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Orbital Spaces

 globe: subdivided into anterior + posterior segments by lens

optic nerve-sheath complex:

optic nerve surrounded by meningeal sheath as extension from cerebral meninges
 intraconal space: orbital fat, ophthalmic a., superior ophthalmic v., nerves I, III, IV, V1, VI
 conus: incomplete fenestrated musculofascial system extending from bony orbit to anterior third of globe, consists of extraocular muscles + interconnecting fascia
 extraconal space: between muscle cone + bony orbit containing fat, lacrimal gland, lacrimal sac, portion of superior ophthalmic v.


=  RETROBULBAR SPACE = cone consisting of extraorbital muscles + envelope of fascia divides retrobulbar space into

(a)  intraconal space

(b)  extraconal space


Ocular globe diameter:  22–25 mm

Wall:   composed of 3 layers:

(a)  fibrous outermost layer

Function:   maintaining shape + pressure of globe

1.  Sclera = collagenous tissue layer continuous anteriorly with cornea posteriorly with dura mater

2.  Cornea

(b)  Uvea = pigmented vascular middle layer

[uvea, Latin = grape]

3.  Iris

4.  Ciliary body (anteriorly)

5.  Choroid (posteriorly) = most vascular structure of globe

Attachment:   tethered to sclera by arteries + veins

◊  Most frequent site of intraocular metastases!

(c) innermost sensory layer

5.  Retina = light-sensitive (sensory) inner layer

Attachment:   firm at anterior margin (= ora serrata) and posteriorly at optic disc

√  various layers of globe are NOT discernable at imaging, especially CT

√  sclera may be separated from choroid at high-res MRI

√  individually visualized in ocular (choroidal / retinal) detachments

Contents:   (a)   anterior segment containing

1.   Aqueous humor subdivided by iris into:

›  anterior chamber

›  canal of Schlemm (see below)

›  posterior chamber

  (b)  posterior segment containing

2.   Vitreous humor

Potential ocular spaces:

(a)  between retina + choroid → retinal detachment

(b)  suprachoroidal space between choroid + sclera → choroidal detachment

(c)  between vitreous + posterior hyaloid membrane → hyaloid detachment

Canal of Schlemm

[Friedrich Schlemm (1795–1858), German anatomist, sentenced for grave robbing, eventually professor at the University of Berlin]

=  SCLERAL VENOUS SINUS [misnomer – not a blood vessel]

=  circular ringlike endothelium-lined tube lymphatic tube resembling a lymphatic vessel

Location:  adjacent to outer angle of anterior chamber in pectinatum iridis (= periphery of cornea) between cornea + iris

Function:  collects aqueous humor from anterior chamber and delivers it into the venous system


=  extension of dura mater


1.   Optic nerve

2.   Ophthalmic artery

3.   Small veins


Histo:   CN II is an extension of brain = retinal ganglion cell axons myelinated by oligodendrocytes + enveloped within meninges


A.  Retinal segment leaves ocular globe through lamina cribrosa sclerae

B.  Orbital segment travels in center of fat-filled orbit

√  surrounded by dural sheath containing CSF

C.  Canalicular segment lies in optic canal below ophthalmic artery; frequently overlooked on radiologic images

D.  Cisternal segment in suprasellar cistern leading to optic chiasm

√  anterior cerebral a. passes over superior aspect of nerve


Exit:   from brainstem anteriorly between posterior cerebral (PCA) + superior cerebellar arteries


(1)  cisternal segment courses through prepontine cistern

(2)  cavernous sinus segment along cephalad portion of the lateral dural wall

(3)  orbital segment through the superior orbital fissure

(4)  division into superior + inferior branches


(a)  motor fibers to

›  levator palpebrae muscle

›  all extraocular muscles except lateral rectus and superior oblique mm.

(b)  parasympathetic fibers to

›  internal eye muscles (pupillary sphincter and ciliary muscles) → constriction of pupil via Edinger-Westphal nucleus

Location:  periphery of CN III → subject to compression by extrinsic masses

Disturbed function:

(1)  “pupil-sparing” oculomotor palsy = loss of motor function in extraocular muscles with relative sparing of pupillary parasympathetic innervation ← compromised blood flow to central microvessels

(2)   compression by posterior communicating artery aneurysm ← close association with posterior cerebral (PCA) + superior cerebellar arteries

(3)   compression during transtentorial uncal herniation ← course over petroclinoid ligament

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Jun 29, 2017 | Posted by in GENERAL RADIOLOGY | Comments Off on of Orbit

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