Orbital imaging is a resource available to strabismus surgeons to help them understand the pathological processes underlying complex ocular motility presentations. This can be particularly useful in adult strabismus when considering various causes of ocular motility disturbances, such as thyroid eye disease, orbital tumors, or orbital inflammation. Imaging also plays a particularly important role in patients with previous trauma when looking for extraocular muscle injury. In the case presented here, computed tomography (CT) proved not to be representative of the actual orbital anatomy encountered at the time of strabismus surgery.
A 36-year-old man presented to a tertiary ophthalmology center for assessment of a long-standing eye turn. He described being involved in an automobile accident approximately 30 years earlier and having an esotropia ever since. The patient was unsure as to whether any surgery was required at the time of injury other than the repair of a skin laceration. He denied having diplopia but described always having poor vision out of his right eye. His best corrected visual acuity was 20/80 in the right eye and 20/20 in the left eye. He had a constant right esotropia measuring greater than 100 prism diopters in both distance and near (Figure 1). He was unable to abduct the right eye to midline. There was a scar above his right lateral canthus at the location of his previous skin laceration. There was no obvious scarring of the conjunctiva and the intraocular examination was unremarkable. A CT scan of the orbits showed absence of the right lateral rectus muscle (Figure 2).
Primary position showing a greater than 100 prism diopter right esotropia.
Computed tomography scan with transverse (left) and coronal (right) images showing an absent right lateral rectus muscle. Close examination shows a dark patch in the area where the lateral rectus muscle should be found.
A surgical plan was devised to perform a maximum right medial rectus recession and a vertical rectus muscle transposition to the location where the lateral rectus muscle should have inserted. Intra-operative forced ductions were tight on attempted abduction and free to adduction. To our surprise, the lateral rectus muscle was present and had a normal appearance. It was identified 6.5 mm from the limbus. The surgical plan was not adjusted and the right medial rectus muscle was recessed 8 mm from its insertion and the entire right superior rectus muscle was transposed adjacent to the insertion of the lateral rectus muscle with an augmentation suture 8 mm posterior to its new insertion. Postoperative magnetic resonance imaging (MRI) was ordered, which showed a present, but extremely atrophic, right lateral rectus muscle (Figure 3).
Magnetic resonance imaging scan with transverse (left) and coronal (right) images showing an atrophic right lateral rectus muscle.
At a 6-month follow-up appointment, the patient had satisfactory ocular alignment with a 16 diopter esotropia and a 5 prism diopter right hypotropia in primary position. His abduction of the right eye had improved to −3.
Modern imaging techniques can be helpful in evaluating ocular motility by allowing a direct evaluation of orbital anatomy. This imaging is typically either CT or MRI. CT is often the initial imaging modality of choice in a public health care system due to ease of access, low cost, and great resolution of bony anatomy.
In the case presented here, clinical examination showed absent function of the right lateral rectus muscle and preoperative CT scanning showed an absent lateral rectus muscle. However, the lateral rectus muscle was present at the time of surgery. We hypothesize that this disconnect between CT scanning and actual anatomy was due to denervation atrophy that likely occurred secondary to the trauma the patient experienced 30 years prior. In denervation atrophy, muscle volume decreases and fatty infiltration of the affected muscle ensues.1–3 This atrophy appears dark on CT and is easily missed unless the radiologist and ophthalmologist are specifically looking for it. Muscle atrophy has been appreciated on CT scans of skeletal muscles after lower motor neuron injury.4 Cranial nerve damage in the head and neck causes a similar picture in the muscles innervated by the affected nerve.5 Hansman et al.6 reported two cases of extraocular muscle atrophy on CT scanning. One case was related to a cavernous sinus lesion and the other was related to ophthalmoloplegia plus and probable Kearns–Sayre syndrome. In our case, an extreme amount of atrophy could have resulted in the lateral rectus muscle appearing to be absent on the CT scan.
Due to the relative superiority of MRI to CT scanning for soft tissue detail, we ordered a postoperative MRI. This scan showed a present but atrophic lateral rectus muscle. Horton et al.7 first reported MRI evidence of superior oblique muscle atrophy in a patient with trochlear nerve palsy. Subsequently, multiple MRI studies have quantitatively demonstrated extraocular muscle atrophy in several forms of paralytic strabismus, including abducens nerve palsy.8–10
This case depicts an example of when orbital imaging should provide assistance in preoperative planning for the strabismologist. Unfortunately, the CT scan that was initially obtained did not provide the correct information and an MRI would have been a better imaging modality to look for soft tissue changes involving the lateral rectus muscle.
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