15.3Fourth Nerve Palsy

In fourth nerve palsy hypertropia, inferior oblique overaction and superior oblique underaction is observed in the a ected eye. In long-standing unilateral cases, a secondary contracture of the superior rectus develops and a pseudo overaction of the superior oblique muscle in the sound eye is observed. Abnormal head posture and a positive Bielschowsky head tilt test are the other findings of fourth nerve palsy. In unilateral cases, the typically observed abnormal head posture is chin down with head tilt toward the una ected side. In bilateral cases, the abnormal head posture may be as in unilateral cases if there is marked asymmetry. If the bilaterality is symmetrical, then the abnormal head posture aims to compensate the “V” pattern. In acquired cases, vertical or torsional diplopia is the main complaint of the patients. Congenital cases do not usually complain about diplopia; however, in decompensated congenital fourth nerve palsy, the patient has vertical diplopia. Some patients may benefit from prisms but most of the patients require surgical treatment.

For a correct surgical plan, one needs to have the correct answers for the following questions:

■What is the amount of deviation in primary position?

■What is the position of gaze with the largest deviation?

■Is it congenital or acquired?

■Is there any superior oblique tendon laxity?

■Is there any superior rectus contracture?

■Is it unilateral or bilateral?

■Is there any torsional diplopia?

What is the amount of the deviation in primary position?

If the vertical deviation in primary position is exceeding 15 prism diopters, two muscle surgeries need to be considered.

What is the position of gaze with the largest deviation?

The surgical treatment should be planned on the EOMs functioning in the field of gaze with the largest deviation. To obtain a reliable data, the measurement of the deviation should be done in nine diagnostic positions of gaze.

Is it congenital or acquired? The reply to this question has a specific importance in fourth nerve palsy.Congenital cases may present with superior oblique tendon abnormalities, such as abnormal tendon laxity, tendon insertion abnormalities, and sometimes even agenesis of the tendon [16–19]. Because of the frequent tendon abnormalities in congenital cases, it was proposed that these cases might have primary developmental abnormality of the superior oblique tendon rather than fourth nerve palsy [16]. However, in a previous MRI study where we looked for the superior oblique muscle size in congenital and acquired cases, we demonstrated that congenital cases with abnormal tendon laxity may have denervation atrophy in the superior oblique muscle bulk and our findings were confirmed in other recent studies [20, 21]. If the abnormality would only be limited with the tendon itself, denervation atrophy would not be expected to develop in those cases with congenital fourth nerve palsy. The differential diagnosis in congenital and acquired cases is not only important for the etiological investigation but also for surgical planning. The clinical clues suggesting that the patient has a congenital superior oblique palsy may be summarized as follows:

■History, old photos

■Absence of a preceding event

■Prominent abnormal head posture

■Facial asymmetry

■Coexistence of amblyopia

■Significant superior oblique underaction

■Large vertical fusional amplitude

■Coexisting horizontal deviation

■Absence of subjective torsion

Is there any superior oblique tendon laxity? Superior oblique tendon laxity can be assessed prior to surgery with traction test that was described by Guyton [22] and modified by Plager [17]. The globe is fixated by two forceps at inferior nasal and superior temporal areas and with retropulsion the globe is elevated on adduction. With this maneuver, the globe is pushed against the superior oblique tendon, and with back and forth movements, the globe the tendon can easily be felt (Fig. 15.6). If there is an agenesis of the superior oblique tendon, the tendon cannot be felt and the globe is totally free with back and forth movements. As an additional finding when the globe is elevated on adduction cornea disappears in total if there is a tendon laxity.

Is there any superior rectus contracture? Superior rectus contracture may develop in long-standing fourth nerve

palsy. In these cases, traction test is positive in depression on adduction. In motility examination, a limitation of depression on adduction and a pseudo overaction of the superior oblique muscle in the sound eye are the clues for superior rectus contracture (Fig. 15.7). Recession of superior rectus muscle is advised in those cases with superior rectus contracture [23, 24] (Fig. 15.8).

Is it unilateral or bilateral? Especially in traumatic cases, masked bilaterality is very common.All of the cases with fourth nerve palsy should be carefully evaluated for the clues of bilateral involvement [25, 26]. The bilateral involvement may be asymmetric but even with marked asymmetry surgery should be planned in both eyes. The clinical clues suggesting bilateral involvement are as follows:

■Bilateral inferior oblique overaction.

■Bilateral superior oblique underaction.

■Positive Bielschowsky head tilt test with the head tilted on both sides. In case of a marked asymmetry, Bielschowsky head tilt test may be positive on the side with marked involvement.

■“V” pattern deviation.

■Abnormal head posture to compensate the “V” pattern.

■Objective torsion exceeding 10° [40].

Is there any torsional diplopia? Torsional diplopia is a symptom that occurs in acquired fourth nerve palsy. The patients with a decompensated congenital fourth nerve palsy has vertical diplopia without a torsional element,

although an excyclotorsion is observed in fundus examination, and this is one of the clues for di erential diagnosis of a congenital and acquired fourth nerve palsy. Some patients may not describe torsional diplopia properly

unless asked specifically and may complain about “blurring” in certain gaze positions.

Surgical methods of treatment may be summarized as follows:

■Inferior oblique weakening procedures

■Superior oblique strengthening procedures

■Superior rectus recession in the a ected eye

■Inferior rectus recession in the contralateral eye

Inferior oblique weakening procedures: Inferior oblique weakening procedures are the most commonly performed operations for treatment of fourth nerve palsy [41, 42]. The weakening procedures are disinsertion, myectomy, recession, and anteroposition of the inferior oblique muscle. Inferior oblique weakening should be performed in all cases with inferior oblique overaction. Our preferred method for inferior oblique weakening is disinsertion. If the deviation in primary position is more than 15 prism diopters, inferior oblique weakening will not be enough to correct the deviation [27]. Anteroposition of inferior oblique muscle should be regarded with caution as it may cause asymmetrical results because of the limitation of elevation and it is not recommended in unilateral cases [24]. Anterior and nasal transposition of inferior oblique muscle is a recently described method to be used in ones with congenital absence of superior oblique tendon [28].

Superior oblique strengthening procedures: Superior oblique strengthening procedures are superior oblique tendon tuck and Fells modified Harada-Ito operation. Superior oblique tendon tuck has a high risk of iatrogenic Brown syndrome in acquired cases with a normal tendon. However, it is a safe and very e ective procedure in congenital cases with abnormal tendon laxity [18, 29]. In cases with marked hypertropia and marked abnormal head posture, superior oblique tendon tuck may be performed alone or usually in combination with inferior oblique weakening. If there is no apparent inferior oblique overaction, superior oblique tendon tuck may be performed without weakening the inferior oblique muscle. To reduce the risk of iatrogenic Brown syndrome, traction test must be performed after tucking with loop sutures (Fig. 15 6). If the traction test is positive then the amount of tuck should be reduced. The triad of indications for superior oblique tendon tuck is large angled vertical deviation, prominent abnormal head posture, and superior oblique tendon laxity.

In acquired cases with marked torsional diplopia, Fells modified Harada-Ito procedure is the method of choice that strengthens the anterior torsional fibers. The anterior fibers of superior oblique muscle are transposed lateral and anteriorly at the upper border of the lateral rectus

muscle [26, 30]. This procedure is usually performed bilaterally and has a minimal e ect on the vertical deviation in primary position and does not alter the esodeviation on downgaze. So, it is only indicated if there is subjective torsional complaint that need to be corrected.

Superior rectus recession in the a ected eye: The indication for superior rectus recession is a vertical deviation exceeding 15 prism diopters in combination with superior rectus contracture [23, 24]. It should be considered as an additional surgery with inferior oblique weakening. The predictability of the recession in a restricted superior rectus muscle will be low and adjustable recession should better be preferred in those cases. In cases with agenesis of the superior oblique tendon, superior rectus recession is the procedure of choice with inferior oblique weakening.

Inferior rectus recession of the contralateral eye: T he cases that do not fit any of the indications specified above and where there is a vertical deviation exceeding 15 prism diopters are the candidates for contralateral inferior rectus recession. It can be performed in combination with inferior oblique weakening of the a ected eye or as a secondary procedure in cases with residual deviation. Progressive overcorrection and lower eyelid retraction are well recognized problems with inferior rectus recession [31].

In summary for an appropriate surgical plan for the individual patient, the diagnosis of a congenital or acquired palsy, the deviation in nine positions of gaze, abnormal head posture, the subjective characteristics of diplopia, and the traction test results are required.

In some particular cases BTXA may be used. Some authors reported encouraging results with BTXA injectionof ipsilateralinferiorobliquemuscle[32].Contralateral inferior rectus injection may be performed during acute or chronic superior oblique palsies. Botulinum toxin is helpful to control postoperative over and undercorrections; ipsilateral inferior rectus injection in the former and contralateral inferior rectus injection in the latter [33]. In our clinical practice, we use BTXA only for inferior rectus muscle in fourth nerve palsy and the patient benefits with BTXA injection if there is no significant torsional element.

Summary for the Clinician

■The pearl is the correct evaluation of a congenital and acquired case. Large vertical fusional amplitudes, facial asymmetry, and absence of torsional diplopia are the major clues for congenital fourth nerve palsy.