Orbit and visual pathway imaging

Introduction

Imaging is often pivotal in making a diagnosis in pediatric ophthalmology. We hope to highlight the way the pediatric ophthalmologist and radiologist can form an effective team when dealing with children with orbital and/or visual pathway problems.

Pre-imaging multidisciplinary collaboration

Effective communication between ophthalmologist and radiologist is crucial to optimizing imaging. This could range from written request to personal conversation. This communication may be very brief, but more complex cases will sometimes need more discussion.

It is not necessary or possible for an ophthalmologist to keep up on every technical development in imaging. Rather than memorizing a few key facts about magnetic resonance imaging (MRI) techniques, it may be more useful to invest energy in the ophthalmology–radiology collaborative relationship.

This multidisciplinary exchange will typically focus attention on the following questions.

1 What is the information we need? What are the clinical questions we are trying to answer?

Every imaging study represents an attempt to answer certain questions. To formulate these questions, it is helpful to consider and share with the radiologist the elements of the ophthalmologic history and examination, including the following.

Patient age

The age of the patient influences the need for sedation, the imaging equipment utilized for image acquisition, and the interpretation. Age-specific differential diagnostic considerations can influence the selection of MRI sequences ( Fig. 9.1 ).

Fig. 9.1, In a 7-month-old girl with proptosis, coronal T2 (A) and T1 postcontrast (B) MRI images show a contrast-enhancing mass in the superolateral orbit displacing the globe (arrows). At this age, a leading consideration would be infantile hemangioma, and the addition of arterial spin label perfusion imaging (C) shows the marked hyperemia of the hemangioma (arrow). In contrast, a similar enhancing mass (arrows) causing proptosis in a 4-year-old girl is seen on coronal T2 (D) and T1 postcontrast (E) images, but at this age a more aggressive neoplasm is suspected. The addition of diffusion-weighted imaging with an apparent diffusion coefficient ADC map (F) shows the dark signal reflecting the reduced water diffusion of a highly cellular neoplasm (arrow), in this case chloroma from leukemia. Patient age influences imaging technique and interpretation. 2

History

Observations regarding events preceding symptoms can aid image planning and interpretation ( Fig. 9.2 ). The patient’s medical history can also be important ( Fig. 9.3 ).

Fig. 9.2, In a 13-year-old boy with acute eye pain, contrast-enhanced axial CT (D) shows post septal inflammation adjacent to the displaced lateral rectus muscle (arrows), concerning for orbital cellulitis. There was also a hyperdense (bright on CT) structure (arrow in B) in the lateral orbital wall with surrounding osseous erosive change (B). With no known history, diagnostic considerations might have to include a bony sequestrum from infectious osteomyelitis or even a hyperdense nidus of an osteoid osteoma in an unusual location. However, this patient was stabbed by his brother with a pencil 6 weeks earlier. At that time, a non contrast CT had been performed (A and C) for “trauma” before anyone admitted to the pencil incident, only orbital hemorrhage was reported (arrow in C), and the hyperdense structure was missed (arrow in A). After the patient gave a more complete history, the second CT scan led to surgery where a small piece of graphite from the pencil was removed from the orbital wall. Clinical history can be crucial to image interpretation. 3 4

Fig. 9.3, When a 13-year-old girl presented with acute diplopia and eye pain, coronal T1 postcontrast MRI (A) showed thickening of extraocular muscles (arrow). She responded to steroids, had a recurrent episode 18 months later, and responded to steroid treatment again. The diagnosis was myositis in idiopathic orbital inflammatory syndrome. Although the coronal T1 postcontrast image of a second patient (B) has similar extraocular muscle thickening (arrow), in this second patient the past medical history of Graves disease led to a diagnosis of thyroid ophthalmopathy. Medical history can lead to different image interpretation even when imaging appearance between cases is similar. 5

Symptoms

Sometimes the symptoms are not specific and require a generic imaging protocol. At times, however, symptoms will be specific and guide both the timing and content of imaging ( Fig. 9.4 ).

Fig. 9.4, This patient undergoing surgical resection of a meningioma at the craniocervical junction woke from anesthesia with bilateral total loss of vision (NLP). An emergency MRI of the brain and orbits was obtained. Axial diffusion-weighted imaging showed striking diffusion restriction with hyperintense signal (arrows) in both optic nerves, consistent with acute posterior ischemic optic neuropathy, a rare and poorly understood but recognized complication of some spine surgeries, particularly when performed with a prone or head down patient position, possibly due to venous stasis and ischemia. 6

Timing

The duration of symptoms and signs influences imaging timing and modality ( Fig. 9.5 ). Knowing that the symptoms are episodic/recurrent or static/isolated may also inform the imaging technique and interpretation ( Fig. 9.6 ).

Fig. 9.5, Image A (normal example) is a high resolution axial T2 image through the pons, showing the two small bumps (the facial colliculi) along the floor of the fourth ventricle (black arrows) where the abducens nuclei are located. The abducens nerves (white arrows) are surrounded by T2 bright CSF in the prepontine cistern. In a 4-year-old girl with chronic abducens nerve palsy and absent facial expression, an elective outpatient sedated MRI with high resolution nerve imaging was the imaging of choice (not CT). Similar high-resolution axial T2 imaging (B) at the same level shows absence of the facial colliculi (black arrows) and absence of the abducens nerves in the cistern (white arrows). High resolution sagittal T2 images (C) through the internal auditory canals show the cochlear and vestibular nerves (white dots), but no facial nerve is present at its expected location (white arrow) in this patient with a congenital cranial nerve dysinnervation syndrome (Mobius syndrome in this case). In contrast, when a 13-year-old boy presented to the ophthalmologist with new diplopia and was found to have abducens nerve palsies, the acute nature of the problem prompted an emergency head CT. Sagittal CT (D) showed a medulloblastoma causing obstructive hydrocephalus and effacement of the prepontine cistern, presumably compressing the abducens nerves. 7

Fig. 9.6, When a 9-year-old boy presented with a “droopy eyelid” of 2 years’ duration, he was found to have an oculomotor nerve palsy. Coronal T2 (A) and T1 post contrast (B) MRI images show an enhancing sub cm nodule (arrows) in the oculomotor nerve as it enters the cavernous sinus, which was unchanged a year later, concerning for a schwannoma given its appearance and stability despite the rarity of schwannomas in children without neurofibromatosis type 2. Although the imaging appearance of the cisternal segment of the left oculomotor nerve is similar in a different patient (C and D), the episodic transient nature of imaging findings on follow-up studies and this patient’s periodic transient headaches with left ocular pain and diplopia led to a diagnosis of ophthalmoplegic migraine. 8 The temporal pattern of the clinical findings is crucial to image interpretation.

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