Cranial Nerve 4 Palsy
Dysfunction of the fourth cranial nerve (trochlear nerve), which innervates the superior oblique muscle (SOM), is one cause of paralytic strabismus. The SOM has different (primary, secondary, and tertiary) actions dependent on mechanical position of the eye. In the primary position, the primary action of the superior oblique muscle is intorsion. In adduction, the superior oblique is primarily a depressor. In abducted gaze, the SOM acts to intort the eye and abducts the eye. Thus, a trochlear nerve palsy causes an ipsilateral higher eye (i.e., hypertropia) and excyclotorsion (the affected eye deviates upward and rotates outward). Patients may report vertical and/or torsional diplopia that is usually worse on downgaze and gaze away from the affected side.
There are several clinically significant features of the trochlear nerve anatomy. It is the thinnest, and longest cranial nerve. Additionally, the fourth cranial nerve exits dorsally, crosses the midline, and innervates the contralateral SOM. The trochlear nucleus is in the midbrain, dorsal to the medial longitudinal fasciculus at the level of the inferior colliculus. The nucleus gives rise to the IV nerve fascicle which decussates at the level of the anterior medullary velum (the roof of the aqueduct) just caudal to the inferior colliculus. The IV nerve then courses around the cerebellar peduncle and travels between the superior cerebellar and posterior cerebral arteries in the subarachnoid space. It progresses through the lateral wall of the cavernous sinus. The trochlear nerve passes adjacent to the ophthalmic division of the trigeminal nerve and the two share a connective tissue sheath. The trochlear nerve gains entry to the orbit via the superior orbital fissure, passes outside the tendinous ring of Zinn and innervates the SOM.
Isolated 4th Nerve Palsy
Congenital Trochlear nerve palsy is a common cause of congenital cranial nerve (CN) palsy. Patients with congenital CN IV palsies may compensate for diplopia with variable head positioning; chin-down head posture is seen in bilateral CN IV palsy and contralateral head tilt is typically seen in unilateral CN IV palsy. Later in life, these patients may experience decompensation of their previously well controlled CN IV palsy from the gradual loss of fusional amplitudes that occurs with aging or after illness or other stress event. Congenital CN IV palsies can have very large hypertropias in the primary position (greater than 10 prism diopters) despite the lack of diplopia or only intermittent diplopia symptoms. These large vertical fusional ranges characteristic of congenital cases.
Trauma CN IV has the longest intracranial course and is vulnerable to damage, even with relatively mild trauma. For trauma-induced trochlear palsy, patients typically report symptoms immediately after injury. Bilateral involvement is rare in non-traumatic cases but is relatively more frequent after trauma (crossed, dorsal exit). Careful examination is necessary in traumatic cases as the CN IV palsies can by asymmetric if bilateral and can be masked or become apparent after strabismus surgery for a presumed unilateral CN IV palsy. Computed tomography (CT) scan is generally the first line imaging study in trauma but is often normal. Magnetic resonance imaging of the head (MRI) is often unremarkable in CNV IV palsy but may show a dorsal midbrain contusion or hemorrhage.
Microvascular disease Microvascular disease can involve CN IV and usually in older patients with cardiovascular risk factors. Sudden onset, of a painless, neurologically isolated CN IV without a history of head trauma or congenital CN IV palsy in a patient with risk factors for small vessel disease implies an ischemic etiology. Some authors recommend following such patients for resolution over time and control of the vasculopathic risk factors alone. Other authors however have suggested that patients with CN IV palsy should undergo neuroimaging and further neurological work-up. MRI may show an infarction in the tegmentum of the midbrain, affecting the fascicle of the fourth nerve. Fourth nerve palsy secondary to microvascular disease will frequently resolve within 4-6 months spontaneously.
Idiopathic In a small subset of patients with acquired trochlear palsy, no etiologic cause can be established even after extensive testing. A recent population-based study finds only 4% of trochlear nerve palsies to be idiopathic, citing increased improved identification of vasculopathic risk factors. Idiopathic cases may improve or completely resolve over a matter of weeks.
Non-isolated 4th Nerve Palsy
Trochlear nerve palsy can also occur as part of a broader syndrome related to causes like trauma, neoplasm, infection, and inflammation. These etiologies are further categorized based on the anatomic location of involvement (midbrain, subarachnoid space, cavernous sinus, orbit). Increased intracranial pressure has also been known to cause CN 4.
Fourth cranial nerve palsies can affect patients of any age or gender. They can present with vertical diplopia, torsional diplopia, head tilt, and ipsilateral hypertropia. Determining the onset, severity, and chronicity of symptoms can be vital in delineating between the various etiologies of a CN 4 palsy. Furthermore, careful history including associated symptoms and other past medical history can help distinguish a CN 4 palsy from other items on the differential.
Examiners should consider obtaining the following: visual acuity, motility evaluation, binocular function and stereopsis, strabismus measurements at near, distance, and in the cardinal positions of gaze, and evaluation of ocular structures in the anterior and posterior segments.
Ipsilateral hypertropia and excyclotorsion are frequently seen due to the superior oblique’s function of intorsion and depression the eye. Patients can also develop a compensatory head tilt in the direction away from the affected muscle.
Patients can present with binocular, vertical or torsional diplopia. The superior oblique causes eye depression in adducted gaze. This can explain the worsening of a patient’s diplopia when they attempt to visualize objects in primary position, especially in down-gaze. Patients with mild or long-standing disease may have blurred vision, difficulty focusing and dizziness instead of diplopia.
Localizing Signs and Symptoms
- Hemisensory loss, ataxia, internuclear ophthalmoplegia, hemiparesis, central Horner syndrome, cranial nerve III palsy
- Frequently due to infarction or hemorrhage
- Presence of an ipsilateral or contralateral rAPD without loss of visual acuity, color vision, or peripheral vision in an apparently isolated CN IV palsy suggests superior colliculus brachium involvement. This suggests a central CN IV palsy. 
- Fever, headache, neck stiffness may be associated with meningitis.
- Aneurysms may manifest as an isolated CN IV palsy
- Signs and symptoms associated with CN III, V, VI and Horner’s syndrome (e.g. ptosis,miosis, etc.)
- Signs and symptoms associated with CN II,III, V, VI and II.
- Proptosis, chemosis, and orbital edema may also be seen
Unilateral CN IV palsy:
Diagnosis is made via the Parks-Bielschowsky three-step test. Ductional testing may be normal however or only show mild depression deficit in adduction with trochlear nerve palsies. The three questions to ask in evaluation of the CN IV palsy are as follows:
- Which is the hypertropic eye?
- There are eight possible muscles that could cause a hypertropia -- the bilateral superior recti, inferior recti, superior obliques and inferior obliques. Determining the hypertropic eye reduces the potentially involved muscles to four. These include the ipsilateral depressors - the superior oblique and inferior rectus or the contralateral elevators - the superior rectus and inferior oblique. For example, with a right hypertropia, the potentially involved muscles include the right superior oblique, right inferior rectus, left inferior oblique and left superior rectus.
- Does the hypertropia worsen in left or right gaze?
- Determining if the hypertropia is worse in left or right gaze helps eliminate two of the possibly affected muscles. The SOM has action that varies depending on the angle between the muscle plane and the visual axis. When the eye is adducted, the muscle plane and the visual axis align and the primary action is as a depressor. When the eye is abducted the visual axis and the muscle plane become more perpendicular and the SOM function is mostly intorsion. So, in a patient with right hypertropia that worsens in left gaze, this suggests either right superior oblique or a left superior rectus involvement.
- Does the hypertropia worsen in left or right head tilt? (Bielschowsky head tilt test)
- Determining if there worsening of the hypertropia in left or right head tilt can identify the involved muscle from the remaining two choices following steps 1 and 2 of the three step test. If the hypertropia is worse in ipsilateral tilt this implicates the ipsilateral superior oblique as the intorsional ability of the superior oblique is weakened. In this head position, the ipsilateral superior rectus will compensate for the weak intorsion of the ipsilateral superior oblique, but will elevate the eye and further worsen the hypertropia. Patients may develop a compensatory head tilt to the contralateral side to reduce their diplopia. In a patient with a right hypertropia that worsens in right gaze and left head tilt is most compatible with a superior oblique palsy.
- Additional “fourth” step to distinguish from skew deviation
- Double Maddox Rod
- In fourth nerve palsy the Double Maddox rod should demonstrate unilateral excyclotorsion.
- Skew deviation may demonstrate bilateral torsion or incyclotorsion, both of which are inconsistent with fourth nerve palsy.
- Bilateral CN IV palsy may have large degree of bilateral excylotorsion (e.g., > 10 degrees) on the Double Maddox rod test
- Fundus Photography
- Could demonstrate that the fundus of the affected eye is excyclotorted.
- Skew deviation may display incyclotorsion of the affected eye or bilateral torsion.
- Bilateral CN IV palsy might show bilateral excyclotorsion.
- Upright - Supine test 
- In a fourth nerve palsy, ocular torsion and hypertropia should be unaffected by positional changes.
- Skew deviation may demonstrate decreasing vertical strabismus with position change from upright to supine. Greater than 50% change in vertical strabismus with position change from upright to supine is a positive test.
- Double Maddox Rod
Features suggestive of a bilateral fourth nerve palsy include:
- Alternating hypertropia on horizontal gaze or tilt
- Positive Bielschowsky head tilt test to either shoulder
- Large degree of excyclotorsion (> 10 degrees)
- Absent or small hypertropia in primary gaze
- Underaction of both superior obliques on duction testing
- A V-pattern esotropia of greater than 25 prism diopters
- Oculomotor palsy
- Guillain Barre Syndrome
- Orbital Pseudotumor
- Brown Superior Oblique Tendon Sheath Syndrome
- Fisher Syndrome
- Chronic Progressive External Ophthalmoplegia (CPEO)
- Vertical one-and-a-half syndrome
- Monocular Supranuclear Gaze Palsy
- Myasthenia Gravis
- Ocular Tilt Reaction/Skew Deviation
- Thyroid Eye Disease
The management of a trochlear nerve palsy depends on the etiology of the palsy. Patients with traumatic or congenital fourth nerve palsies may be considered for patch, prism, or surgical treatment, especially if they are symptomatic in primary gaze. Patients with an acquired trochlear nerve palsy may respond to treatment of the underlying disease. Microvascular causes may spontaneously resolve over the course of weeks or months. When these palsies persist, they are typically responsive to prism treatment as they tend to cause comitant deviations.
Prism therapy is a reasonable treatment option for patients amenable to therapy. Strabismus surgery can be used in patients who do not respond or tolerate prisms. Patching is also an acceptable alternative for patients who defer prisms or surgery.
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