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16.5.2 Orbit and Eye
188.8.131.52 Orbit and associated structures Eyelids (palpebrae)
-The eyelids are two thin, flexible folds which cover and (with the eyelashes) protect the exposed anterior surface of the eyeball.
-The upper lid normally just overlaps the iris whereas sclera is often visible between the lower lid and iris. Clinical: sclera visible between the iris and the upper lid may indicate either retraction of the eyelid or protusion of the globe (exophthalmos).
-Two tarsal plates form a connective tissue 'skeleton' for the lids; their inner surfaces are lined with conjunctiva.
-Between the tarsal plates and conjunctiva are the vertically oriented tarsal glands, modified sebaceous glands, the ducts of which open on to the margin of the eyelids. Clinical: if the tarsal glands become blocked and inflamed, cystic swellings can form which may require surgical excision. If the eyelash follicles become infected, 'styes' may form which make the outer surface of the eye red and swollen. The tarsal plate is attached to the margins of the orbit by the orbital spetum and the medial and lateral palpebral ligaments.
Core: levator palpebrae: innervations by III and by sympathetic fibres
-The movement of the upper eyelid is under the control of levator palpebrae superioris.
-It arises from the posterior aspect of the roof of the orbit and inserts into the skin and tarsal plate of the upper eyelid.
-Most of its fibres are striated and innervated by the oculomotor nerve (III). Some of the deep fibres are smooth and supplied by sympathetic neurons derived from the carotid plexus. Core: ptosis: failure of sympathetic or oculomotor nerve (III); lesions of sympathetic supply (Horner's syndrome)
-Damage to either of these nerve supplies can lead to ptosis (drooping of the eyelid)
-The oculomotor nerve can be damaged anywhere between the brainstem and the orbit.
-The sympathetic innervations originates from pre-ganglionic neurons in C7-T1 segments of the cord. Their axons pass out with the T1 spinal nerve, join the sympathetic chain, leave the thorax and run up through the neck to the superior cervical ganglion. Here, they synapse to give rise to the postganglionic fibres which form the carotid nerve which runs with the internal carotid artery and its ophthalmic branch to reach the orbit.
-Ptosis due to sympathetic lesions can therefore be caused by lesions anywhere between the spinal root at T1 to the orbit.
-Damage to the oculomotor nerve is more severe - the eyelid cannot be elevated and remains completely closed.
-Damage to sympathetic fibres only causes an abnormal drooping of the eyelid but the eye is not completely closed.
-The drooping seen in sympathetic damage is often caused by Horner's syndrome, which is a condition in which the rostral parts of the sympathetic chain are damaged. Extension: carcinoma of apex of lung (Pancoast's tumour)
-Pancoast's tumour is a carcinoma of the apex of the lung that is a common cause of Horner's because it compresses the sympathetic chain. Core: blink reflex: afferent - ophthalmic V; efferent - facial to orbicularis oculi
-The blink reflex is stimulated by potentially dangerous stimuli such as sudden flashes of light (conveyed by the optic nerve), emotional input (conveyed from higher brain centres) and stimulation to the cornea (conveyed by the ophthalmic nerve) or conjunctiva as well as during sleep.
-The facial nerve contains efferent fibres that cause orbicularis oculi to contract, closing the eyelids transiently. Core: lacrimal gland, nasolacrimal duct
=The lacrimal gland is located at the upper lateral aspect of orbit, under the upper eyelid and secretes aqueous tear fluid into the conjunctival sac through a number of small ducts which pierce the upper fornix of the conjunctiva.
-The fluid passes continuously over the surface of the eye during blinking; it helps to keep the cornea moist and is an antiseptic.
-Rapid evaporation of the tear fluid and drying of the exposed surface of the eyeball is prevented by the oily secretions of the tarsal glands which form a thin film over the tear fluid.
-It drains from the conjunctival sac into the lacrimal punta by capillarity (capillary action) and passes through the superior and inferior canaliculi to the lacrimal sac and from there to the inferior meatus of the nose via the nasolacrimal duct.
-During blinking, drainage is facilitated by contraction of a few (lacrimal) fibres of orbicularis oculi which pulls open the lacrimal sac.
-If the production of lacrimal fluid becomes too great, if the lids become everted or if the drainage system becomes blocked, tear fluid will spill on to the cheeks.
-Blinking is needed for the circulation of the lacrimal fluid.
-Clinical: in severe cases of exophthalmos, which often occurs in hyperthyroidism, the eyelids cannot close properly, the cornea dries out and is susceptible to infection and opacity. Core: lacrimation reflex: afferent - ophthalmic V (emotional stimuli), efferent - parasympathetic facial VII, pterygopalatine ganglion
-The lacrimation reflex is triggered by irritation of the cornea or conjunctiva. Afferent sensory fibres run in the ophthalmic division of the trigeminal nerve to its sensory nuclei in the brainstem. Emotional stimuli project caudally from other areas of the brain.
-This leads to stimulation of parasympathetic fibres in the superior salivatory centre of the pons.
-Parasympathetic fibres run in the facial nerve, synapse in the pterygopalatine ganglion and stimulate the lacrimal gland.
-Sympathetic fibres in the upper thoracic spinal cord are also stimulated and may increase secretion.
Control of the position of the eyeball
-A fibro-elastic membrane, the fascial sheath of the eyeball (Tenon's capsule_ partially encloses the eyeball posteriorly and - with the orbital fat lying behind it - acts as a socket in which movements of the eyeball take place.
-Between the fascial sheath and the eyeball is a layer of loose connective tissue that facilitates surgical removal of the eyeball (enucleation).
-The fascial sheath is attached posteriorly to the optic nerve coverings and anteriorly the margin of the cornea and is continuous with the fascial sheaths of the extrinsic ocular muscles as they pierce the capsule to insert into the sclera.
-From these sheaths, connective tissue expansions pass outward.
-The expansion from the sheaths of the medial and lateral rectus muscles form distinct triangular medial and lateral check ligaments which attach to the adjacent wall of the orbit and to the medial and lateral palpebral ligaments. The check ligaments may help to stabilise the horizontal position of the eyeball.
Core: suspensory ligament of the eyeball
-The suspensory ligament of the eyeball lies inferiorly to the eyeball. It is a condensed sheet of fascia that supports the eyeball and is involved in controlling its rostro-caudal (vertical) position.
-It is continuous with the sheaths of the inferior rectus and inferior oblique muscles and narrows on either side to merge with the medial and lateral check ligaments, gaining attachment to the bony margins of the orbit.
Clinical - ruptured suspensory ligament
-A blow to the orbit eg. from a cricket ball can cause a 'burst fracture' of the relatively fragile orbital floor, disrupting the suspensory ligament.
-This can also occur by a blow to the lateral margin of the orbit if its attachment to the small tubercle on the inner aspect of the zygoma is displaced.
-Suspensory ligament damage allows the eyeball to drop with the orbit causing severe diplopia ('double vision') as the brain is unable to fuse the two overlapping imagines which are no longer in register. Core: attachments and function of external ocular muscles
-The eye can be rotated within certain limits by the action of the four rectal muscles: lateral, medial, inferior and superior; and the two oblique muscles: inferior and superior.
-Contraction of individual extraocular muscles would produce some rotation of the eyeball about an antero-posterior axis. Such rotation is not seen in normal movements of the eyeball which are the result of coordinated contractions and relaxations of a number of extraocular muscles.
-Clinical: if, however, one or more of the muscles is paralysed then the action of the remaining muscles may cause rotation of the globe. Rectal Muscles
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