Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

a

14.33.12

12.11

Fig. 2.19.a Axial T1-weighted native MRI of the region of the orbital roof. b Corresponding contrast-enhanced view. c Corresponding diagram: 7.2 = frontal sinus, 10.3 = superior rectus muscle, 10.8 = trochlea of the superior oblique muscle, 10.13 = superior ophthalmic vein, 10.15 = lacrimal gland, 12.3 = optic tract, 13.21 = third ventricle, 14.9 = MCA

b

Fig. 2.20.a Coronal T1-weighted native MRI of the region of the cavernous sinus. b Corresponding contrast-enhanced view. c Corresponding diagram: 3.10 = oval foramen, 3.12 = sphenoid sinus, 12.2 = chiasm, 12.8 = oculomotor nerve (N III), 12.9 = abducent nerve (N VI), 12.10 = maxillary nerve (N V2), 12.11 = mandibular nerve (N V3), 12.12 = ophthalmic nerve (N V1), 13.1 = temporal lobe, 13.2 = frontal lobe, 13.11 = pituitary gland, 13.17 = ventricle, 14.2 = ICA, 14.3 = siphon of ICA

a

c

a

13.213.2

intracranial 10.11 marrow (fat)

c

b

Fig. 2.21.a Coronal T1-weighted native MRI of the region of the pituitary stalk. b Corresponding contrast-enhanced view. c Corresponding diagram: 3.10 = oval foramen, 3.12 = sphenoid sinus, 12.2 = chiasm, 12.11 = mandibular nerve (N V3), 12.14 = trigeminal (Gasserian) ganglion, 13.11 = pituitary gland, 13.12

=pituitary stalk, 13.17 = ventricle, 14.3 = siphon of ICA, 14.8

=anterior cerebral artery (ACA), 14.9 = MCA

b

Fig. 2.22.a Coronal T1-weighted native MRI at the level of the prechiasmatic intracranial optic nerves. b Corresponding con- trast-enhanced view. c Corresponding diagram: 3.12 = sphenoid sinus, 10.11 = optic nerve (prechiasmatic intracranial portion), 13.1 = temporal lobe, 13.2 = frontal lobe, 13.17 = ventricles, 14.2 = ICA (“knee” of the siphon), 15.1 = cavernous sinus

13.2

10.11 in 3.2

13.1

3.6

3.9

c

a

c

gyrus rectus

13.2

10.11 in 3.2

3.12 marrow of 3.1 Corresponding diagram: 2.8 = vomer, 3.1 = anterior clinoid

3.6 process, 3.2 = optic canal, 3.6 = superior orbital fissure, 3.9 =

b

Fig. 2.24.a Coronal T1-weighted MRI at the middle of the optic canal. b Corresponding T2-weighted view. c Corresponding diagram: 3 = sphenoid bone, 3.2 = optic canal, 3.6 = superior orbital fissure, 3.12 = sphenoid sinus, 10.11 = optic nerve, 13.1 = temporal lobe, 13.2 = frontal lobe

10.5med. orbital gyrus

1.21.2

c

Fig. 2.26.a Coronal T1-weighted MRI of the midorbital region. b Corresponding T2-weighted view. c Corresponding diagram: 1.2 = maxillary sinus, 10.1 = inferior rectus muscle, 10.2 = medial rectus muscle, 10.3 = superior rectus muscle, 10.4 = lateral rectus muscle, 10.5 = superior oblique muscle, 10.7 = levator palpebrae muscle, 10.10 = orbital fat, 10.11 = optic nerve, 10.13 = superior ophthalmic vein, 10.14 = ophthalmic artery (note again the mucoid inflammation of the ethmoid and maxillary sinus)

2.4

The Extraocular Muscles, Intraand Extraconal Space (Figs. 2.30, 2.31)

The four rectus muscles and the superior oblique muscle are long, stretched, fusiform muscles with a slightly thickened middle region. Together with the levator palpebrae muscle, they originate from a common annular tendon, Zinn’s ligamentous ring, attached to the bone of the orbital apex (Fig. 2.10a). Anterior to the orbital opening of the optic canal, this annular tendon encircles the optic nerve with its dural sheath and the ophthalmic artery eccentrically. Furthermore, the annular tendon crosses the inferomedial region of the inferior orbital fissure, where the distal oculomotor and abducent nerves enter the annular tendon. The superior and inferior ophthalmic veins pass the superior and the inferior orbital fissure, respectively, outside the annular tendon. The superior ophthalmic vein then passes in a stretched S-like course directly below the anterior region of the superior rectus muscle from posterolateral to medioanterior (Figs. 2.5, 2.18). Usually it collects the blood from the face via the angular vein with an antegrade flow direction to the cavernous sinus. However, in anatomic variations there can be reversed flow, if the middle cerebral and uncal veins both drain into the cavernous sinus (Servo 1982). The four rectus muscles insert on the globe posterior to the corneoscleral border. The superior oblique muscles course forward superomedially of the medial rectus muscle, forming a small, round tendon anteriorly which is deflected posterolaterally by a pulley, the trochlea (Figs. 2.5, 2.18, 2.19), and is then attached to the sclera posterior of the equator. The thin and broad inferior oblique muscle originates from the medial orbital floor and passes posterolaterally to insert into the inferolateral sclera (Figs. 2.1, 2.13, 2.14, 2.28, 2.33). The levator palpebrae muscle runs above the globe and terminates anteriorly in an aponeurosis (Figs. 2.12, 2.27, 2.28). Some of these tendinous fibers are attached to the tarsus, while the remnant fibers radiate directly to the skin of the upper eyelid.

2.4.1

Lacrimal Gland

The lacrimal gland and system represent a functional subunit of the orbit, located in the extraconal space. The lacrimal gland consists of two parts, incompletely separated by the aponeurosis of the levator palpebrae muscle: a superior larger orbital and an

inferior smaller palpebral lobe, connected only at the lateral edge of the aponeurosis (Fig. 2.33). The orbital part is nestled to the lacrimal fossa in the anterior aspect of the superolateral orbital wall (Figs. 2.5, 2.13, 2.28). The structure of the lacrimal gland resembles the salivary glands with secretory cells (acini), granules, and ducts (Bron et al. 1997). The lacrimal ducts open into the superior conjunctival fornix. At the medial end of the lid, two lacrimal puncta, one superior and one inferior, open to short (2 mm), primary vertical, then horizontal superior and inferior lacrimal canaliculi, which join to the membranous lacrimal sac, the widest portion of the drainage system (Fig. 2.34). The lacrimal sac (Fig. 2.1) lies in the lacrimal fossa, a bony hollow formed by the lacrimal bone and the superior and anterior maxillary processes. Normally 10–12 mm in length, it has a downward continuation into the lacrimal duct, which lies in the osseous nasolacrimal canal with a length of about 10–15 mm, formed by the maxillary bone (Figs. 2.13, 2.34). As the posterolateral direction is the exit into the inferior concha, the valve of Hasner marks the endpoint of the excretory lacrimal system (Bron et al. 1997) (Fig. 2.34).

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Fig. 2.29. Diagram of the four compartments of the orbital space

2.5

Optic Nerve (Fig. 2.35)

The optic nerve can be divided into an intraocular, an intraorbital, a canalicular, and an intracranial segment. Although the optic nerves are part of the cranial nerves, their structure differs from that of the other cranial nerves (to simplify matters, in the rest of this article the optic nerves are cited in the singular). Phylogenetically, the optic nerve and the retina represent an evagination of the brain. Therefore, the optic nerve contains the same tissue components as the brain, and thus meningiomas or optic gliomas can develop in the optic nerves, analogous to intracranial tumors, whereas schwannomas or neurofibromas develop from the other cranial nerves. About 4 mm nasal to the central part of the posterior pole of the globe, the superficial nerve fiber layers converge, exit, and pierce the outer retina, the choroid, and the lamina cribrosa, forming the optic nerve head and intraorbital optic nerve. As it traverses the annular tendon near the optic foramen,the attachments of the superior and medial rectus muscles are adherent to its dural sheath, explaining the characteristic orbital pain in retrobulbar neuritis (Bron et al. 1997).

As part of the brain, the entire intraorbital optic nerve is embedded in meningeal and arachnoidal sheaths in continuation with the respective intracranial layers. As already mentioned, the outermost sheath corresponds to the dura mater, tightly adherent to the bone within the optic canal. In the orbit, this sheath divides into two layers, one of which remains the outer sheath, blending with the sclera of the eyeball, the other becoming the orbital periosteum (periorbita) (Bron et al. 1997). The intermediate arachnoid layer is attached inside, while the inner arachnoid layer is attached to the pia mater that envelops the nerve and also the branches of the central retinal vessels.

The central retinal artery is the most important branch of the ophthalmic artery and pierces the optic nerve inferomedially, approximately 10 mm behind the globe, then running to the globe centrally inside of the nerve. The path of the central retinal vein also runs through the center of the optic nerve, but it exits the nerve 1–2 mm dorsal of the entry point of the central retinal artery, then running dorsally below the optic nerve. It may join the superior ophthalmic vein and can drain either into this, into another orbital vein, or directly into the cavernous sinus. It is assumed that inside the bulb no functionally sufficient venous collateralization to the central retinal vein exists, explaining the retinal bleeding and sec-

Fig. 2.35. Diagram of the orbital compartment optic nerve

ondary glaucoma in case of a venous occlusion (Lang 1981).

A tube-shaped subarachnoid space extending between the intermediate and the inner arachnoid layer communicates with the intracranial subarachnoid space.A slight or pronounced bulbous widening of the arachnoid sheath of the proximal optic nerve is a normal appearance (Fig. 2.36) and should not be misinterpreted as some form of optic nerve CSF disturbance, although according to histologic studies on this segment, one expects the most dural bulging in the case of intracranial hypertension at the optic nerve head (Helmke and Hansen 1996) (Figs. 6.185, 6.186).

The intraorbital optic nerve has a length of about 4–4.5 cm, and its course is slightly tortuous (Fig. 2.37). It runs posteromedially through the muscle cone of the four rectus muscles to the orbital apex and enters the optic canal. Passing the optic canal, the tube-shaped subarachnoid space surrounding the optic nerve shows a remarkable narrowing, which explains why no perioptic CSF is visible intracanalicularly on T2-weighted coronal MRI (Figs. 2.24, 2.36).

2.5.1

Intracanalicular Optic Nerve, Prechiasmal Area

(Fig. 2.36)

The length of the optic canal is about 5 mm, its width about 3–4 mm, and the course of the intracranial prechiasmatic optic nerve has an extension of about 10 mm (Fig. 2.38). The internal carotid artery lies laterally and below it, giving rise to the ophthalmic artery at the anteromedial wall of the upper carotid siphon (Figs. 6.196, 7.30, 7.46, 7.47, 7.51). At the intracranial end of the optic canal, the ophthalmic artery is located inferolaterally to the optic nerve inside the subarachnoid space. Within the orbital end of the optic canal, the ophthalmic artery exits the dura laterally (Rhoton and Natori 1996). The intradural segment of the internal carotid artery starts a little more proximally than the origin of the ophthalmic artery (Oikawa et al. 1998). The A1 segment of the anterior cerebral artery crosses above the prechiasmatic optic nerve (Fig. 2.39). The relation of the vessels of the circle of Willis is shown by two different views of the vascular anatomy, demonstrated with MRA (Figs. 2.40, 2.41).

2.6

Chiasm, Postchiasmal Area

(Figs. 2.36, 2.38, 2.39, 2.42)

The intracranial optic nerves then run posteromedially to the midline,where they merge together to form the caudocranially flat X-shaped optic chiasm. The posteromedial chiasm, containing the posterior part of the decussating fibers, partially forms the anterior floor of the third ventricle (Fig. 2.42). The optic recess of the third ventricle is bordered inferoposteriorly by the superoposterior aspect of the chiasm and anteriorly by the inferior lamina terminalis, which merges inferiorly with the chiasm. Posterior to the chiasm, the tuber cinereum and the pituitary stalk extend inferiorly. The bone below the chiasm forms a groove between the optic canals, the chiasmatic sulcus, which opens anterolaterally into the optic canals and is limited posteriorly at the midline by a ridge of the tuberculum sellae. Dorsal to this, the diaphragm of the sella covers the pituitary fossa. The localization of the chiasm varies between the position in the chiasmatic sulcus and more posteriorly near the sellar diaphragm (prefixed, postfixed chiasm) (Renn and Rhoton 1975).

a

13.19

13.26

13.16

b

Fig. 2.36.a Axial T2-weighted MRI of the orbit and neighboring brain area, demonstrating the (normal finding of a) widening of the proximal optic nerve sheath (ampulla). b Corresponding diagram: 3.2 = optic canal, 9.7 = vitreous body, 12.1 = prechiasmatic optic nerve, 13.1= temporal lobe, 13.16 = (inferior colliculi of the) quadrigeminal plate, 13.19 = temporal horn, 13.26 = aqueduct, 14.2 = ICA