Orbit

=**Orbit**=


 * 1. Describe the surface features of the eye and orbit.**

The //palpebral fissure// is the space between the eyelids; it can decrease or increase in size, depending on how open or closed the eyelids are. The //medial canthus// and //lateral canthus// are the medial and lateral angles, respectively, where the upper and lower eyelids unite. The //lacrimal caruncle// is a fleshy, yellow mass in the medial canthus that contains sweat and sebaceous glands to help keep the cornea hydrated. The //iris// is the circular, pigmented muscular structure posterior to the cornea that regulates the amount of light entering the eye (aperture). The //pupil// is the opening through the iris through which light enters the eye. It is usually black because of the dark melanin pigment on the back of the eye, which helps reduce reflection. The //periorbita// is a special name for the periosteum on the bones inside the orbit. It is continuous with the periosteum on the rest of the skull.


 * 2A. Describe the bones that make up the orbit, its major foramina, and openings.**

The roof of the orbit is formed by the frontal bone and lesser wing of the sphenoid. The floor is formed by the maxilla, zygomatic, and palatine bones. The medial wall is formed by the ethmoid, frontal, lacrimal, and sphenoid bones. The ethmoid in this region is called the lamina papyracea and is very thin and prone to fracture. The lateral wall of the orbit is formed by the zygomatic bone and the greater wing of the sphenoid.

The optic canal transmits the optic nerve (CN II) and the ophthalmic artery (through the lesser wing of the sphenoid). Both structures pass through the common tendinous ring. The medial, inferior, lateral, and superior rectus muscles all originate on the common tendinous ring, surrounding the optic nerve at its entrance at the apex of the orbit.

The superior orbital fissure is between the greater and lesser wings of the sphenoid. The superior orbital fissure is divided by the common tendinous ring into 3 portions, one part superior to the ring, another through the ring, and a third part inferior to the ring. In the part above the ring, the lacrimal and frontal nerves (branches of CN V1), trochlear nerve (CN IV), and the superior ophthalmic vein pass through. Within the common tendinous ring, the superior and inferior divisions of the oculomotor nerve (CN III), nasociliary nerve (branch of CN V1), and abducens nerve (CN VI) pass through. Lastly, the inferior ophthalmic vein passes through the third part inferior to the common tendinous ring.

The inferior orbital fissure is between the greater wing of the sphenoid and the maxilla bone. The infraorbital nerve (branch of CN V2) and the infraorbital artery and vein pass through the inferior orbital fissure.

There is an anterior and an inferior ethmoidal foramina that is right on the suture between the frontal and ethmoid bones. The anterior and posterior ethmoidal arteries and nerves (branches of the nasociliary, branch of CN V1) pass through these two foramina.


 * 2B. Clinical Question: What is a blow-out fracture?**

Because the medial and inferior walls of the orbit are so thin, a blow to the eye may fracture the orbit. Indirect traumatic injury that displaces the orbital walls is called a “blow-out” fracture. Patients typically present with pain/tenderness around the eye, bleeding into sinuses, and double vision. Double vision or diplopia results because the oblique or rectus muscles of the injured eye are trapped by fractured bone, limiting the range of visional gaze.


 * 3. Explain the significance of the shape of the orbit as it relates to the direction of primary gaze.**

The orbits are roughly pyramidal in shape with medial walls parallel to each other and lateral walls 90 degress from each other. Thus, the axis of the orbit faces away from each other. However, the orbit axis is not the same as the visual axis. The visual axis of the two eyeballs in primary gaze (eyes focused at infinity and looking straight ahead) are parallel to each other and displaced medially from the orbit axis by approximately 23 degrees.

This angle is important for extra-ocular muscle attachment. The superior and inferior rectus muscles actually originate 23 degrees lateral from the visual axis. As a result, superior rectus not only elevates the eye, but also partially adducts. Conversely, the inferior rectus not only depresses the eye, but also partially abducts. As a result, the superior rectus muscle requires the inferior oblique muscle and the inferior rectus muscle requires the superior oblique muscle to counteract adduction and abduction, respectively, in order to allow pure elevation or depression of the eye.


 * 4A. Describe the components of the upper eyelid and their function.**

__Skeletal Muscle__ Orbicularis oculi is a muscle of facial expression (innervated by facial nerve CN VII) that extends from its orbital part down through a palpebral part in the eyelid. These fibers extend to the connective tissue deep to the skin of eyelids but superfical to the tarsal plates; in the palpebral part, the tendons of levator palpebrae superioris create little compartments of orbicularis muscle as they insert to the skin of the upper eyelid.

__Tarsal Plates__ The superior and inferior eyelids are supported by dense ands of connective tissue called the superior and inferior tarsus or tarsal plates which act as a “skeleton” for the eyelids. The superior and inferior tarsal plates are attached horizontally by the lateral and medial palpebral ligaments to the anterior wall of the orbit. An orbital septum composed of fascia of the tarsal plates attaches the eyelids to the rim of the orbit and is continuous with the periorbita.

__Tarsal Glands__ The tarsal plates are full of tarsal glands which are modified sebaceous glands that lubricate the edges of the eyelids (prevent sticking together when closed), forms a barrier to lacrimal fluid, increases tear viscosity, and decreases evaporation from the eyeball surface.

__Superior and Inferior Tarsal Muscles__ The two tarsal plates are also attached by the superior and inferior tarsal muscles. The superior tarsal muscle originates from the levator palpebral superioris tendon and inserts into the superior tarsal plate. The tarsal muscles are smooth muscles that set the tone of the eyelids and react to autonomic impulses (surprise triggers these two muscle to help open the eyelids wider). Tarsal muscles are under sympathetic control.


 * 4B. How is the eyelid affected in Horner's syndrome.**

In Horner’s syndrome, the sympathetic innervation to the head and neck is interrupted. This can cause ptosis, or drooping of the eyelids resulting from the paralysis of the tarsal muscles (smooth muscles) and is especially noticeable in the upper eyelid with the paralysis of the superior tarsal muscle.


 * 5. Describe the conjunctiva and its fornices? How is it innervated?**

The conjunctiva covers the eyelids internally and is divided into two parts. The palpebral conjunctiva lines the inner side of the eyelids. The bulbar conjunctiva is actually adherent to the eyeball, extending from the fornices to the corneoscleral junction. The superior and inferior fornices are recesses formed by reflections of the palpebral and bulbar conjunctiva on the superior and inferior eyelids and represent potential spaces. The superior palpebral conjunctiva receives innervation from the ophthalmic nerve (CN V1) while the inferior palpebral conjunctiva receives its innervation from the maxillary nerve (CN V2). The superior and inferior bulbar conjunctiva receives its innervation from the ophthalmic nerve (CN V1).


 * 6. Describe the oribital fascia and Tenon's capsule. What are the check and suspensory ligaments of the eye?**

The deep fascia of the orbit includes Tenon’s capsule, and the check and suspensory ligaments. Tenon’s capsule is a thin fascia the envelopes the eye, extending from the surface of the optic nerve to the corneoscleral junction. It is perforated by the tendons of the six extra-ocular muscles and becomes continuous with the deep fascia of the extra-ocular muscles.

The medial and lateral check ligaments are strong expansions of fascial sheaths of the medial and lateral rectus muscles that attach to the bony orbit. They limit the movement of the horizontal rectus muscles. Note that the check ligaments are //not// the palpebral ligaments. The suspensory ligament is composed of the blended fascial sheaths of the inferior oblique and inferior rectus muscles. They are continuous with Tenon’s capsule and attach to the check ligaments, forming a continuous fascial hammock below the eye.


 * 7. Describe the location of the lacrimal gland, its innervation, and blood supply. What is the pathway of tears, from production to collection in the nasal cavity.**

The lacrimal gland is located in the fossa for the lacrimal gland in the superolateral part of each orbit. A branch off the ophthalmic artery, the lacrimal artery supplies blood to the lacrimal gland.

The innervation to the lacrimal gland has parasympathetic, sympathetic, and sensory components. Presynaptic, parasympathetic fibers are conveyed from the facial nerve (CN VII) by the greater petrosal nerve which passes through the pterygoid canal. Postsynaptic sympathetic fibers from the superior cervical ganglion are brought by the internal carotid plexus along the internal carotid artery. They travel from the internal carotid plexus to the deep petrosal nerve and join up with the presynaptic parasympathetic fibers to become the nerve of pterygoid canal and enter the pterygopalatine ganglion (PT ganglion). The presynaptic parasympathetic fibers synapse at the PT ganglion while the sympathetic fibers continue on (since they already synapsed back at the superior cervical ganglion). The sympathetic and parasympathetic fibers then hitchhike from the PT ganglion along the maxillary nerve (CN V2) and follow its zygomatic branch to join the lacrimal branch of the ophthalmic nerve (CN V1) where they continue into and innervate the lacrimal gland. The parasympathetic fibers are secretomotor to the lacrimal gland while the sympathetic fibers control the vasculature of the lacrimal gland.

The lacrimal branch of the ophthalmic nerve (CN V1) also caries sensory components to //only// the capsule of the gland (and not the parenchynma). It continues through the gland to supply sensory innervation to the lateral upper eyelid, and upper palpebral and bulbar conjunctiva.

__The Path of Tears__ =*( Tears produced by the lacrimal gland flow from approximately 12 lacrimal gland excretory ducts in the superior conjunctival fornix toward the bulbar conjunctiva and over the cornea. Tears accumulate at the lacrimal lake near the medial canthus. The superior and inferior lacrima puncta in the medal edges of the eyelids draw tears from the lacrimal lake as the eye blinks. The puncta then empty into the lacrimal canaliculi, just deep to the medial palpebral ligament. Tears then drain into the lacrimal sac and continue into the nasolacrimal duct and into the inferior nasal meatus (nasal cavity).


 * 8A. Describe the extra-ocular muscles (EOMs or extrinsic eye muscles).**

The 6 muscles of the eye work in concert to move the eye:


 * 8B. Understand EOMs origin, insertion, and action(s).**


 * Muscle || Origin || Insertion || Action ||
 * Superior Rectus || Common tendinous ring || Superior sclera || Elevation and Partial Adduction ||
 * Inferior Rectus || Common tendinous ring || Inferior sclera || Depression and Partial Adduction ||
 * Lateral Rectus || Common tendinous ring || Lateral sclera || Abduction ||
 * Medial Rectus || Common tendinous ring || Medial sclera || Adduction ||
 * Inferior Oblique || Maxilla (anterior floor of orbit) || through trochlea to posterotemporal surface || Partial Abduction and Intorsion (Medial Rotation) ||
 * Superior Oblique || Sphenoid bone (posterior roof of orbit) || scleral surface between the inferior rectus and lateral rectus || Partial Abduction and Extorsion (Lateral Rotation) ||

Rotational (torsional) movements help maintain a visual horizon when the head is tilted and are //not// voluntary.

Spiral of Tillaux: insertions of rectus muscles move further from the pupil in a clockwise direction around the eye with medial rectus the closest and superior rectus the furthest.


 * 8C. Clinical Question: How would you test the extra-ocular muscles?**

See Objective 9C


 * 9A. What are the action(s) of extra-ocular muscles with the eye in primary gaze?**

__Adduction:__Medial rectus muscle contracts (minor component from Superior and Inferior Rectus) __Abduction:__Lateral rectus contracts (minor component from Superior and Inferior oblique) __Elevation:__Superior rectus and Inferior oblique (use Cross-pairs Rule) __Depression:__Inferior rectus and Superior oblique (use Cross-pairs Rule) __Rotation:__minor motion to maintain horizon (involuntary)


 * 9B. Understand the distinction between the "anatomical functions" of each extra-ocular muscle (i.e., how each muscle would move the eye if acting by itself) versus the "clinical function" of each muscle (i.e., in the context of all extra-ocular muscles).**

The movements of the eye in primary gaze use multiple muscles to achieve movement since there orientation is 23 degrees offset from the angle of primary gaze.


 * 9C. How would you test each extra-ocular muscle?**


 * **Muscle** || **Test** || **Nerve** ||
 * Superior Rectus || Look laterally 23 degrees and look up || CN III ||
 * Inferior Rectus || Look laterally 23 degrees and look down || CN III ||
 * Lateral Rectus || Look laterally || CN VI ||
 * Medial Rectus || Look medially || CN III ||
 * Inferior Oblique || Look medially look up || CN III ||
 * Superior Oblique || Look medially look down || CN IV ||


 * 10A. Describe the nerves of the orbit, and understand how each enters and exits the orbit.**


 * **Nerve** || **Entry** || **Exit** || **Innervation** ||
 * __**CN II:**__ || Optic canal ||  || **Special Sense:** Retina ||
 * __**CN III:**__ || Superior orbital fissure within the common tendinous ring ||  ||   ||
 * //superior branch// ||  ||   || **Motor:** Levator palpebrae superioris, and Superior rectus muscles ||
 * //inferior branch// ||  ||   || **Motor:** Inferior rectus, Medial rectus, Inferior oblique muscles; **Parasympathetic:** Ciliary ganglion ||
 * __**CN IV:**__ || Superior orbital fissure superior to the common tendinous ring ||  || **Motor** Superior oblique m. ||
 * __**CN V1:**__ || Superior orbital fissure ||  ||   ||
 * __Lacrimal branch__ || Outside common tendinous ring ||   || **Sensory:** enters the lacrimal gland and gives rise to branches to the lacrimal gland, conjunctiva, and skin of the lateral upper eyelid ||
 * __Frontal branch__ || Outside common  tendinous ring ||   || Splits into supraorbital and supratrochlear branches ||
 * //supraorbital branch// ||  || exits throught the supraorbital foramen || **Sensory:** innervating the medial portion of the upper eyelid, scalp, forehead and frontal sinus ||
 * //supratrochlear branch// ||  || exits between the trochlea of the Superior oblique and the supraorbital foramen || **Sensory:** innervating the central portion of the upper eyelid, scalp, forehead and frontal sinus ||
 * __Nasociliary branch__ || Inside common tendinous ring ||   || Splits into anterior and posterior ethmoidal branches, infratrochlear branch, and long ciliary branch ||
 * //anterior ethmoidal branch// ||  || exits the anterior ethmoidal foramen || **Sensory:** innervating the anterior ethmoidal air cells and divides into internal and external nasal branches ||
 * //posterior ethmoidal branch// ||  || exits the posterior ethmoidal foramen || **Sensory:** innervating the sphenoid and posterior ethmoidal sinuses ||
 * //infratrochlear branch// ||  ||   || **Sensory:** innervates the eyelids, conjunctiva, skin of nose and lacrimal sac, nasocilliary ||
 * //long ciliary branch// ||  ||   || **Sensory and Sympathetic:** innervates iris and cornea ||
 * __**CN VI:**__ || Superior orbital fissure Inside common   tendinous ring ||   || **Motor:** lateral rectus muscle ||

__Comments__ A branch of CN V2 provides sensory innervation to the lower palpebral conjunctiva may pass through the orbit.

The short ciliary nerves coming off the ciliary ganglion carry information from the sensory root (off the nasociliary branch of CN V1), sympathetic root (from the carotid plexus), and parasympathetic root (off the oculomotor nerve CN III).

A key difference between the short and long ciliary nerves is that the short ciliary nerves contain sensory, sympathetic, and parasympathetic components while the long ciliary nerve contains only sensory and sympathetic components (and no parasympathetic component).

The short ciliary nerves supply parasympathetic innervation to the sphincter muscle of the pupil and ciliary muscle and sympathetic innervation to the dilator muscle of the pupil.


 * 10B. Clinical Questions: What are the consequences of injury to each nerve?**

Injury to nerves carrying sensory supply would result in loss of sensation to the areas that they supply. For example, injury to the frontal branch of the ophthalmic nerve (CN V1) would result in the loss of sensation from areas innervated by both the supraorbital and supratrochlear nerves (innervating the medial and central portion of the upper eyelid, scalp, forehead and frontal sinus). Injury to nerves carrying motor supply would result in paralysis of muscles that they supply. For example, injury to the trochlear nerve (CN IV) would result in paralysis of the superior oblique extra-ocular muscle (Note: To test, have patient look medially and downward).

Injury to the inferior branch of the oculomotor nerve (CN III) will result in the paralysis of the inferior rectus, medial rectus, and inferior oblique muscles. However, if the injury occurs before the parasympathetic root to the ciliary ganglion branches off the inferior branch of CN III, it could also sever the preganglionic parasympathetic fibers that lead to the ciliary ganglion. As a result, the parasympathetic component that would normally travel through the short ciliary nerves to the sphincter muscle of the pupil and ciliary muscle would no longer be functional and the eye would not be able to constrict the pupil or keep the lens from thickening. However, the pupil can still dilate because dilator muscle of the pupil will not be affected since it receives its sympathetic innervation from the sympathetic root of the ciliary ganglion and is not related to both the parasympathetic root of the ciliary ganglion and the inferior branch of the oculomotor nerve (CN III).


 * 11. Describe the location and functional components of the ciliary ganglion.**

The ciliary ganglion is located in the posterior of the orbit. The postganglionic axons innervate two eye muscles: the sphincter pupillae constricts the pupil, and the ciliaris muscle changes the shape of the lens. The ganglion contains sympathetic fibers from the superior cervical ganglion and parasympathetic synapses from preganglionic fibers originating in the brain stem.


 * 12A. Describe the arterial supply to the orbit.**

The blood supply to the orbit is from branches of the ophthalmic a., whose branches include Dorsal nasal, Supratrochlear, Central retinal, Lacrimal, cilliary branches, and Ophthalmic a. The orbital arteries emerge around the orbit and supply the skin surrounding the eye.


 * 12B. Clinical Question: What is the central retinal artery, where does it arise, and why is it important?**

The central retinal a. is a branch of the ophthalmic a. and enters the eye with the optic nerve. It is the sole blood supply to the optic n. and if damaged results in blindness.


 * 13. Describe the venous drainage of the orbit. How are the ophthalmic veins connected to various venous plexuses and dural sinuses of the head?**

The venous drainage of the orbit occurs via the superior opthalmic vein which drains to the angular vein and cavernous sinus, and the inferior ophthalmic vein which drains to the cavernous sinus and Pterygoid plexus.


 * 14A. Describe the cavernous sinus. Where would you find it and how is it related to the orbit?**

The cavernous sinus is deep to the orbit and is connected via the superior and inferior opthalmic veins. The cavernous sinus is bordered by the sphenoid bone and the temporal bone of the skull and contains the Hypophysis, CN VI, III, IV, V1, and V2 and the internal carotid artery.

cavernous sinus, and why is this clinically important?**
 * 14B. Clinical Question: How do the nerves that supply EOMs related to the internal carotid artery in the

The internal carotid artery travels along with CN VI in the cavernous sinus. CN III, IV, V1, and V2 also travel with the artery but are separated from it by part of the cavernous sinus. If there is an aneurysm in the carotid artery it would impinge on the nerves.