is chosen if there is no evidence of superior rectus restriction or superior oblique tendon laxity. This is an especially useful procedure when deviation is greatest in downgaze. For deviation greater than 15 prism diopters, 2-3 muscle surgeries probably will be required. Two muscle surgery generally includes weakening of ipsilateral inferior oblique, as well as a procedure on ipsilateral superior rectus, superior oblique, or contralateral inferior rectus. For large deviations, 3-muscle surgery should be considered. Inferior oblique and contralateral inferior rectus should be weakened. Then, the surgeon may choose to operate on superior oblique or superior rectus, based on intraoperative findings. Modified Harada-Ito procedure is useful for patients with large excyclotorsional deviation. This is likely to be the case for patients with bilateral superior oblique palsy, and bilateral surgery should be performed. In this procedure, the superior oblique tendon is split and anterior fibers are advanced anteriorly and laterally. Preoperative Details
Intraoperative DetailsPatients with congenital superior oblique palsy often have abnormally lax superior oblique tendon. Exaggerated, forced duction test described by Guyton can be performed intraoperatively to determine if there is any degree of tendon laxity relative to normal eye.
Any surgeon who performs oblique muscle surgery should be familiar with anatomy, landmarks, and appropriate approaches to these muscles. Visualization is more difficult than with rectus muscle surgery, and injury to adjacent nerves, blood vessels, and other extraocular muscles may occur. Use of headlight can improve visualization. Postoperative Details
COMPLICATIONSSection 7 of 10
OUTCOME AND PROGNOSISSection 8 of 10
Prognosis of trochlear nerve palsy varies depending on etiology. Best information regarding outcome comes from cases collected at the Mayo Clinic over the past 40 years.
Because patients have good fusional abilities, surgery generally produces excellent results. Plager reported a nearly 90% success rate with his surgical algorithm. Mitchell and Parks also reported excellent results in correcting excyclotorsion using modified Harada-Ito procedure. MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
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