A fourth nerve palsy typically causes diplopia that is worse in downgaze; hence, patients almost always report diplopia (or the tendency to close 1 eye) while reading. In some cases, examination of the affected eye reveals limited downgaze in the adducted position, but, in most cases, ocular motility appears grossly normal. Accordingly, it is essential to perform cover-uncover or Maddox rod testing to demonstrate a hypertropia that worsens on contralateral downgaze. Ipsilateral head tilting usually increases the vertical strabismus, and, therefore, patients typically (subconsciously) tilt their head to the opposite side to avoid diplopia.
The Parks-Bielschowsky 3-step test is a time-honored algorithmic approach to identifying patterns of ocular motility that conform to dysfunction of specific vertically acting extraocular muscles. The 3 steps are
Find the side of the hypertropia.
Determine if the hypertropia is greater on left or right gaze.
Determine if the hypertropia is greater on left or right head tilt.
Beyond these 3 steps, it is also useful to determine if the vertical separation is greater in upgaze or downgaze (a fourth step) and check for relative cyclotropia.
The 3-step test is most helpful in determining whether a vertical strabismus conforms to the pattern of a fourth nerve palsy; for example, a right fourth nerve palsy shows right hyperdeviation that worsens on left gaze, right head tilt, and downgaze, with relative excyclotropia of the right eye. (More specific details on this testing are described in BCSC Section 6, Pediatric Ophthalmology and Strabismus.) Occasionally, a skew deviation mimics a fourth nerve palsy on the 3-step test but can distinguish itself by nonconformity to these rules. Practically speaking, the specific muscle(s) involved and the etiology of a vertical strabismus not due to a fourth nerve palsy is often not resolved by the 3-step plus fourth step test, because acquired vertical strabismus is often the result of the dysfunction of more than one muscle. In particular, thyroid eye disease, myasthenia gravis, or dysfunction of multiple ocular motor cranial nerves produces a wide variety of nonspecific patterns of ocular motility. The reliability of the 3-step test in identifying patterns of vertical strabismus lessens somewhat over time because of the phenomenon known as “spread of comitance” (see Chapter 7).
Bilateral fourth nerve palsy should always be considered whenever a unilateral palsy is diagnosed, especially after head trauma. Bilateral fourth nerve palsy presents with
crossed hypertropia (ie, the right eye is higher on left gaze, and the left eye is higher on right gaze)
excyclotorsion of 10° or greater (each eye rotates outwardly; best measured with double Maddox rod testing)
. Palsies of the trochlear nerve: diagnosis and localization—recent concepts.. 1993;68(5):501–509.
a large (≥25 D) V pattern of strabismus
Fourth nerve palsies are often congenital. An anomalous superior oblique tendon, an anomalous site of its insertion, or a defect in the trochlea are now recognized as causes of some congenital fourth nerve palsies; similarly, some cases of presumed congenital fourth nerve palsy are secondary to a benign tumor (eg, schwannoma) of the fourth nerve. Patients are often asymptomatic until the fourth to sixth decades of life, when their vertical fusional amplitudes diminish and diplopia develops. Most patients maintain a chronic head tilt. The long-standing nature of the head tilt can often be confirmed by reviewing old photographs (Fig 8-12). Patients with a long-standing fourth nerve palsy have a relatively large vertical fusional range (>3 prism diopters).
, KrachD, PlagerDA, EllisFD. A new classification of superior oblique palsy based on congenital variations in the tendon.. 1992;99(10):1609–1615.
In patients older than 50 years, an isolated fourth nerve palsy is typically caused by microvascular ischemic disease, and function always improves and typically resolves within 3 months. The fourth nerve is particularly vulnerable to closed-head cranial trauma due to the unique dorsal midbrain crossing anatomy. In addition, the fourth nerve can be damaged by disease within the subarachnoid space or cavernous sinus.
Diagnostic evaluation for an isolated, nontraumatic fourth nerve palsy usually yields little information because most cases are congenital, ischemic, or idiopathic. In patients in the vasculopathic age group, a full medical evaluation looking for vascular risk factors, including diabetes, hyperlipidemia, and hypertension is appropriate. Older patients should be followed to ensure recovery. Lack of recovery after 3 months should prompt neuroimaging directed toward the base of the skull to search for a mass lesion. Other possible causes of an acquired vertical strabismus include orbital restrictive syndromes (eg, thyroid eye disease or previous trauma). Skew deviation, partial oculomotor nerve palsy, or myasthenia gravis should be considered in atypical cases.