Fourth Nerve Palsies
What Is the Topographic Anatomy of the Fourth Nerve?
The fourth nerve nucleus is located in the midbrain beneath the inferior colliculus. The fourth nerve is the only cranial nerve that exits dorsally from the brainstem, it has the longest intracranial course, and it crosses in the anterior medullary velum. It passes between the superior cerebellar artery and the posterior cerebellar artery, runs in the subarachnoid space, travels within the lateral wall of the cavernous sinus, and enters the orbit via the superior orbital fissure to innervate the superior oblique muscle.
What Are the Clinical Features of Fourth Nerve Palsies?
Fourth cranial nerve palsies may cause the following (von Noorden, 1986):
1. Incomitant hypertropia is demonstrated with the three-step maneuver. The hypertropia increases on head tilt toward the paralyzed side (positive Bielschowsky’s test). Usually the unaffected eye is fixating and the hypertropia occurs in the involved eye. Hypotropia may occur in the normal eye if the affected eye is fixating. The hypertropia is usually most prominent in the field of gaze of the involved superior oblique muscle, especially in cases of acute or recent onset. The hypertropia may also be most prominent in the field of gaze of the ipsilateral overacting inferior oblique muscle in subacute or chronic cases. In palsies of longer duration, the hypertropia may be relatively equal in the various gaze positions (spread of comitance).
2. Duction testing may variably reveal underaction of the ipsilateral superior oblique muscle, overaction of the ipsilateral inferior oblique muscle, or overaction of the contralateral superior oblique muscle.
3. Pseudo-overaction of the superior oblique in the uninvolved eye may occur with spread of comitance. Secondary contracture of the superior rectus muscle in the involved eye may cause hypertropia involving the entire lower field of gaze. In a patient with a superior oblique muscle paralysis who habitually fixates with the paretic eye and in whom overaction of the ipsilateral inferior oblique muscle has developed, less than the normal amount of innervation will be required when the patient looks up and to the contralateral side. Because the innervation flowing to the opposite superior rectus is “determined” by the overacting ipsilateral inferior oblique (Hering’s law), the opposite superior rectus muscle will seem paretic (inhibitional palsy of the contralateral antagonist). In these cases, the head tilt test will correctly determine which of the two eyes is paretic.
4. Excyclotropia due to loss of incyclotorsion function of the superior oblique muscle. This torsion may be evident on fundus exam and can be measured using double Maddox rod testing. The excyclotropia is usually symptomatic in acquired cases but is often asymptomatic in congenital cases.
5. An anomalous head tilt eliminates the hypertropia or less commonly the cyclotropia. This head tilt is present in approximately 70% of patients and is usually away from the involved side but may be paradoxical (toward the involved side) in about 3%.
It is important to differentiate patients with decompensation of a congenital fourth nerve palsy (FNP) from those with an acquired FNP. In patients with congenital FNPs:
1. Old photos may show a long-standing head tilt.
2. Patients usually are noted to have cyclotropia on examination but often do not complain of cyclotropia (subjective image tilting) as do some patients with acquired FNPs.
3. Large vertical fusional amplitudes (> 8 prism diopters) in primary gaze are characteristic of congenital cases.
4. Facial asymmetry (hypoplasia on side of head turn) suggests a congenital lesion.
Bilateral FNPs are suggested by the following:
1. A right hypertropia in left gaze and left hypertropia in right gaze (a reversing hypertropia).
2. A positive Bielschowsky test on tilt to either shoulder (“double Bielschowsky test”).
3. Large excyclotropia (> 10 degrees).
4. V-pattern esotropia (15 prism diopters or more difference in esotropia between upward and downward gaze). The V pattern is caused by a decrease of the abducting effect of the superior oblique(s) in depression and secondary overaction of the abducting effect of the inferior oblique muscle(s).
5. Underaction of both superior oblique muscles and/or overaction of both inferior oblique muscles on duction testing.
6. In general, bilateral FNPs tend to have a smaller hypertropia in primary position than do unilateral FNPs.
The criteria for the diagnosis of FNPs are listed in Table 12–1. FNP may be categorized as either isolated or nonisolated. For diagnostic classification based on topographic localization, nonisolated FNP may be grouped into the following four syndromes:
1. Midbrain (nucleus/fascicle syndrome) FNP
2. Subarachnoid space FNP
3. Cavernous sinus FNP
Binocular vertical and/or torsional diplopia or misalignment
Ipsilateral hyperdeviation in primary position, worsened by contralateral gaze and ipsilateral head tilt (the three-step test)
Variable ipsilateral excyclotorsion
Anomalous compensatory head or face position
Weakness of the involved superior oblique muscle on ductions
4. Orbital FNP
Nonisolated FNP (type 1; see below), with findings that localize to the brainstem, subarachnoid space, cavernous sinus, or orbit, should undergo a directed neuroimaging study (Berlit, 1991; Brazis, 1993; Burde, 1992; Celli, 1992; Elliot, 1991; Kim, 1992; Richards, 1992; Vanooteghem, 1992).
Is the FNP Due to a Midbrain Lesion?
A midbrain (i.e., nuclear/fascicular) FNP is defined by the “company it keeps”; other brainstem signs usually present, including hemisensory loss, hemiparesis, a central Horner’s syndrome, or other brainstem cranial neuropathies (e.g., third nerve palsy). The differential diagnosis includes midbrain ischemia, hemorrhage, demyelination, and neoplasm. Neuroimaging (preferably magnetic resonance imaging [MRI]) should be directed to the midbrain (class II–III, level B).
Is the FNP the Result of a Subarachnoid Space Lesion?
Lesions of the subarachnoid space are rarely associated with an isolated FNP. Patients with subarachnoid space lesions usually have associated signs and symptoms including headache, stiff neck, and other cranial neuropathies. Neuroimaging (MRI) should be directed to the brainstem and subarachnoid space. Computed tomography (CT) imaging should be considered in cases of acute trauma, to evaluate bone lesions, or in the evaluation of acute vascular processes (e.g., subarachnoid hemorrhage). Lumbar puncture following negative neuroimaging should be considered in these cases (class II–III, level B).
Is the FNP Due to a Cavernous Sinus Lesion?
Cavernous sinus lesions are usually associated with other cranial nerve signs (e.g., third, fifth, or sixth nerve paresis) or a Horner’s syndrome. Neuroimaging (preferably MRI) should be directed to the cavernous sinus (class II–III, level B).
A: Lesions affecting the trochlear nucleus and/or fascicles (superior oblique palsy contralateral to lesions)
Isolated trochlear palsy (rare)
Vertical gaze palsy (dorsal midbrain syndrome)
Superior cerebellar peduncle
Dysmetria on side of lesion
Descending sympathetic fibers
Horner’s syndrome on side of lesion
Medial longitudinal fasciculus (MLF)
Ipsilateral paresis of adduction with nystagmus of contralateral abducting eye
Brachium of superior colliculus
Contralateral relative afferent pupillary defect (RAPD) without visual impairment
Anterior medullary velum
Bilateral trochlear nerve palsies
B: Lesions affecting the trochlear nerve within the subarachnoid space (superior oblique palsy usually ipsilateral to lesion unless mesencephalon compressed)
Trochlear nerve alone
Isolated trochlear palsy
Superior cerebellar peduncle
C: Lesions affecting the trochlear nerve within the cavernous sinus and/or superior orbital fissure
Trochlear nerve alone
Isolated trochlear palsy (rare)
Cranial nerves III, VI, sympathetic
Ophthalmoplegia, pupil small, large, or spared, ptosis
Cranial nerve V (ophthalmic division)
Facial/retro-orbital pain; sensory loss (forehead)
Increased venous pressure
D: Lesions affecting the trochlear nerve within the orbit
Trochlear nerve, trochlea, superior oblique muscle or tendon
Superior oblique palsy
Mechanical restriction of superior oblique tendon
Brown’s superior oblique tendon sheath syndrome
Other ocular motor nerves/extraocular muscles
Ophthalmoplegia, ptosis, restricted ocular movements
Visual loss; optic disc swelling/atrophy
Proptosis (occasionally enophthalmos), Chemosis, eyelid swelling, etc.
Source: Modified from Brazis, 2001, with permission from Lippincott Williams & Wilkins.
Is the FNP Caused by an Orbital Lesion?
Orbital lesions usually produce signs such as proptosis, Chemosis, and orbital or conjunctival edema. Neuroimaging (preferably MRI) should be directed to the orbit (class II–III, level B).
We define six types of FNP, as shown in Table 12–4.
Midbrain (nuclear/fascicular) (Elliot, 1991; Thömke, 2000)
Aplasia of the nucleus
Arteriovenous malformation (Gonyea, 1990; Kim, 1992)
Demyelination (Jacobson, 1999)
Hemorrhage (Galetta, 1998; Kim, 1993; Mon, 1996; Muri, 1995; Tachibana, 1990; Thömke, 1999)
Ischemia/infarction (Kim, 1993; Thömke, 1999; Ulrich, 1998)
Tumor (e.g., glioma) (Barr, 1997; Landolfi, 1998; Mielke, 2001)
Trauma (including surgical)
Sarcoidosis (Leiba, 1996)
Arachnoid cyst of quadrigeminal cistern (Ohtsuka, 1998)
Aneurysm (e.g., superior cerebellar artery) (Agostinis, 1992; Collins, 1992)
Infections (mastoiditis, meningitis) (Carter, 1997; Ferreira, 1997; Sadun, 1999)
Wegener’s granulomatosis (Newman, 1995)
Sarcoidosis (Frohman, 2001)
Superficial siderosis of central nervous system (CNS) (Hashimoto, 1996; Shinmei, 1997)
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