Ординатура / Офтальмология / Английские материалы / Clinical Neuro-ophthalmology A Practical Guide_Schiefer, Wilhelm , Hart_2007

Table 19.11. Neuroradiologic signs associated with cerebral causes of visual damage (excluding tumors)

Periventricular leukomalacia

Infarcts in the parieto-occipital region

Infarcts in the occipital lobes

■ Table 19.11 lists the typical neuroradiologic findings in such cases. If the damage is present at birth, or appears in the first 3 to 6 months of life, fundus changes such as optic disc atrophy or pathological cupping will often be present, caused by descending transsynaptic degeneration.

Periventricular Leukomalacia

:Definition

Table 19.12. Congenital brain lesions with ocular manifestations (excluding tumors)

Disorders of induction

λArnold-Chiari malformation

λHoloprosencephaly

λSeptooptic dysplasia

Disorders of cell migration and proliferation

λFetal alcohol syndrome

λFetal hydantoin syndrome

λPhacomatoses

λLissencephaly (agenesis of cerebral gyri)

λMicrocephaly

Other congenital anomalies of the brainstem and cerebellum

λMoebius’ syndrome

λJoubert’s syndrome

λDandy-Walker cysts

Hydrocephalus

Delayed Visual Maturation

Periventricular leukomalacia is a softening of the white matter in the regions surrounding the ventricles of the brain.

Radiologically demonstrable periventricular leukomalacia is an important sign of cerebral damage in premature infants and those suffering from perinatal cerebral asphyxia. It can present with optic disc cupping that should be distinguished from the sort caused by congenital glaucoma. Associated disturbances of vision can escape detection when only spatial acuity is tested, since acuities can be largely normal in the face of significant visual loss. Other problems associated with damage to visually associated regions of cerebral cortex become apparent only later, when learning disabilities are exposed in primary school–aged children.

Delayed visual maturation can occur as an isolated entity. Typically such children are noted at an age of 2 to 3 months to behave as blind, and yet seem to have normal pupillary light responses. (A differential diagnosis should include Leber’s congenital amaurosis [LCA], which can be confirmed by absent or severely impaired pupillary light reactions and an undetectable or severely reduced electroretinogram [ERG].) Often, a response of the child to facial appearances may be present when light responses seem to be absent. By the age of 6 months this feature will typically become inapparent. Neither morphological nor electrophysiologic studies will show any detectable disease. Delayed visual maturation can also occur in company with retinal or intracranial diseases.

Congenital Brain Tumors and Other Lesions

Ocular manifestations of congenital CNS diseases can also be found, attributable to disturbances of cellular induction, migration, and proliferation. Other congenital anomalies of the brainstem and cerebellum often accompany these disorders, as summarized in ■ Table 19.12. In addition, a variety of brain tumors lead to ocular symptoms. A detailed discussion of congenital intracranial tumors is beyond the scope of this chapter, but it is dealt with more completely in Chap. 12. In all such cases the neuroradiologic findings are conclusive.

Differential Diagnosis of Unexplained

Visual Loss – Psychogenic Disturbances

of Vision (Functional Visual Loss)

Unexplained acquired visual loss in children can result from macular diseases that at first escape detection. This is particularly true in the early stages of Stargardt’s disease. Typically, a psychogenic cause is suspected during the early stages of discovery. The diagnosis is often made by multifocal ERG (or by pattern ERG), and more recently by the detection of increased fundus autofluorescence at a wavelength of 488 nm. Correct diagnosis of an X-linked retinoschisis can be particularly difficult when only macular

changes are present. Only with precise optical examinations can schisis be found. This ordinarily requires the use of optical coherence tomography (OCT). Electrophysiology (see Chap. 7) can confirm the diagnosis.

Migraine equivalents produce the most highly varied forms of visual hallucinations, and can often be early signs of cerebral diseases that can be found only by appropriate neuroradiologic study. Psychogenic (functional) disorders of vision often pose serious problems with differential diagnosis (see Chap. 15). They constitute a diagnosis by exclusion. A high level of suspicion of a psychogenic disturbance should accompany findings, such as highly variable visual responses – as reflected, for instance, in the spiraling of isopters plotted during kinetic perimetry. If a profound unilateral loss of vision is claimed, conflicting data such as normal stereoacuity, absence of a relative afferent pupillary defect, or intact acuity demonstrated with polarizing filter isolation of test characters to one eye or the other during binocular reading usually permits a decisive determination of the psychogenic character of the visual loss.

• Pearl

It is important to remember that such problems are almost never because of conscious simulation on the child’s part, but are rather the byproduct of some unresolved difficulty.

Conclusion

Neuro-ophthalmic diseases during childhood include both a number of age-independent diseases and a variety of specific disorders that are variably expressed in an age-depen- dent fashion. Neuro-ophthalmic investigation of children with unexplained visual disorders requires the use of methods that are appropriate to the study of children, with consideration given to age-corrected measures of function. A decisive neuro-ophthalmic investigation often provides important insights into childhood diseases, facilitating their management by physicians in other branches of pediatric medicine.

Further Reading

Brodsky MC (ed) (1996) Pediatric neuroophthalmology. Springer, Berlin Heidelberg New York

Kaufmann H (ed) (2003) Strabismus. 3rd completely rev. edn. Springer, Berlin Heidelberg New York

Taylor D (ed) (1997) Paediatric ophthalmology, 2nd edn., Blackwell Scientific, Oxford

Wright KW (ed) (2003) Pediatric ophthalmology and strabismus. 2nd edition, Mosby, St. Louis

Wright KW, Spiegel PH, Thompson, LS (eds) (2006) Handbook of Pediatric Neuroophthalmology. Springer, Berlin Heidelberg New York

Conventional Radiologic Diagnosis

The use of conventional radiologic imaging in ophthalmology has been reduced to its role in the detection of metallic foreign bodies; for a more detailed study of soft tissues, tomographic images have completely replaced them.

Tomographic Imaging

Knowledge of the anatomic planes in which the results are depicted is a primary requirement for understanding the use of modern tomographic imaging techniques. The three principal tomographic planes are orthogonally arranged as depicted in ■ Fig. 20.1: The coronal and sagittal planes are oriented along the same planes defined by the skull sutures of the same names. (The plane of the sagittal suture bisects the two halves of the skull, while the coronal suture lies in the dividing plane between the frontal and parietal bones). The transaxial plane is orthogonal to the first two and lies athwart the long axis of the body in an orientation described as “parallel to the hat brim.”

Computed Tomography

:Definition

Computed tomography (CT) is a digital tomographic procedure in which beams of X-rays pass through the tissue being examined, and an anatomical image of the plane being examined is reconstructed by means of computations that start with the measured variations in tissue absorption of the energy. Multiple, evenly spaced, parallel beams form single sets that are then repeated at regularly varying rotational orientations, viewing the body through many sets for each tomographic plane being imaged. Complex matrix calculations are then used to derive highly resolved values for the radiodensity maps in each cross-sectional layer.

Indications for CT

Cross sectional tomograms for transaxial orbital CT are best when parallel to the plane of the optic nerves (■ Fig. 20.1), with a thickness of 1 to 3 mm.

Fig 20.1. a Overview of the tomographic planes used in computed tomography (CT). b Normal findings in an orbital CT scan: topographic image showing the orientation of a tomographic plane of

examination that is parallel to the optic nerves. c Normal findings in an orbital CT scan, transaxial plane showing both optic nerves

The various tissue density levels are assigned values (expressed as Hounsfield units [HF]) on a grayscale, and are displayed on an analog, or digital, video monitor. Air has a value of about –1,000 HF; fat has a value of about –60 HF; hemorrhages about +70 HF; and bones, +1,000 HF. By choosing appropriate data values collected from the scan and arranging them according to their density values, images can be displayed in a variety of windows (ranges of density values), some ideal for studying bone (■ Fig. 20.2 a), others for soft tissues (■ Fig. 20.2 b).

A coronal plane of scanning is ideal, when examining the orbital bones, the paranasal sinuses, and/or the extraocular muscles (■ Fig. 20.3). However, patient positioning for coronal scans has the distinct disadvantage of requiring a prone position with the head fully extended (■ Fig. 20.3 a), which is impossible for those with arthritic or mechanically unstable cervical vertebrae. In addition, metallic dental fillings produce strong artifacts that can obscure the regions of interest, since the coronal plane crosses the levels of the maxilla and mandible. For those with supple necks

Fig. 20.2. a Skull/brain trauma with fracture of the lateral orbital wall on the right side: bone window showing displaced fragment of bone (arrow), transaxial-plane orientation. b Skull/brain trauma with fracture of the lateral orbital wall on the right side: soft tissue window with edema of the facial tissues and exophthalmos (arrow), transaxial-plane orientation

Fig. 20.3. a Coronal image of the orbital bones following a fracture of the orbital floor (arrow) and the lateral wall of the right maxillary sinus (arrow): initial coronal plane with sharply defined orbital floor and good demonstration of the fracture lines. b Coronal image of the orbital bones following a fracture of the orbital floor (arrow) and the lateral wall of the right maxillary sinus (arrow): secondary reconstruction of spiral CT data with an unsharp but still recognizable image of the fracture (arrow)

and minimal dental reconstructions, however, coronal images are the most revealing.

Alternatively, the data sets collected from spiral CT scans can be processed by mathematical reconstruction methods to produce calculated tomograms of whatever plane of imaging is desired (■ Fig. 20.3 b). This method is ideal for the study of critically injured patients and those with extensive dental work, since the patients can lie comfortably in a supine position. The transaxial tomographic data, collected in the planes just above the maxilla, can be used to calculate so-called secondary reconstructions (■ Fig. 20.3 b). A relative disadvantage of this ap-

proach is that significantly greater radiation doses must be used.

CT is the imaging method of choice in emergency rooms that care for patients with acute, multisystem injuries. For patients with skull/brain injuries, the presence and course of fractures in the orbit can be studied, using the windows that allow the best definition of bones’ anatomy (■ Fig. 20.3). The effects of direct ocular trauma, or retrobulbar hemorrhages, are best studied when using the window settings for soft tissues. Thus, ophthalmic vein distension (■ Fig. 20.4) may be detected because of a traumatic carotid–cavernous fistula.

Fig. 20.4. Distention of the superior orbital vein (arrow) on the right side following a traumatic carotid–cavernous fistula, transax- ial-plane orientation

Another indication for the use of computed tomography is the suspicion of hemorrhage or ischemia in the tissues supplied by the posterior cerebral arteries (■ Fig. 20.5). The presence of an intracranial hemorrhage is immediately demonstrable, and an area of infarction can be detected within a few hours, based on the development of a decrease in tissue radiodensity.

In the study of intraorbital and intracranial space-oc- cupying lesions, additional information for correct classification is often obtained by CT scanning after intravenous administration of radiodense contrast material.

!Note

The following contraindications for the use of contrast materials must be observed:

■Allergy to iodinated compounds

■Hyperthyroidism

■Poor renal function

■Paraproteinemia

An important consideration is the question of total radiation exposure in patients undergoing CT studies. A cumulative radiation dose associated with 50 to 100 thin-section scans can result in radiation-induced cataract formation.

• Pearl

Another important role of CT is in the detection of tu- mor-related alterations in bony anatomy, as when osteolytic or hyperostotic changes have been induced (■ Fig. 20.6).

Fig. 20.5. a Ischemic disease of the posterior visual pathways: Freshly infarcted tissue (approximately 12 h) in the distribution of the right posterior cerebral artery with hemorrhage formation (arrow). In comparison with an older infarct, shown in b, there is mild swelling with erasure of the cortical sulci, transaxial-plane orientation. b Ischemic disease of the posterior visual pathways: Older, hypodense, and sharply demarcated insults in the visual cortex of both sides (arrows), transaxial-plane orientation

Advantages of CT as compared with MRI

The advantages of CT scanning, as compared with MRI imaging, are summarized as follows:

■Shorter examination times (2 to 5 min for CT scanning,

20 to 45 min for an MRI)

■Facilitated monitoring of rapidly changing findings

■Better assessment of bone structure

■Faster and safer detection of intracranial hemorrhages

■Lower cost

Fig. 20.6. a Sphenoid wing meningioma: tumor-induced hyperostosis (arrow) of the greater sphenoid wing (bone window), transaxial-plane orientation. b Sphenoid wing meningioma: soft tissue components of the tumor (arrow) in the lateral orbit, producing an exophthalmos (soft tissue window), transaxial-plane orientation.

Most Important Indications for CT Scanning

■Skull/brain trauma

■Stroke: differential diagnosis of hemorrhage/infarction

■Foreign-body detection

■Space-occupying lesions with bony involvement or soft tissue calcification

Due to its better soft tissue differentiation, elective MRI has increasingly replaced CT scanning of the brain and orbits. Avoidance of radiation exposure has also played a big role in these decisions.

Magnetic Resonance Imaging

:Definition

Magnetic resonance imaging is a tomographic process that permits the tomographic imaging in any chosen plane within the body. This is done with the help of a strong magnetic field parallel to the long axis of the patient’s body and an additional, freely variable, loca- tion-dependent magnetic field (a gradient field). In place of X-rays, the MRI uses nonionizing, radiofrequency energy.

With few exceptions, MRI today is the method of choice for the elective study of the soft tissue structures of the orbit, the optic nerve, and the intracranial portions of the visual system. In some places, the availability of MRI scanning remains somewhat limited, as compared with the availability of CT scanners. More important, though, are the following contraindications.

Contraindications to MRI

!Note

Absolute contraindications (patient endangerment):

■Cardiac pacemakers

■Incorporated ferromagnetic foreign bodies/ implants

■Shrapnel wounds

■Aneurysm clips of uncertain origin

Relative contraindications (no patient endangerment, but with comparatively poorer imaging quality):

■Cosmetics, such as mascara or eyeliner, with metallic content

■Nonferromagnetic metal implants, such as the metallic plates used in plastic surgical reconstructions of the face or orbit

■Cochlear implants (loss of function)

■Claustrophobia

■Inadequate patient cooperation

The minimal requirements for patient cooperation are that he/she must be able to remain immobile with no head or eye movement for the time needed to complete one measuring sequence (about 3 min).

Fig. 20.7. a Tomographic orientation for magnetic resonance imaging (MRI) study of the orbits: parasagittal T1-weighted (T1w) image with a linear mark parallel to the optic nerve. b Normal optic chiasm with the typical “suspenders’ shape” (black arrow), main trunk of the middle cerebral artery (superior white arrow). Posterior cerebral artery (inferior white arrow), transaxial T2w image

Fig. 20.8. a Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: T1w contrast-enhanced study of a pituitary macroadenoma (superior arrow) in a transaxial orientation, showing contact between the tumor and the optic chiasm (inferior arrow). b Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: coronal tomographic plane illustrating the relationship of the mass to the distal (supraclinoid) carotid (arrow). (Continuation see next page)

Advantages of MRI

If the prerequisite conditions noted above have been met, MRI scanning offers significant advantages as compared with CT scanning. Chief among them is the improved differentiation between retrobulbar soft tissues (fat, muscle, and optic nerve), between normal components of the brain (gray and white substances) and between differing forms of pathological change (infarction, hemorrhage, inflammation, and neoplasms). Additional advantages of MRI imaging lie in the use of high-frequency radio waves with no ionizing radiation at all, which is particularly important for protecting the ocular lens. Another, perhaps more important, advantage is the freedom with which

data can be acquired and displayed, to study whatever tomographic section is desired without having to move the patient (■ Fig. 20.7). Also, the contrast material used in MRI scanning, usually containing a gadolinium–DTPA complex, is better tolerated than the iodinated CT contrast dye, lowering the risk of an allergic reaction substantially. Another important advantage of MRI scanning is the ease with which the sellar region (■ Fig. 20.8), the posterior fossa, and the course of the afferent visual pathways can all be clearly delineated, without the interference of image artifact caused by the dense bony structures, as is frequently encountered during CT scanning of these spaces.

Indications for MRI

In routine diagnostic settings, a plane that is parallel to that of the optic nerves and with a thickness 2 to 3 mm provides the most useful information (■ Fig. 20.7). If additional study of intracranial contents is desired, an orientation of the plane of interest that is parallel to the line connecting the anterior and posterior commissures of the corpus callosum is ideal. A thickness of 3 to 5 mm is best. The total time for the procedure will vary from 20 to 60 min.

The most important advantage of MRI scanning lies in the fact that the signal strength in the image is not determined by measures of radiodensity, as in CT scanning, but is instead determined by tissue-specific parameters, the T1 and T2 relaxation times. This produces a high-resolution image with excellent tissue identification (■ Table 20.1). In the so-called T1-weighted (T1w) images, the cerebrospinal fluid (CSF) appears hypointense (dark), while fat, subacute hemorrhages and gadolinium-containing contrast materials appear hyperintense (bright) (■ Figs. 20.9 a and 20.10 a).

Fig. 20.9. a Cavernous hemangioma (arrow): retrobulbar tumor in the left orbit between the optic nerve and the medial rectus muscle (T1w), transaxial tomographic plane. b Cavernous hemangioma: T2w image illustrating a common feature of this type of tumor, i.e., hemosiderin deposits (arrow), which appear as a hypointense ring surrounding the tumor, transaxial tomographic plane

Fig. 20.10. a Choroidal melanoma: Contrast-enhanced study of a choroidal melanoma in the left eye, arising from the temporal fundus quadrant, near the equator of the globe, as a mushroomshaped tumor that takes up the contrast material. Transaxial tomographic plane, T1w, contrast-enhanced. b Choroidal melanoma: strong T2w image, transaxial tomographic plane. Note the reversal of contrast: In the T1w image (a) the vitreous is dark and the tumor is light, whereas in this T2w image, the vitreous cavity is light, and the tumor appears dark

In the T2w images, the CSF and edematous tissues appear hyperintense (■ Fig. 20.9 b), but hemosiderin deposits (such as one finds in cavernomas or at the margins of old, reabsorbed hemorrhages) appear hypointense. Of particular interest for the diagnosis of orbital disease are the so-called fat-saturated (“fat-sat”) images. These use a special method for the excitation of the signal given off by the retrobulbar fatty tissues, suppressing their signal strength, to allow for better definition of more subtle soft tissue features. For example, a retrobulbar lymphangiosar-

Fig. 20.11. a Dysthyroid ophthalmopathy: pronounced enlargement of the superior and inferior rectus muscle (arrows). Oblique parasagittal tomographic plane. b Dysthyroid ophthalmopathy: coronal T1w, fat-sat, contrast-enhanced image of massive swelling of the medial, superior, and inferior rectus muscles in both orbits (asterisks). Unrelated finding of a left maxillary sinusitis (arrow)

coma, or changes in the extraocular muscles produced by a dysthyroid ophthalmopathy, can be more easily identified (■ Fig. 20.11 b).

Aside from the standard sequences used in T1w and T2w scans, more recently developed methods (such as MR angiography for studying blood vessels, and diffusionweighted MRI for the detection of strokes) have enhanced the usefulness of MRI imaging. Diffusion-weighted scanning allows for a high level of sensitivity in the rapid detection of tissue infarction, and surrounding areas of brain ischemia, only 1 to 2 h after a stroke or stroke-like episode (■ Fig. 20.12). Other methods, such as perfusion imaging for the measurement of cerebral perfusion, and functional MRI for the determination of metabolic brain activity, are for the time being not yet generally available for routine clinical diagnosis.