From the Departments of Ophthalmology (JMK, J-MH) and Radiology (JHK), Seoul National University College of Medicine, Seoul National University Bundang Hospital, Seongnam, Korea.
Supported by Seoul R&BD Program (ST090841) and a grant of Korea Health 21 R&D Project, Ministry of Health, Welfare and Family Affairs, Republic of Korea (A080299).
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Jeong-Min Hwang, MD, Department of Ophthalmology, Seoul National University College of Medicine, Seoul National University Bundang Hospital, 166, Gumiro, Bundang-gu, Seongnam, Gyeonggi-do 463-707, Korea. E-mail: firstname.lastname@example.org
Aplasia of the inferior rectus muscle is a rare congenital disorder1–6; therefore, it is usually not considered in the differential diagnosis of vertical strabismus. Monocular elevation deficiency or double elevator palsy is the inability to elevate one eye. Our patient presented with left hypertropia and inability to elevate the right eye. Repeated examinations performed over a few months revealed limitation of depression in the left eye. The patient was finally diagnosed as having left inferior rectus aplasia on the basis of magnetic resonance imaging (MRI) findings. This case illustrates that inferior rectus aplasia can manifest as contralateral elevation deficit. Careful extraocular motility examination and MRI can be essential for the differential diagnosis of monocular elevation deficiency.
An 11-month-old male infant presented with inability to elevate the right eye and left hypertropia, which was detected when the patient was approximately 6 to 7 months old. The medical and family histories were not significant. His parents denied any ocular trauma to both eyes.
The patient fixed and followed a near 5-inch object well with both eyes. Krimsky test revealed left hypertropia of 35 prism diopters (PD) in the primary position. In addition, the patient showed limitation of elevation in both adduction and abduction in the right eye (Fig. A). Downward eye movement was difficult to examine because he cried when we tried to elevate the upper eyelid or head to check the down gaze. Cycloplegic refraction revealed −0.25 diopter sphere −1.00 diopter cylinder × 180A in the right eye and +2.25 diopter sphere −1.50 diopter cylinder × 180A in the left eye. Bell’s phenomenon could not be determined owing to poor cooperation of the patient. He did not show an abnormal head posture.
Figure. (A) Gaze Photographs Show Limited Elevation in the Right Eye and Left Hypertropia. (B–E) Coronal and Sagittal T2-Weighted Orbital Images do not show the Left Inferior Rectus Muscle (B, C, and E). Arrows Indicate Normal Right Inferior Rectus Muscle (B–D). (F and G) Axial T2-Weighted Brain Stem Images Show Normal Right and Left Oculomotor Nerves (arrows).
Spectacles to fully correct the refractive errors and 3 hours of occlusion of the right eye per day failed to improve the left hypertropia. Repeated examinations performed for a few months revealed limitation of depression in the left eye. T2-weighted coronal and sagittal imaging of the orbit and high-resolution T2-weighted axial imaging of the brain stem were performed using a 3-tesla MRI system (Intera Achieva; Philips Healthcare, Best, the Netherlands) for extraocular muscles and cranial nerves, respectively. The entire length of the left inferior rectus muscle could not be visualized in the orbital MRI (Figs. B, C, and E). Other extraocular muscles in both eyes appeared normal on MRI (Figs. B–D). High-resolution brain stem images showed normal right and left oculomotor nerves (Figs. F and G).
Occasionally, performing extraocular motility examination, especially in down gaze, can be difficult in infants who do not cooperate well because the upper eyelid needs to be elevated to evaluate the downward eye movement. Without the evaluation of the downward eye movement, this could be a typical case of monocular elevation deficiency.
The patient with inferior rectus aplasia may prefer to fixate with the affected eye, producing the apparent decreased elevation of the contralateral eye because of the increased innervation caused by secondary deviation. In this case, patching the affected eye and assessment of the ductions of the contralateral eye should be performed to avoid misdiagnosis.
There are two more differential entities to consider when dealing with inferior rectus aplasia. One condition could be contralateral superior oblique overaction because the largest angle of deviation would be in depression and abduction in patients with inferior rectus aplasia.7 Something else to consider would be a partial oculomotor nerve palsy; however, patients with inferior rectus aplasia would not show other findings of oculomotor nerve palsy such as limitation of adduction and elevation, internal ophthalmoplegia, or aberrant regeneration.
Congenital abnormalities of extraocular muscles causing ocular motility problems showed a spectrum from accessory additional rectus muscles to aplasia of extraocular muscles. Thin-section imaging study can be helpful to differentiate such a spectrum.1–6
Aplasia of extraocular muscles can occur in isolation; however, occasionally it may be associated with a systemic condition such as craniofacial syndrome or neurofibromatosis, and aplasia of two or more extraocular muscles can occur simultaneously.1–6 In addition, an accessory extraocular muscle can be present with a bilateral hypoplasia of the inferior rectus muscle.5 Therefore, these three possibilities should be carefully examined in patients with aplasia of extraocular muscles. Our patient did not show any evidence of these possibilities.
There is no clinical test to differentiate aplasia from palsy of the extraocular muscles. In superior oblique aplasia, the presence of amblyopia, hypertropia, or horizontal deviation can be a clue7,8; however, there are no specific clinical characteristics of aplasia of other extraocular muscles. Therefore, surgical exploration or imaging study can be essential for the diagnosis of aplasia of extraocular muscles.
The oculomotor nerves in both eyes of our patient were normal and symmetrical. This finding suggests that inferior rectus aplasia can be caused by a different mechanism from congenital cranial dysinnervation disorders including Duane retraction syndrome, congenital fibrosis syndrome, congenital oculomotor palsy, and congenital superior oblique palsy.9–13 A possible mechanism for inferior rectus agenesis could be that the abnormality of the fascicle, undetectable by MRI, innervates the inferior rectus muscle among the many fascicles of the oculomotor nerve by MRI.
Inferior rectus aplasia could masquerade as contralateral elevation deficit because of inhibitional palsy of the elevator of the contralateral eye. Careful extraocular motility examination and imaging study are essential for the diagnosis of inferior rectus aplasia.
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