Sixth nerve palsy is the most common ocular motor nerve paralysis in adults. The sixth cranial nerve controls lateral rectus function. Acquired sixth nerve palsies cause binocular diplopia due to abduction deficit and incomitant esodeviation due to the exertion of the antagonist medial rectus muscle. Acquired sixth nerve palsies in adults can be due to microvascular ischemia, neoplasm, aneurysm, trauma, infection, inflammation, demyelination, and idiopathic etiologies [1]. Treatment of sixth nerve palsy usually requires a solution based on the underlying etiology and waiting for recovery of the cranial nerve for at least six months. Treatment of diplopia includes occlusion, base-out prisms, botulinum toxin, and strabismus surgery. Strabismus surgery is usually performed after stable angle of deviation. Muscle transposition surgery is performed in cases of complete sixth nerve palsy where there is no or less than 50% of lateral rectus muscle function.

Basilar artery aneurysm occurs in about 3–5% of intracranial aneurysms [2]. A large or giant basilar aneurysm may compress the midbrain and pons. The patient with an unruptured basilar aneurysm usually presents with headaches and neurological signs including sixth nerve palsy. We report a case with an unruptured and thrombosed mid-basilar aneurysm who presents with complete sixth nerve palsy. We performed a double-under muscle transposition (DUT) procedure and can correct the angle deviation of 66 prism diopters (PD).

A 41-year-old male presented with severe headache and binocular horizontal diplopia. He was a heavy smoker and quit smoking for two years. He had no history of any underlying disease or medications. Initial computed tomography (CT) scan from a regional hospital showed an unruptured giant mid-basilar aneurysm. He underwent implantation of a blood flow diverter and clot removal surgery. Cerebral angiography revealed complete healing of the basilar aneurysm. He received a 6-month dual antiplatelet therapy following stent implantation. Follow-up magnetic resonance imaging (MRI) brain was found no significant change of a giant thrombosed basilar artery aneurysm (Figure 1). He still had diplopia after 18 months of primary treatment and he was referred for ophthalmologic consultation.

On eye examination, the best-corrected visual acuity (BCVA) was 20/20 in both eyes. The intraocular pressure (IOP) was 15 mmHg in the right eye and 13 mmHg in the left eye. The orthoptic examination revealed complete left abduction deficit with left esotropia. The primary deviation, measured by the alternate prism cover test with right eye fixation, was esotropia 66 prism diopters (PD) at distance (Figure 2). Slit-lamp examination showed normal anterior and posterior segment in both eyes. The pupil size was 3 mm both react to light without relative afferent pupillary defect (RAPD). The forced duction test (FDT) was positive in left medial rectus muscle. Preoperative MRI orbit showed an atrophic change of left lateral rectus muscle (Figure 3).

Surgical treatment

Preoperative botulinum toxin injection 5.0 units have been added at left medial rectus muscle. The patient had 50% of left adduction, but he still had esotropia 66 PD at one month after botulinum toxin injection. We planned to do both DUT and medial rectus recession procedures in one-time surgery under general anesthesia. Intraoperatively, the FDT was positive in left medial rectus muscle. We performed 3.0 mm recession of left medial rectus muscle with the fornix-based conjunctival incision. The DUT technique was initiated by a 240° limbal conjunctival peritomy. The Tenon capsule and the intramuscular septum surrounding the superior rectus, lateral rectus, and inferior rectus muscles were dissected free. The superior and inferior rectus muscles were split nasal half-tendon width (about 5 mm) up to approximately 10 mm from the insertion. The transposed nasal half-tendon width of vertical rectus muscles crossed under the remaining temporal halves of vertical rectus muscles with 5-0 vicryl double-armed sutured near the left lateral rectus muscle insertion by hang back technique (Figure 4 and Figure 5). A limbal-based conjunctival incision was sutured by 8-0 vicryl.

Postoperative follow-up and alignment after surgery

The patient had improvement of alignment in the immediate postoperative period (Figure 6). At one week follow-up, he had the improvement to slightly abduct the left eye and he had orthotropia at near and distance (Figure 7). Slit-lamp examination revealed conjunctival chemosis of the left eye (Figure 8). We prescribed 1% prednisolone acetate every 2 hours for one week then tapered to four times a day for two weeks and twice a day for one week. At one month follow-up, he had 50% to abduct the left eye and he had esophoria 4 PD at near and 6 PD at distance (Figure 9). After one month, we noticed that he still had some chemosis, so we switched to 0.1% fluorometholone four times a day for two weeks. The patient received a course of topical corticosteroids for six weeks and he did not have steroid-induced ocular hypertension or glaucoma. He had IOP 14 mmHg of the left eye by Goldmann applanation technique. At postoperative three months, he was found to be orthotropia at near and exotropia 8 PD at distance (Figure 10).

The principle of muscle transposition surgery is to transfer force from other muscles to help the paralyzed muscle, a change in the vector forces of the transposed muscles will improve the ocular misalignment in the primary position [3]. In 1907, the partial vertical rectus muscle split transposition technique was first described by Hummelsheim for the management of complete sixth nerve palsy. Hummelsheim procedure will split and disinsert the temporal half-tendon width of the superior rectus and inferior rectus muscles to the insertion of the lateral rectus muscle [4]. The results of Hummelsheim procedure have been reported to correct the angle deviation from 43 ± 5 PD before surgery to 6 ±7 PD after surgery. In 1946, Jensen et al. described Jensen procedure, in this procedure the split half-tendon width without dis-inserted of the superior rectus and inferior rectus muscles is tied with the lateral rectus muscle [5]. In 2003, Nishida et al. described Nishida procedure, in this procedure the split temporal half-tendon width of the superior rectus and inferior rectus muscles is sutured 8 mm posterior to the lateral rectus muscle insertion [6]. The results of Nishida procedure have been reported to correct the average angle deviation of 42.4 PD.

Recently, Furuse et al. described the double-under muscle transposition (DUT) procedure, in this novel procedure the split nasal half-tendon width of the superior rectus and inferior rectus muscles is crossed under both the remaining temporal halves with suture anchored near the lateral rectus muscle insertion. To increase the effectiveness of DUT procedure, recession of medial rectus muscle can be added along with this procedure in the case with a positive force duction test. The results of DUT procedure have been reported to correct the average angle deviation of 60 ± 6 PD [7]. However, cases of overcorrection have been reported.

There are several techniques of muscle transposition surgery for sixth nerve palsy. For the large angle deviation in sixth nerve palsy, the surgical technique can correct the esodeviation of 58.3 ± 10.8 PD [8],[9],[10]. Surgeons usually perform surgical technique depending on the residual lateral rectus function, degree of palsy, and lateral incomitance. In the case presentation, we performed combined DUT procedure and medial rectus recession because the patient had a positive FDT. Moreover, we believed that the split nasal half-tendon width instead of the split temporal half-tendon width of the vertical rectus muscles can enhance the force of the transposed muscles. We can correct the large angle deviation up to 66 PD. There was no postoperative complication including anterior segment ischemia. In complete palsy, DUT procedure may be an alternative procedure of choice.

We demonstrated that a DUT technique can be a procedure of choice in patients with large angle deviation from paralytic strabismus. This procedure can be performed simultaneously with medial rectus surgery, especially in cases with large angle of deviation with an evidence of medial rectus restriction. Our patient restored the binocular field and obtained satisfactory outcomes with the DUT procedure.

Take home messages

Always do force duction test of antagonist muscle in restrictive strabismus.

Muscle transposition is essential for correction in case of negative for force generation.

Double-under muscle transposition technique may take a role in large deviation case.