Sir,

We present signs of superior oblique underaction in a patient with orbital plexiform neuroma with radiological evidence of trochlear displacement as the primary cause of underaction. We postulate mechanisms that account for the features seen on ocular motility examination in this particular tumour.

We present unusual signs in a case of unilateral medial orbital plexiform neuroma in a 22-year-old white male with neurofibromatosis type 1.

The patient presented with significant mechanical ptosis of the left upper lid associated with a rubbery tumour in the superomedial orbit. MRI scanning showed a large plexiform neuroma in the left superomedial orbit causing ptosis by anteroinferior displacement of the tarsal plate. He underwent uncomplicated levator dehiscence repair to elevate the upper lid with deliberate avoidance of the tumour. He made a good recovery, with a good cosmetic and excellent functional result.

It had been noted that examination of eye movements revealed striking features of left inferior oblique overaction, with gross hyperdeviation of the left eye on right gaze (Figure 1). There was no associated diplopia as the bulk of tumour at the nasal bridge obscures the view from the left eye on right gaze (Figure 2). Of note, the patient was not concerned by his eye movement abnormalities (for the reason outlined above), and was purely interested in correction of his ptosis for functional purposes.

Figure 1
figure 1

Eye movements demonstrating left superior oblique underaction on right gaze simulating inferior oblique overaction. Notice that tumour bulk effectively occludes left eye on right gaze.

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Figure 2
figure 2

Axial MRI showing plexiform neuroma at medial canthus extending posteriorly into orbit.

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Comment

It was initially thought that the plexiform neuroma was inducing a left fourth nerve palsy; however, the trochlear nerve enters the superior oblique muscle approximately one-third the way along its length from its origin at the body of sphenoid, above the tendinous ring, and the trochlea. The MRI scan shows that the tumour does not extend much further posteriorly than the trochlea (Figure 3).

Figure 3
figure 3

Axial MRI showing posterior displacement of the left trochlea, with increase in angle between the two limbs of the superior oblique tendon.

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Additionally, the trochlear nerve enters the muscle from the orbital aspect, rather than the side adjacent to the ethmoid and frontal bone. The tumour is not seen on the orbital side of the muscle (Figure 4). The MRI scan shows the tumour extending posteriorly into the orbit and beneath the trochlea, detaching it from the frontal bone. The trochlea is seen at the angle of the superior oblique tendon. Comparison with the contralateral anatomy shows distinct posterior displacement of the trochlear complex with the trochlea being attached to the tumour rather than the frontal bone. The amount of displacement does not, however, appear consistent with the degree of superior oblique underaction.

Figure 4
figure 4

Axial MRI. Note that there is no posterior invasion of the orbit by the tumour but that the trochlea has been displaced posteriorly and laterally.

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Plexiform neuromas have unusual mechanical properties, with firm, rubbery characteristics. It may be that in addition to inducing displacement of the trochlea, the tumour also acts like an elastic band, allowing the trochlea to travel posteriorly into the orbit upon contraction of the superior oblique muscle, and pulling the complex forward again during relaxation. This would ensure two things:

  1. 1

    The force of contraction is not transmitted to the globe, inducing the appearances of inferior oblique overaction (superior oblique underaction).

  2. 2

    The tendon-muscle complex does not undergo contracture. Contracture of the complex would eventually partially reverse the effects of trochlear displacement by taking up the slack in the tendon.

Posterior displacement of the trochlea complex also redirects the superior oblique tendon, increasing its incyclotorsion vector and reducing the depression vector (Figure 4). This acts synergistically to increase the observed ocular motility abnormality.

While numerous case reports and case series of fronto-orbital mucocoeles and other tumours exist, this case is the first report of trochlear displacement by this rare orbital tumour. It is interesting to consider that the mechanical properties of the tumour substance may influence the forces it exerts and has exerted upon it by surrounding tissues.