of Orbit



























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.

B.  POSTERIOR COMPARTMENT


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


(a)  intraconal space


(b)  extraconal space


GLOBE


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


OPTIC NERVE SHEATH


=  extension of dura mater


Content:


1.   Optic nerve


2.   Ophthalmic artery


3.   Small veins


OPTIC NERVE (CN II)


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


Segments:


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


OCULOMOTOR NERVE (CN III)


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


Segments:


(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


Function:


(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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