MR imaging of the globe and optic nerve

The superb contrast resolution of MR imaging has significantly improved radiologists' ability to evaluate the anterior visual pathways and globe. Optimal imaging of the orbit requires the radiologist to have a working understanding of the clinical questions the ophthalmologist needs answered. In this article, basic orbit anatomy, MR imaging protocols for the orbit, and common clinical scenarios encountered when imaging the orbit are reviewed, with a focus on the optic nerve and globe.     

Magnetic resonance imaging of the optic nerve in optic neuritis

Magnetic resonance imaging (MRI) of the optic nerves using the STIR (short inversion time inversion recovery) sequence was performed in 37 adult patients with a recent or past attack of optic neuritis. MRI revealed high-signal regions in 84% of symptomatic and 20% of asymptomatic nerves. The mean longitudinal extent of lesions was 1 cm. Slow or poor visual recovery was associated with more extensive lesions, or lesions within the optic canal. Disk swelling was usually associated with anterior lesions but also occurred with lesions in the canal. Visual evoked potentials were even more sensitive than MRI in detecting lesions and are still the investigation of choice in suspected demyelinating disease involving the optic nerve.     

Diffusion-weighted imaging of the spinal cord and optic nerve

The optic nerve and spinal cord are technically challenging to investigate with any magnetic resonance imaging (MRI) technique due to the effect of the surrounding cerebrospinal fluid and lipid, and the presence of nearby bony structures. Motion and the relatively small cross-section of the structures make diffusion-weighted imaging even harder. With careful choice of pulse sequence and parameters, however, apparent diffusion coefficient (ADC) measurements are now possible in the optic nerve, and both ADC and diffusion tensor imaging (DTI) measurements are becoming available in the spinal cord.     

Spectral Doppler imaging of vessels in the optic nerve of children

Elevation and blur of the optic disc margin with hyperemia and flame hemorrhages are classic features of papilledema that may not be present with mild elevations of the cerebral spinal fluid pressure. In children, the disc can be dramatically elevated with indistinct margins in pseudopapilledema. Children with equivocal disc features are sedated for neuroimaging and lumbar puncture to measure opening intracranial pressure.     

Confocal imaging of glial cells in the intact rat optic nerve

Astrocytes and oligodendrocytes in the isolated intact mature rat optic nerve have been computer imaged in three dimensions by laser scanning confocal microscopy of single cells, dye-filled with lysinated rhodamine dextran (LRD). Our results illustrate the first application of these techniques to an intact CNS white matter tract and provide comparative data for previous studies on neonatal rat optic nerve (Butt and Ransom: Glia 2:470–475,1989; Butt and Ransom: J Comp Neurol 338:141–158, 1993). The combined use of intracellular injection of LRD and confocal imaging significantly improves the resolution of glial cell structure, particularly that of mature astrocytes, for a number of reasons. (1) Single mature dye-filled glia can be imaged, because LRD does not pass through gap junctions. (2) The entire process field of astrocytes can be visualized in a single two-dimensional image. (3) Cell images can be rotated through 360° in all planes to provide a new perspective of glial cell structure in the intact tissue. (4) Reconstruction of optical sections, within a narrow focal plane, provides a high definition and resolution of the finer details of glial morphology. Using these techniques, three astrocyte subclasses were distinguished on morphological criteria. It is the conclusion of this study that the majority of these forms represent a single population of fibrous astrocytes which are well-suited to perform the multiple functions attributed to astrocytes in the CNS. The morphology of mature myelin-forming oligodendrocytes was also described. The results confirm that the structural characteristics of both astrocyte subtypes and oligodendrocytes described in younger rat optic nerves by Butt and Ransom (Glia 2:470–475,1989; J Comp Neurol 338:141–158, 1993) accurately reflect those of the mature glial population. © 1994 Wiley-Liss, Inc.     

Magnetic resonance imaging of luxury perfusion of the optic nerve head in anterior ischemic optic neuropathy

ABSTRACT:: A 49-year-old woman with painless reduction in visual acuity in her left eye was found to have nonarteritic anterior ischemic optic neuropathy (NAION). Fluorescein angiography revealed optic disc capillary leakage consistent with "luxury perfusion." Contrast-enhanced FLAIR magnetic resonance imaging (MRI) showed marked enhancement of the left optic disc. Resolution of the optic disc edema and the MRI abnormalities followed a similar time course. This report appears unique in documenting the MRI findings of luxury perfusion in NAION.     

Advances in imaging of the optic disc and retinal nerve fiber layer.

In the past decade, three technologies for imaging the optic disc and retinal nerve fiber layer have become commercially available: 1) confocal scanning laser tomography with the Heidelberg retinal tomograph; 2) confocal scanning laser polarimetry with the GDx VCC; and 3) optical coherence tomography with the Stratus OCT. Each uses different principles of physics. Understanding the merits and limitations of each of these technologies requires familiarity with the principles of operation of each device. This knowledge should be considered a prerequisite for the appropriate clinical utilization of these devices and for accurate interpretation of their results.     

Magnetic resonance imaging in optic nerve lesions with multiple sclerosis

Magnetic resonance imaging (MRI) of the optic nerves was performed in 10 patients with multiple sclerosis (MS) using short inversion time inversion recovery (STIR) pulse sequences, and the results were compared with the visual evoked potentials (VEP). The 10 patients had optic neuritis in the chronic or remitting phase together with additional symptoms or signs allowing a diagnosis of clinically definite or probable MS. Sixteen optic nerves were clinically affected and 4 were unaffected. MRI was performed using a 0.5 tesla superconducting unit, and multiple continuous 5 mm coronal and axial STIR images were obtained. A lesion was judged to be present if a focal or diffuse area of increased signal intensity was detected in the optic nerve. In VEP, a delay in peak latency or no P 100 component was judged to be abnormal. With regard to the clinically affected optic nerves, MRI revealed a region of increased signal intensity in 14/16 (88%) and the VEP was abnormal in 16/16 (100%). In the clinically unaffected optic nerves, MRI revealed an increased signal intensity in 2/4 (50%). One of these nerves had an abnormal VEP and the other had a VEP latency at the upper limit of normal. The VEP was abnormal in 1/4 (25%). In the clinically affected optic nerves, the degree of loss of visual acuity was not associated with the longitudinal extent of the lesions shown by MRI. The mean length was 17.5 mm in optic nerves with a slight disturbance of visual acuity and 15.0 mm in nerves with severe visual loss.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathology of the optic nerve

This article focuses on pathologic processes affecting the optic nerve, optic nerve/sheath complex, and chiasm. Pathology of the visual pathways posterior to the chiasm is beyond the scope of this article and is discussed only briefly.     

Age-dependent extrasynaptic modulation of axonal conduction by exogenous and endogenous GABA in the rat optic nerve.

To ascertain whether endogenous gamma-aminobutyric acid (GABA) exists and exerts physiological effects in the optic nerve, we compared the effects of GABA and related drugs on the neonatal (1 to 22 days of age) and adult (greater than 6 months) rat optic nerve in vitro. GABA (10(-4)-10(-3) M) reversibly depressed the amplitude and increased the latency of compound action potentials in the neonatal optic nerve. In the adult optic nerve, GABA (10(-4)-10(-3) M) had no significant effect on the compound action potential. The GABA-A receptor agonist, isoguvacine (10(-4)-10(-3) M), mimicked these GABA effects on the neonate and adult optic nerve. Lower concentrations (10(-5) M) of GABA increased excitability of the neonatal optic nerve but produced no discernible effects on the adult optic nerve. The GABA-uptake inhibitor, nipecotic acid (10(-3) M), mimicked the effects of GABA (10(-5) M) on the neonatal optic nerve. The GABA-A receptor blockers, picrotoxin and bicuculline (10(-6)-10(-3) M), decreased the latency of compound action potentials in the neonatal optic nerve. Membrane potential recordings indicate that while GABA (10(-5)-10(-3) M) depolarized the neonatal optic nerve dose-dependently, picrotoxin hyperpolarized the axons. In the adult optic nerve, neither GABA-uptake inhibitors nor GABA-A receptor blockers had significant effects on the compound action potential. These results suggest that functional GABA-A receptors and GABA are present in the neonatal rat optic nerve and depolarize axons under physiological conditions. However, this does not appear to be the case in the adult optic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Magnetization transfer magnetic resonance imaging of the brain, spinal cord, and optic nerve

Magnetic resonance imaging is highly sensitive in revealing CNS abnormalities associated with several neurological conditions, but lacks specificity for their pathological substrates. In addition, MRI does not allow evaluation of the presence and extent of damage in regions that appear normal on conventional MRI sequences and that postmortem studies have shown to be affected by pathology. Quantitative MR-based techniques with increased pathological specificity to the heterogeneous substrates of CNS pathology have the potential to overcome such limitations. Among these techniques, one of the most extensively used for the assessment of CNS disorders is magnetization transfer MRI (MT-MRI). The application of this technique for the assessment of damage in macroscopic lesions, in normal-appearing white and gray matter, and in the spinal cord and optic nerve of patients with several neurological conditions is providing important in vivo information—dramatically improving our understanding of the factors associated with the appearance of clinical symptoms and the accumulation of irreversible disability. MT-MRI also has the potential to contribute to the diagnostic evaluation of several neurological conditions and to improve our ability to monitor treatment efficacy in experimental trials.     

The contribution of magnetic resonance imaging in the differential diagnosis of optic nerve damage

In this paper we review the findings of magnetic resonance imaging (MRI) in optic neuritis and visual dysfunction due to other optic neuropathies. With advances in MRI technology, it has become possible to visualise optic nerve pathology. STIR and RARE sequences, contrast-enhanced sequences, and phased array surface coils are technical developments that provide fine anatomical detail and that are sensitive to pathological changes. MRI can offer information in the differential diagnosis of optic neuropathies, the monitoring of their treatment, and in some instances should provide new insights into the underlying pathophysiological mechanisms.     

Assessment of optic nerve damage in multiple sclerosis using magnetic resonance imaging

The MR imaging-based assessment of the optic nerve in optic neuritis and multiple sclerosis provides information that is complementary to clinical and electrophysiological methods. The standard and more tissue destruction specific methods can be used in strategies to measure treatment efficacy and for understanding the mechanisms of relapse, recovery, and failure of recovery.     

Magnetic resonance imaging, magnetisation transfer imaging, and diffusion weighted imaging correlates of optic nerve, brain, and cervical cord damage in Leber's hereditary optic neuropathy

OBJECTIVES: Leber's hereditary optic neuropathy (LHON) is a mitochondrial disease leading to bilateral loss of central vision and severe optic nerve atrophy. A subtype of LHON presents additional clinical and MRI aspects indistinguishable from those of multiple sclerosis (MS) (LHON-MS). In patients with LHON or LHON-MS, an assessment was made of (a) the severity of optic nerve damage, using MRI and magnetisation transfer imaging (MTI), and (b) the presence and extent of macroscopic and microscopic pathology in the brain and cervical cord, using MRI and MT ratio (MTR) and mean diffusivity (D) histogram analysis. METHODS: Ten patients with LHON, four with LHON-MS, and 20 age and sex matched healthy controls were studied. For the optic nerve and the brain, dual-echo turbo spin echo (TSE), T1 weighted spin echo, and MT images were obtained. For the brain, fast fluid attenuated inversion recovery (fast FLAIR) and diffusion weighted images were also obtained. For the cervical cord, fast short tau inversion recovery (STIR) and MT images were obtained. The volume and the average MTR value of both the optic nerves were measured. MTR and histograms of the normal appearing brain tissue (NABT) and MTR histograms of the whole cervical cord tissue were created. RESULTS: The mean values of optic nerve volumes and MTR were significantly lower in patients with LHON than in healthy controls. Mean NABT-MTR histogram peak height was significantly lower in patients with LHON than in controls, whereas no significant difference was found for any of the cervical cord MTR histogram derived measures. Average diffusivity (D) was higher in patients with LHON than in controls. Optic nerve volume and MTR value and mean NABT-MTR were lower in patients with LHON-MS than in those with LHON. CONCLUSIONS: The severity of optic nerve pathology in LHON is measurable in vivo using MRI and MTI. MTR and histogram analysis suggests that microscopic brain damage occurs in LHON and that it is more severe in the MS-like form of the disease.     

Ultrastructural changes in the optic nerve and capillary vessels during early stages of optic nerve injury★

BACKGROUND: Capillaries are the only blood supply for optic nerves, which makes the system more vulnerable to impaired blood circulation. OBJECTIVE: To observe the ultrastructural changes in the optic nerves and capillaries in rabbits following intracanalicular segment injury to the optic nerve. DESIGN, TIME AND SETTING: Comparative, observational, pathological morphology was performed at the Department of Anatomy, Weifang Medical College from September to November 2007. MATERIALS: Models of intracanalicular segment injury to the optic nerve were induced in the right eye of thirty healthy, adult rabbits by a flee-falling metal cylinder. The H-7500 transmission electron microscope was provided by Hitachi, Japan. METHODS: All rabbits were randomly assigned into experimental (n = 25) and control (n = 5) groups. Optic nerve specimens were obtained from the experimental group at 0.5, 6, 12, 48, and 96 hours, respectively, following injury. Uitrastructural changes to the optic nerves and their capillaries were observed by electron microscopy. Optic nerve injury was not established in the control group, but optic nerve specimens were collected similarly to the experimental group. MAIN OUTCOME MEASURES: Ultrastructural changes in the injured optic nerves and their capillaries. RESULTS: Thirty rabbits were included in the final analysis. In the control group, cross-sections of the optic nerves exhibited varied thicknesses with regularly arranged fibers. The axons appeared to be smooth with condensed myelin sheaths and oval mitochondria. The microtubules and mierofilaments were clearly seen. The lumens of the capillaries were regular with densely arranged endothelial cells and visible mitochondria. In the experimental group, 30 minutes after injury to the optic nerves, swollen axons, sparse myelin sheath, disordered microtubules and microfilaments, swollen mitochondria, and a decreased number of pinocytosis vesicles and microfilaments in endothelial cells of the capillaries were observed. At 6 hours, medullary and vacuolar degeneration in the mitochondria, and swollen endothelial cells in the capillary, were visible. At 12 hours, these changes were more obvious. At 48 hours, granular dissolution of microtubules, micro filaments, and mitochondria, as well as diffuse degeneration of mitochondria in the endothelial cells, were observed. At 96 hours, axonal disintegration, vacuolar degeneration, and dilated capillaries were observed. CONCLUSION: During early stages, the injured intracanalicular optic nerve exhibited swollen axons with vacuolar degeneration, swollen and degenerated mitochondria, decreased number of microtubules and microfilaments, and dilated capillaries with increased permeability.     

Intraorbital optic nerve signal hyperintensity on magnetic resonance imaging sequences in perioperative hypotensive ischemic optic neuropathy.

A 61-year-old man experienced severe bilateral posterior ischemic optic neuropathy after cardiac bypass surgery. Routine magnetic resonance imaging sequences were normal, but diffusion-weighted and fluid-attenuated inversion recovery (FLAIR) sequences showed abnormal hyperintensity within both intra-orbital optic nerves. This imaging abnormality has not been previously reported in this setting.     

Regeneration of the frog optic nerve

Developing and regenerating frog optic axons grow within optic pathways and form connections only with optic targets. However, unlike normal development, many regenerating optic axons in the adult frog are misrouted within optic pathways, including axons that grow into the opposite retina. Many of the axons misrouted during regeneration appear to be collaterals of axons that grow in normal directions. Ganglion cell loss of up to 60% occurs after optic nerve damage, beginning prior to reinnervation of optic targets. Massive axonal collateralization also takes place near the point of nerve damage, causing the normal order found within the nerve to be lost. Collaterals are eliminated as selective reinnervation is completed, and the smaller complement of optic cell axons remaining after regeneration form an expanded projection within optic targets. Evidence is reviewed that suggests that factors involved in axonal guidance and target recognition during development remain intact in the adult frog brain. Additional conditions resulting from nerve injury causes axonal guidance to be less successful during regeneration.     

Cysticercosis of the optic nerve.

Cysticercosis of the optic nerve has been reported only twice in the literature. A case of optic nerve cysticercosis in a 50-year-old woman with atypical optic neuritis is reported. Computerized tomography showed a thickened left optic nerve with a ring-enhancing lesion containing an eccentric nodule. An enzyme-linked immunosorbent assay test for cysticercosis further established the diagnosis. The patient was treated with oral prednisolone and albendazole, with no improvement in vision.