5 : The Eye ποΈ Flashcards
How is a Meibomian cyst different from a stye?
Meibomian cyst is due to a blocked tarsal gland which lies just posterior to the eyelash within the lids. A stye is an infection of a sebaceous gland situated at the base of the eyelash. The glands secrete an oily material that readily forms a cystic swelling if their drainage is blocked.
Explain two different causes of a drooped eyelid (ptosis).
Ptosis may be congenital. However there are two anatomical causes due to two different muscles involved in keeping the upper eyelid raised.
The greater portion of the muscle fibres are the voluntary fibres of levator palpebrae superioris but the smooth muscle fibres of the superior tarsal muscle also contribute to the normal width of the palpebral fissure.
The voluntary muscle is innervated by the oculomotor (III) cranial nerve whilst the smooth muscle is controlled by sympathetic fibres from the superior cervical ganglion. Damage to either of these nerves will cause ptosis.
What are the effects on lacrimation of damaged orbicularis oculi?
The orbicularis oculi muscle is the sphincter of the palpebral fissure. Upon contraction (as in blinking), tears are forced from the lateral aspect of the fissure where they are secreted by the lacrimal gland to the medial aspect where they drain via the lacrimal puncta. Damage to the muscle causes the eyelid to fall away from the eye. A stagnant pool of tears will then form in the lower fornix, which will eventually spill over the paralysed lower lid. Infection may occur in this pool and the resultant conjunctivitis will increase the secretion of tears with further weeping. Paralysis of the orbicularis oculi will result in the lack of protection of the cornea which will also dry out and subsequently may ulcerate.
A patient comes to an eye clinic with ptosis, a dilated pupil and an eyeball that looks down and laterally. Explain the cause of this disorder?
The ptosis is likely to be due to sympathetic or oculomotor (III) cranial nerve problems. The dilated pupil may be due to loss of parasympathetic control and the position of the eyeball indicates a loss of those muscles(medial rectus, inferior oblique, superior and inferior rectus) supplied by the oculomotor nerve. All the signs can be explained by damage to the oculomotor nerve (III) and its parasympathetic fibres.
During eye examination, which side of the fundus would you find the βblind spot in a normal eye?
It lies on the nasal side of the fundus. The blind spot is where the fibres of the optic (II) cranial nerve pass through the retinal layers. It is a pale area seen on opthalomoscopy as the optic disc and is also the point of entry of the retinal artery.
A patient complains that he cannot turn his right eye to the right side. Explain which nerve is likely to be affected.
The nerve most likely to be affected is the abducent (VI) nerve which supplies the lateral rectus. This is the only motor supply to the muscle.
Why is the central artery to the retina termed as an βend arteryβ?
The central artery to the retina is the only arterial supply to the retina. Thrombosis or blockage of blood flow through the central retinal artery causes instant blindness
Explain why thrombophlebitis of the facial veins may spread into the interior of the cranial cavity.
The facial vein has connection with the cavernous sinus (a venous channel of the dura mater covering the brain) through the ophthalmic vein. Usually the blood in the facial vein drains inferiorly but because the facial vein has no valves, blood may pass in the opposite direction and may enter the cavernous sinus. In patients with thrombophlebitis of the facial vein, inflammation of the vein with secondary thrombus formation, blood clots may pass into the intracranial venous system.
Why might a lesion of the ophthalmic division of the trigeminal nerve be dangerous to the eye?
A lesion of the ophthalmic division will result in the cornea being insensitive to touch and so specks of dust or grit will not be felt in the eye. This quickly leads to corneal ulceration and scarring which if not controlled may cause blindness. It is for this reason that after anaesthetizing an eye, it is most important to cover the patientβs eye to prevent collection of more foreign material.
What do you understand by the term βecchymosisβ?
The term refers to the discoloured patch produced by extravasation of blood into the subcutaneous tissues.
Which division of the common carotid artery supplies the area of the skin of the forehead and eyeβ?
The supra-orbital artery supplies the skin of the forehead and upper eyelid
γ
The supra-orbital artery is a branch of the ophthalmic artery which in turn is a branch of the internal carotid artery
In which layer of the scalp would you find blood vessels supplying it?
The blood vessels of the scalp run in the dense subcutaneous layer between the skin and the epicranial aponeurosis (a stong tendinous sheet that covers the bones of the vault between the occipitalis and frontalis muscles). The way they are held by the connective tissue that they tend to remain open when severed/lacerated; thus bleeding from the scalp is profuse.
S β Skin
C β Connective Tissue (vessels & nerves)
A β Aponeurosis (epicranial)
L β Loose connective tissue
P β Periosteum(pericranium)
Blood and fluid collecting in the subcutaneous layer does not track into the subtemporal or the occipital regions of the skull but passes forward into the orbital region as in the case this patient. Explain this occurrence.
Tracking of fluid into the subtemporal region is prevented because the epicaranial aponeurosis is continuous with the temporal fascia which is attached to the zygomatic arch.
Tracking into the occipital region is prevented because the occipitalis muscle is attached to the occipital bone and the mastoid process.
Thus fluid enters the orbital region and collects in the upper eyelid because the frontalis muscle inserts into the skin and subcutaneous tissue and does not attach to the bone; some fluid may also enter the lower eyelid.
