54Chapter 3: Congenital anomalies or acquired disease?

look a bit “blurry” when she looked to the left. An MR scan of brain and orbits was unrevealing.

What feature identifies this woman’s abduction deficit as a congenital, rather than acquired, sixth nerve palsy?

In addition to incomplete abduction of the left eye there is narrowing of the palpebral fissure and globe retraction when that eye is adducted. This motility pattern indicates a Type I Duane’s syndrome, i.e. a form of congenital sixth nerve palsy. The patient was reassured as to the benign nature of this defect.

Discussion: Duane’s syndrome is a congenital brainstem anomaly in which there is aberrant innervation of the lateral rectus muscle with resultant co-firing of the ipsilateral medial and lateral rectus muscles. Pathologic studies in this syndrome have shown agenesis of the sixth nerve nucleus with innervation to the lateral rectus muscle supplied instead by branches of the third nerve. This anomalous innervation produces a synkinesis on attempted adduction due to simultaneous activation of the medial and lateral rectus muscles resulting in globe retraction and fissure narrowing. Three subtypes have been defined based on clinical and electromyographic features. Type I is the most common form and is characterized by incomplete abduction, as exemplified in this patient. In Type II, adduction is impaired, and in Type III, both adduction and abduction are incomplete. Duane’s syndrome is more common in women and most often affects the left eye, for reasons that are unknown. Other features that may be present in Duane’s syndrome include exaggerated elevation or depression of the eye in adduction and A-, V-, or X-pattern ocular deviations. Occasional patients have other forms of anomalous innervation such as a Marcus Gunn jaw-wink or paradoxical- gustatory-lacrimal reflex (“crocodile tears”) and some others have developmental anomalies including Goldenhar syndrome, Klippel–Feil anomaly, sensorineural deafness and Wildervanck syndrome (cervico-oculo-acoustic anomaly).

Despite loss of abduction, patients with Type I Duane’s syndrome usually remain well aligned in primary position. This clinical feature is extremely helpful for distinguishing this condition from acquired sixth nerve palsy and from other causes of acquired abduction deficit which typically produce esotropia.

Diagnosis: Type I Duane’s syndrome

Tip: An abduction deficit that is acquired should produce diplopia. When it does not, a congenital anomaly should be suspected. Look carefully for narrowing of the palpebral fissure and globe retraction when the eye is in adduction.

Intermittent vertical diplopia

Case: A 50-year-old salesman presented with a one-year history of intermittent vertical binocular diplopia unassociated with head or eye pain. His double vision usually occurred at the end of the day when he was tired. He denied other focal neurologic deficits including dysphagia, dysarthria, limb weakness, hoarseness and fatigue with chewing. An MRI of brain and orbits, MRA of the cranial vessels, thyroid function tests, acetylcholine receptor antibodies and results of a lumbar puncture were all normal or negative.

Neuro-ophthalmic examination demonstrated a 10 diopter left hyperphoria in primary position, worse on right gaze and with left head tilt. There was subtle underaction of the left superior oblique with otherwise full extraocular movements. Testing with double Maddox rods indicated five degrees of excyclotorsion OS. Lids and pupils were normal including no fatigability or lid twitch. His examination was consistent with a left superior oblique (fourth nerve) palsy, but investigation for specific causes had been unrevealing. Because of the intermittent nature of his symptoms and worsening of diplopia with fatigue, myasthenia was suspected. A Tensilon (edrophonium chloride) test, however, was negative.

Table 3.3 Causes of fourth nerve palsy

Trauma

Congenital

Idiopathic

Tumor

Brainstem stroke

Demyelinating disease

Increased intracranial pressure

Dural-cavernous fistula

Aneurysm

What other causes of fourth nerve palsy should be considered?

The most common types of fourth nerve palsy are traumatic, congenital and idiopathic (See Table 3.3). At this point, the possibility of a congenital fourth nerve palsy was considered.

How would you pursue a diagnosis of congenital fourth nerve palsy in this patient?

His vertical fusional amplitudes were assessed with a prism bar, yielding a measurement of 12 diopters. Such supra-normal fusional capacity is, for practical purposes, diagnostic of a congenital fourth nerve palsy. Inspection of childhood photographs confirmed a pre-existing and long-standing right head tilt. Based on these observations, a diagnosis of congenital right fourth nerve palsy was made. He was treated with spectacle correction that included a total of 6 diopters of prism (3 base up in the right eye, 3 base down in the left) and enjoyed relief of his intermittent diplopia.

Discussion: Common causes of vertical diplopia include cranial nerve palsy (third or fourth), skew deviation, restrictive orbitopathy and myasthenia. Fourth nerve palsy can usually be distinguished from other mechanisms by means of the Bielschowski three-step test, also known as the head tilt test (Figure 3.14). This useful technique is designed to determine the paretic muscle in

patients with vertical diplopia and is performed as follows:

Step 1 Record which is the higher eye in primary position. If the left eye is higher, for example, there is either failure of elevation of the right eye (weakness of the superior rectus or inferior oblique muscle) or failure of depression of the left eye (weakness of the superior oblique or inferior rectus muscle).

Step 2 Note whether the misalignment is greater on left gaze or right gaze. This step is based on the principle that any ocular motor deviation will be worse in the field of action of the paretic muscle. A left hypertropia that is worse on right gaze must be due to weakness of a muscle acting in that field of gaze, thus narrowing down the choices to the right superior rectus or the left superior oblique.

Step 3 Record the direction of head tilt that worsens the deviation. Tilting to the left normally evokes excyclotorsion of the right eye and incyclotorsion of the left eye. Incyclotorsion is normally accomplished by both the superior rectus and superior oblique, whose vertical actions cancel each other. In the presence of superior oblique weakness, however, head tilt toward the involved side evokes the unopposed vertical action of the intact superior rectus, thus further elevating that eye. It is this mechanism that accounts for the third step of the head tilt test. Thus, a patient with left fourth nerve palsy will have a left hyperdeviation in primary position that is worse on right gaze and with left head tilt.

The most common cause of an acquired fourth nerve palsy is trauma (see Table 3.3). The most common non-traumatic mechanism is a congenital anomaly of the superior oblique muscle or its tendon. Congenital fourth nerve palsy often presents in mid-life rather than childhood, not because of worsening muscle weakness, but rather due to progressive loss of fusion. The range of fusional capacity is variable and tends to decline over a lifetime. Individuals with a congenital fourth nerve palsy initially have sufficiently large fusional capacities to