Fourth Cranial Nerve Palsy

An acute CN IV palsy typically causes diplopia that is worse in contralateral gaze, ipsilateral head tilt, and downgaze; hence, patients almost always report diplopia (or a 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 PACT or Maddox rod testing to demonstrate a hypertropia that worsens on contralateral gaze and ipsilateral head tilt. Ipsilateral head tilting usually increases the vertical strabismus; thus, patients typically (subconsciously) tilt their head to the opposite side to avoid diplopia.

The Parks-Bielschowsky 3-step test is an algorithmic approach to identifying ocular motility patterns that conform to dysfunction of a specific vertically acting extraocular muscle. The 3 steps are as follows:

1. Find the side of the hypertropia in primary gaze.

2. Determine if the hypertropia is greater on left or right gaze.

3. Determine if the hypertropia is greater on left or right head tilt.

Excyclotropia of the affected eye should be assessed by double Maddox rod testing and indirect fundoscopy. The 3-step test is most helpful in determining whether a vertical strabismus conforms to the pattern of a CN IV palsy. For example, a right CN IV palsy shows right hyperdeviation that worsens on left gaze and right head tilt (positive 3-step test) and causes excyclotropia of the right eye. (For more on the 3-step test, see BCSC Section 6, Pediatric Ophthalmology and Strabismus.) Occasionally, a skew deviation mimics a CN IV palsy on the 3-step test. However, in a skew deviation, the hypertropic eye will often be intorted as opposed to extorted, as it is in a CN IV paresis. In addition, moving the patient from sitting to a supine position (by reclining the examination chair) often reduces the magnitude of the hypertropia in skew deviation but has little or no effect on a CN IV palsy. Practically speaking, acquired vertical strabismus not from a CN IV palsy is often the result of the dysfunction of more than 1 muscle and will not generate a positive 3-step test. In particular, TED, myasthenia gravis, or simultaneous dysfunction of multiple CNs can produce a variety of nonspecific 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 called “spread of comitance” (see the section Comitant and Incomitant Deviations).

Bilateral CN IV palsy should always be considered whenever a unilateral palsy is diagnosed, especially after head trauma. Patients with bilateral CN IV palsy present with

• crossed hypertropia (ie, the right eye is higher on left gaze, and the left eye is higher on right gaze)

• extorsion of ≥10° (best measured with double Maddox rod testing)

• large (≥25 prism diopters [Δ]) V-pattern esotropia

• habitual chin down posture

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 confirmed by reviewing old photographs (Fig 7-11). Patients with a longstanding CN IV palsy have relatively large vertical fusional amplitudes (>3 Δ). Comparing superior oblique size on MRI between the affected and unaffected sides does not reliably distinguish acquired from congenital palsies.

In patients older than 50 years, an isolated CN IV palsy is typically caused by microvascular ischemia. Function always improves and typically recovers within 6 months. CN IV is particularly vulnerable to closed head trauma because of the unique dorsal midbrain-crossing anatomy. In addition, CN IV can be damaged by disease within the subarachnoid space or cavernous sinus.

Figure 7-11 Congenital left CN IV palsy. A, Note the left hypertropia and right head tilt as a child. B, The right head tilt is still present 40 years later, but the patient describes more difficulty maintaining single, binocular vision. C, After eye muscle surgery, the diplopia and head tilt have resolved.

(Courtesy of Lanning B. Kline, MD.)

Diagnostic evaluation for an isolated, nontraumatic CN IV palsy usually yields little information because most cases have congenital, ischemic, or idiopathic causes. In older patients, who are at increased risk for vasculopathy, a full medical evaluation is appropriate to assess for vascular risk factors (eg, diabetes mellitus, hyperlipidemia, and hypertension). Occasionally GCA can cause a CN IV palsy. Older patients should have follow-up evaluation to ensure recovery. Lack of improvement after 3 months should prompt neuroimaging directed toward the skull base to search for a mass lesion. Other possible causes of an acquired vertical strabismus include orbital restrictive syndromes (eg, TED or previous trauma). Skew deviation, partial CN III palsy, or myasthenia gravis should be considered in atypical cases.

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Helveston EM, Krach D, Plager DA, Ellis FD. A new classification of superior oblique palsy based on congenital variations in the tendon. Ophthalmology. 1992;99(10):1609–1615.

Uchiyama E, Matsuo T, Imai S, Itoshima E. Paretic side/normal side ratios of cross-sectional areas of the superior oblique muscle vary largely in idiopathic superior oblique palsy. Am J Ophthalmol. 2010;149(3):508–512.

Wong AM, Colpa L, Chandrakumar M. Ability of an upright-supine test to differentiate skew deviation from other vertical strabismus causes. Arch Ophthalmol. 2011;129(12):1570–1575.

Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.