Neuroanatomy, Cranial Nerve 4 (Trochlear)


Article Author:
Seung Kim
Mahsaw Motlagh


Article Editor:
Imama Naqvi


Editors In Chief:
Jasleen Jhajj
Cliff Caudill
Evan Kaufman


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Abdul Waheed
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James Hughes
Beata Beatty
Richard Ciresi
Hajira Basit
Phillip Hynes
Kavin Sugumar


Updated:
6/1/2019 12:21:14 PM

Introduction

The trochlear nerve is the fourth cranial nerve (CN IV), and one of the ocular motor nerves which control eye movement. The trochlear nerve, while the smallest of the cranial nerves, has the longest intracranial course. It originates in the dorsal midbrain and extends laterally and anteriorly to the superior oblique muscles.[1]

Structure and Function

The trochlear nerve is a solely general somatic efferent (motor) nerve, which with the oculomotor and abducens nerve controls eye movement. The trochlear nerve through its sole innervation, the superior oblique, controls abduction and intorsion of the eye.[1]

Embryology

The trochlear nerve as with the abducens (VI), hypoglossal (XII), and oculomotor (III) nerves are homologs of the ventral roots of the spinal nerves. The somatic efferent columns of the brainstem give rise to these cranial nerves. The muscles that they innervate derive from the head (preoptic and occipital) myotomes in early skeletal muscle development.

The trochlear nerve derives from the somatic efferent column of the posterior part of the midbrain, where the nerve emerges from the brainstem dorsally and passes ventrally to the superior oblique muscle.[2]

Nerves

 The trochlear nerve pair originates from a pair of symmetrical trochlear nuclei within the midbrain. The left and right nerves then travel dorsally surrounding the periaqueductal gray matter, decussating before exiting the dorsal midbrain below the inferior colliculus. The two nerves run on contralateral sides extend laterally, then anteriorly around the pons, before penetrating the dura above the trigeminal nerve.

It enters the cavernous sinus where it runs anteriorly/ventrally above the abducens nerve and ophthalmic branch of the trigeminal nerve. Here in the cavernous sinus, a few sympathetic fibers join the trochlear nerve with the possibility of some sensory fibers from the trigeminal nerve. Then into the orbit, through the superior orbital fissure above the origin of the levator palpebra muscle and continues to extend anteriorly to the superior oblique muscle.[1][3]

Muscles

The only muscle the trochlear nerve innervates, the superior oblique muscle, is the longest and thinnest muscle among the extraocular muscles. The muscle belly originates from the back of the roof of the orbit, and the tendon extends between the roof and medial wall to the pulley (known as the trochlea). The tendon then reaches backward and downward passing the superior rectus muscle to the upper back of the eye. The trochlea of the superior oblique is a U-shaped cartilage attached to the upper front of the orbit.

Because of the unusual shape and direction of the superior oblique muscle afforded by the trochlea, the muscle can depress, abduct, and intorts the eye. Because of the muscle’s placement at the upper back of the eye, the muscle elevates the posterior of the eye, causing the front of the eye to become depressed. The muscle also causes abduction of the eye, moving the pupil away from the nose, and intorsion, rotating the eye such that the top of the eye moves toward the nose.

The superior oblique muscle is the only extraocular muscle that can lower the pupil with the eye adducted. Thus, to isolate the function of the superior oblique muscle from the other extraocular muscles, the muscle can be tested by requesting the patient to adduct the eye and then ask to depress the eye. Failure to depress the eye during adduction indicates a problem with the superior oblique muscle or the trochlear nerve.[4]

Clinical Significance

The trochlear nerve because of its fragility and its extensive intracranial course is especially vulnerable to trauma compared to most cranial nerves. Thus, the most common cause of an acquired defect of the trochlear nerve is head trauma. Traumatic trochlear nerve palsies are associated with motor vehicle accidents and boxing, as they involve rapid deceleration of the head. Because the trochlear nerve is so fragile, this can occur in minor head injuries that do not involve loss of consciousness or skull fracture. 

Other significant causes are microvascular, congenital, and idiopathic. Microvascular trochlear nerve palsy usually occurs in the setting of diabetes in patients who are 50 years of age and older. The nerve, however, is less affected compared to the other ocular motor nerves (CN III and CN VI). These nerve injuries are relatively transient with symptom resolution in a few months without treatment.

Congenital trochlear nerve palsies are almost always unilateral. These nerve palsies in children can be initially mistaken for torticollis because of the head tilt many of these children display. Diplopia is not usually present in these patients, and in fact, these patients may compensate for the nerve palsy until adulthood, when diplopia and/or blurry vision may result in a supposed new onset of nerve palsy.[5][6]

Other more minor causes of the trochlear nerve palsy include Lyme disease, Meningioma, Guillain-Barre Syndrome, Herpes zoster, and Cavernous Sinus Syndrome.

Most trochlear nerve palsies are unilateral. However, because the decussation of the trochlear nerve pair happens when the nerve pair is near each other, a single lesion at the dorsal midbrain can cause bilateral trochlear nerve palsy.[6]

Lesions of the trochlear nerve can either involve the nucleus or the nerve, but both virtually present with similar symptoms. The only difference is that a unilateral trochlear nuclear lesion affects the contralateral nerve and superior oblique muscle, while a fascicular lesion affects the ipsilateral nerve and muscle.

If other ocular motor nerves are involved in the patients’ symptoms, a lesion in the cavernous sinus or midbrain is most likely, as these nerves are relatively near each other in this space.

Trochlear nerve palsy can present in patients as diplopia, but can also present as blurry vision or a minor vision problem when looking down like reading a book or going down the stairs. The diplopia presented in trochlear nerve palsy is either vertical or diagonal and is worse with downward gaze. Compensation for the nerve palsy usually includes a head tilt to the opposing side and tucking in the chin, so the affected eye’s pupil can move up and extort, instead of downwards and intort. During clinical examination, the eyes will display hypertropia with the affected eye being slightly elevated relative to the other normal eye. Under cover, the affected eye will show an upward drift relative to the other eye.[7]

Treatment of trochlear nerve palsies includes surgical procedures, the most common one being the weakening of the inferior oblique muscle. Diabetic nerve palsies virtually do not need additional treatment as they resolve spontaneously.

Another condition that involves the trochlear nerve is superior oblique myokymia which causes spasms of the superior oblique muscle. Symptoms include transient vertical diplopia. The etiology is not known but may be related to other conditions that cause microtremors. Rarely this can cause superior oblique muscle weakness.[5][6]


  • Image 7286 Not availableImage 7286 Not available
    Contributed by Seung Yun Kim and Imama Naqvi, MD
Attributed To: Contributed by Seung Yun Kim and Imama Naqvi, MD

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Neuroanatomy, Cranial Nerve 4 (Trochlear) - Questions

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Which of the following is incorrect about the trochlear nerve?



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Which nerve emerges at the dorsal brainstem?



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Which cranial nerve exits from the dorsal brainstem?



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Cranial nerve IV is also known as the:



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The cranial nerve which arises on the back of the brain and therefore has the longest intracranial course is the:



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The smallest of the cranial nerves is the:



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Which of the following cranial nerves has total decussation?



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Which of the following nerves is most superior to the lateral wall of the cavernous sinus as they approach the superior orbital fissure?



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Which of the following statements is not correct in describing the trochlear nerve?



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Which is the only cranial nerve that leaves the dorsal surface of the brainstem?



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Which of the following cranial nerves is not related to each other in some way?



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The trochlear nerve controls specific aspects of eye movement. What other nerves are involved in the control eye movement?



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The trochlear nerve controls what muscle, and what is the muscle’s function?



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A 34-year-old woman comes into the emergency department after a motor collision. She appears anxious and reports a violent shake of her body and head during the accident. Otherwise, the patient is conscious and alert. On physical exam, the patient’s left eye is found to be slightly elevated compared to the right eye. Where would the damage in on the trochlear nerve pathway to cause this physical finding?



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A 55-year-old woman makes an office visit after noticing increasingly blurry vision in the past few days. During the physical exam, the patient displays symptoms of both horizontal and vertical diplopia. She has trouble depressing her right eye but also elevating her right eye. Where in the trochlear nerve pathway would the damage most likely occur?



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Neuroanatomy, Cranial Nerve 4 (Trochlear) - References

References

Laine FJ, Cranial nerves III, IV, and VI. Topics in magnetic resonance imaging : TMRI. 1996 Apr     [PubMed]
Park HK,Rha HK,Lee KJ,Chough CK,Joo W, Microsurgical Anatomy of the Oculomotor Nerve. Clinical anatomy (New York, N.Y.). 2017 Jan     [PubMed]
Demer JL, Compartmentalization of extraocular muscle function. Eye (London, England). 2015 Feb     [PubMed]
Adler FH, Some Physiologic Factors in Differential Diagnosis of Superior Rectus and Superior Oblique Paralysis. Transactions of the American Ophthalmological Society. 1945     [PubMed]
Cooper ER, THE TROCHLEAR NERVE IN THE HUMAN EMBRYO AND FOETUS. The British journal of ophthalmology. 1947 May     [PubMed]
Adamec I,Habek M, Superior oblique myokymia. Practical neurology. 2018 Oct     [PubMed]
Brazis PW, Isolated palsies of cranial nerves III, IV, and VI. Seminars in neurology. 2009 Feb     [PubMed]

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