| Symptoms | TEST YOURSELF |
An acute 4th CN palsy typically causes diplopia that is worse in downgaze and looking medially; hence, patients almost always report diplopia (or a tendency to close one eye) while reading.
Diplopia will also worsen if the patient tilts their head towards the side of the lesion.
Signs
The hallmark of a 4th nerve palsy is a vertical misalignment that is greatest with the affected eye looking downwards and medially. A fourth nerve palsy will cause a compensatory head tilt to the contralesional side (eg, right fourth nerve palsy, left head tilt).
The degree of misalignment changes with the direction of gaze, that is, the deviation is noncomitant. The affected eye is higher and the vertical misalignment is greatest with the affected eye is adducted and depressed (in the direction of action of the superior oblique), and also when the head is tilted toward the side of the higher eye (toward the side of the lesion).
Weakness of superior oblique may result in the eye being paretic for downgaze in the adducted position (ie, looking toward the tip of the nose).
The relative direction of the torsion in the elevated eye, intorsion with skew and extorsion with a superior oblique palsy, may be helpful. The direction of torsion can be determined by examining the fundus with the ophthalmoscope, by the bucket test for SVV examining each eye separately, or using visual field perimetry to detect the location of the blind spot relative to the fovea.
This is often assocated with overactivity and contracture of the inferior oblique muscle.
Essential features of a 4th nerve palsy are:
1. Binocular vertical/oblique diplopia.
2. An ipsilateral hypertropia.
3. Difficulty looking down when the eye is adducted.
4. The hypertropia is greater when they look to the opposite side.
5. The hypertropia is greater when they tilt their head toward the same side.
6. They spontaneously tilt their head to the contralateral side to compensate for their diplopia.
NB:
A hypertropia is a form of vertical strabismus where one eye is deviated upwards in comparison to the fellow eye. The term of hypertropia is relative to the fellow eye which, by analogy is the hypotrpoic eye- meaning that is deviated downwards. Depending on which eye is fixing, a hypertropia of one eye is the same entity as a hypotropia of the fellow eye, according to Hering’s law. By convention, the designation of the vertical strabismus is made according to the hypertropic eye. If it behaves the same in all fields of gaze or differently in different fields of gaze, it is classified as comitant or incomitant, respectively1.
Anatomical correlations
Intraparenchymal lesions (either nuclear or intra-axial) of the 4th CN IV are rare, given the relatively short course of the nerve within the brainstem. A lesion of the 4th CN nucleus is clinically identical to a fascicular lesion.
Microvascular, inɻammatory, neoplastic, or demyelinating lesions may involve the central course of the 4th CN. Occasionally, a 4th CN palsy may be accompanied by a contralateral Horner syndrome (first-order neuron lesion) because of the proximity of the descending sympathetic pathway to the caudal portion of the nucleus. A relative afferent pupillary defect may also be associated with a 4th CN palsy due to the pupillary fibers running in the nearby brachium of the superior colliculus.
Combined 3rd and 4th nerve assessment
Evaluation for a fourth nerve palsy when a third nerve palsy is present requires examination of the affected eye for intact intorsion since depression of the eye is already impaired by the third nerve palsy. The patient should be asked to abduct the eye and then look down; the examiner typically tracks a vessel on the limbus of the cornea to ascertain for the presence or absence of intorsion2.
Oculomotor nerve palsy can also cause a vertical deviation along with weakness of adduction. In this case, however, the affected eye is usually lower (due to the spared superior oblique muscle), and ptosis or pupillary dilatation is often present3.
Causes
- Congenital
- Microvascular ischemia: In patients older than 50 years, this is a typical cause of an isolated CN IV palsy. Function always improves and typically recovers within 6 months.
- Closed head trauma because of the unique dorsal midbrain-crossing anatomy.
- Disease within the subarachnoid space or cavernous sinus.
- Giant cell arteritis
- Skew Deviation
There is a right hypertropia in primary position that worsens in adduction (left gaze) and also in right head tilt. There is right superior oblique underaction and right inferior oblique overaction.
There is a compensatory left head tilt.
Distinguishing 4th nerve palsy from a skew deviation
Skew deviations tend to be relatively concomitant, ie, the degree of misalignment changes little with different directions of gaze.
Note that in skew deviation abnormal torsion and vertical misalignment are head position dependent, in that they decrease substantially or disappear when the head changes from an upright to a supine position as the patient lies flat.
By contrast, in unilateral peripheral trochlear nerve palsy, there are minimal changes in the torsional or vertical deviation between the two head positions4.
From: Wong AM. Understanding skew deviation and a new clinical test to differentiate it from trochlear nerve palsy. J AAPOS. 2010;14(1):61-67. doi:10.1016/j.jaapos.2009.11.019
Parks three-step test has been commonly used for identifying the paretic muscle in vertical diplopia.
The steps are as follows:
- Determine which eye is hypertropic in the primary position of gaze. Example: If the right eye is hypertropic, one of four muscles must be paretic; right eye depressors (right superior oblique and right inferior rectus) or left eye elevators (left inferior oblique and left superior rectus).
- Determine whether the hypertropia increases with right or left horizontal gaze. Example: If the hypertropia is worse on left gaze, the affected muscles have now been limited to two muscles; right superior oblique and left superior rectus muscles.
- Determine whether the hypertropia is worse on head tilting to left or right (Bielschowsky test). Example: If the hypertropia is worse on head tilting to the right, only one possible paretic muscle remains; right superior oblique muscle. However, this step does not always show the difference and other clues should be used if this step is negative
If the 3-step test is negative for trochlear nerve palsy and is suggestive of weakness of other oculomotor muscles, a brain MRI with gadolinium enhancement should be performed because skew deviation can mimic palsy of any of the cyclovertical muscles3. Practically speaking, acquired vertical strabismus not from a CN IV palsy is often the result of the dysfunction of more than one muscle and will not generate a positive 3-step test. In particular, thyroid eye disease, myasthenia gravis, or dysfunction of multiple CNs may produce a range of nonspeciɹc patterns of ocular misalignment. The reliability of the 3-step test in identifying patterns of vertical strabismus lessens somewhat over time because of the phenomenon of “spread of comitance” .
When this patient with a right 4th nerve palsy tilts her head to the right, the right eye deviates upwards when the left eye fixates. The elevating action of the superior rectus is unopposed by the palsied superior oblique in the right eye
From: Brainkart.com. Ophthalmoplegia and Paralytic Strabismus. Retrieved from http://www.brainkart.com/article/Ophthalmoplegia-and-Paralytic-Strabismus_26155/
Congenital 4th nerve palsy
CN IV palsies are often congenital. Recognized causes of congenital CN IV palsies include an anomalous superior oblique tendon, an anomalous site of its insertion, or a defect in the trochlea. Similarly, some cases of presumed congenital CN IV palsy are secondary to a benign tumor (eg, schwannoma) of the nerve. Patients may be asymptomatic until adulthood, when their vertical fusional amplitudes diminish and diplopia develops. Most patients maintain a chronic head tilt. The longstanding nature of the head tilt may cause facial asymmetry and can often be conɹrmed by reviewing old photographs.
- On the left, a head tilt to the right is present.
- There is failure to depress the left eye in adduction (looking right and down).
- With gaze to the right (image on the right), the left eye elevates, due to inferior oblique overactivity.
