BLOCK 12 WEEK 5 Flashcards
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What is Posture?
A
- Relative position of the various parts of the body with respect to one another, to the
environment, and to gravity. - Postural systems exist to stabilise and maintain position of the body
Definitions:
Postural Equilibrium: state in which all the forces acting on the body are balanced
Static Equilibrium: is maintaining balance when the body is still
Dynamic Equilibrium:
maintaining
balance during movement
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Cranial nerves
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- 12 paired nerves
- Arise directly from the brain
- Fist 2 arise from cerebrum
- Remaining arise from the brainstem
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Where the cranial nerves arise from?
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Olfactory nerve - cribriform plate
Optic nerve - optic canal
3,4,6 v1 - Superior orbital fissure
v2 - foramen rotundum
v3 - foramen ovale
7 and 8 - Internal Acoustic Meatus
9,10,11 - jugular foramen
12 - hypoglossal canal
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Brainstem
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- Brainstem is made up of the midbrain, pons and medulla.
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Posterior Brainstem
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Cranial nerves emerge from the brainstem.
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- All of the cranial nerves emerge from the brainstem except CN1 (olfactory nerve)
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ANATOMY & PHYSIOLOGY OF THE EAR
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- Ears help us to hear and to balance in space
Outer ear: Pinna/Auricle (earrings on) + Ear canal (aka external acoustic meatus)
Middle ear: tiny ear bones: MALLEUS, INCUS, STAPES
Inner ear:
COCHLEA - converts sound waves into electrical impulses for the brain
SEMICIRCULAR CANALS - help with balance
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Q
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- Ear canal is about 1-2cm long and ends at the tympanic membrane (aka EAR DRUM). The tympanic membrane separates the external ear from the middle ear.
- When sound waves reach the eardrum, it vibrates and transmits those vibrations to the tiny bones in the middle ear.
- On the inside of the ear canal we have CERUMINOUS glands - secrete CERUMEN (ear wax) - stops foreign objects like bugs getting into the ear.
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MIDDLE EAR
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-Middle ear is an air-filled cavity inside the temporal bone
- The eardrum makes up the lateral wall of this cavity
- Internal wall makes up the medial wall- that separates the middle ear from the inner ear.
The internal wall has two windows: an oval window above, and a round window below
-The two other walls of the middle ear are the posterior wall - towards the back of our head - and the anterior wall - towards the front.
- The posterior wall has an opening called the mastoid antrum, and it connects the middle ear with the mastoid cavity within the temporal bone.
- The anterior wall has an opening for the eustachian tube, which connects the middle ear to the nasopharynx.
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EUSTACHIAN TUBE FUNCTIONS
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The eustachian tube has three main functions:
-equalizing pressure across the tympanic membrane
-protecting the middle ear from reflux of fluids going up from the nasopharynx
- clearing out middle ear secretions.
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WHAT IS THE ROOF OF THE EAR CALLED AND WHAT DOES THE FLOOR OF THEEAR SIT ONTOP OF?
A
- Roof: Epitympanic recess
- Floor: the floor of the middle ear is a thin layer of bone that sits right above our friendly basement neighbor - the jugular vein.
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Q
Label the 3 bones in the middle ear?
A
When the eardrum vibrates the vibrations are transmitted from the malleus to the incus, then to the stapes, and finally to the oval window, which transfers the vibrations over to the inner ear.
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INNER EAR
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- On the outside, the inner ear has a tough bony shell - the bony labyrinth; and inside the bony labyrinth, there is the membranous labyrinth.
- Now, both of these sections are filled with fluid - the bony labyrinth contains a fluid called perilymph, while the membranous labyrinth contains endolymph.
3 parts of inner ear:
- VESTIBULE: which is like a hallway that leads up to two other parts or rooms
- COCHLEA: hearing
- SEMICIRCULAR CANALS: balance
Movement of both perilymph and endolymph within the labyrinth forms the basis for both hearing and balance.
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Q
COCHLEA
A
The cochlea is shaped like a snail’s shell, and inside it, and in cross-section there are three parts: from top to bottom, there’s the scala vestibuli, which is connected to the middle ear through the oval window, and contains perilymph.
Then there’s the cochlear duct, which is filled with endolymph, and houses the organ of Corti.
The organ of Corti is the mastermind of our hearing sense, and it contains our hearing receptors, or hair cells.
Finally, there’s the scala tympani, which is connected to the middle ear through the round window, and it also contains perilymph.
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HOW SOUND TRAVELS TO THE VESTIBULOCOCHLEAR NERVE (CN8)
A
When she meows, the sound vibrations travel through the external ear and the ossicles in the middle ear.
When the foot of the stapes beats against the oval window - it transfers the vibrations over to the perilymph inside the scala vestibuli, forcing the fluid into motion. This motion transmits to the organ of Corti inside the cochlear duct.
The hearing receptors convert the vibrations into an electrical impulse. And the electrical impulse is sent to the brain via the auditory branch of the eight cranial nerve, and the perilymph inside the scala tympani is also set in motion. This makes the round window bulge back out towards the middle ear, relieving the pressure.
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Q
FOR BALANCE
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Engages the second part of your inner ear - the vestibular apparatus.
Now the vestibular apparatus - has two parts - the first part, the three semicircular canals, and the second part, which includes both the utricle and saccule - both of which deal with different aspects of balance.
The three semicircular canals are shaped like three letter U’s oriented in the three directions of space, with each of them forming a 90 degree angle with the other two - kinda like the corner of box, where the 3 sides meet.
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DECEREBRATE RIGIDITY
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Decerebrate posturing is a position your body may assume automatically because of damage to or disruptions in your brain. It’s sometimes called “decerebrate rigidity.”
- It causes certain muscles in your body to tense up
- Most people this happens to are in a coma
- Not treatable
SIGNS:
- Wrists flexed away from your body.
- Fingers curled
- Legs extended and rigid.
- Arched or stiff back
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STATIC EQUILLIBRIUM
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Our static equilibrium is a job for the utricle and saccule - also known as the otolith organs.
They contain endolymph, as well as special balance receptors that detect changes in our head position in relation to horizontal or vertical acceleration.
Now, inside the utricle, there’s a region called the macula - which looks like like a bean-shaped shaggy rug lying on the floor.
The macula is where our balance receptors, called the hair cells, can be found.
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Ok, so the utricular macula senses changes in our head position in the horizontal axis - like when we feel pushed towards the back of a seat in a speeding race car.
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HAIR CELLS
- Each hair cell has multiple finger-like projections - called stereocilia - and a single kinocilium.
- These stereocilia are arranged in rows, depending on their length - the longer ones are closer to the kinocilium, and the shorter ones are further away from the kinocilium
- Now, the tips of these cilia are imbedded in a gel, called the otolithic membrane - which has a layer of tiny ear stones called otoconia on top of it, making the otolithic membrane heavier than the endolymph.
- Finally, the bottom of each hair cell is connected to sensory neurons that make up the vestibular branch of the vestibulocochlear cranial nerve, that carries balance information to the central nervous system.
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SACCULA
The saccular macula has the same structure as the utricular macula - hair cells embedded in an otolith membrane, but there’s a twist. This time, the macula is vertically oriented, so basically hanging on the saccular wall like a tapestry.
The saccular macula also has a striola, which divides the hair cells in a superior and an inferior group.
However, the cilia on these hair cells are arranged so that the kinocilium of each hair cell is further away from the striola than the stereocilia - so all the kinocilia point away from the striola.
So for hair cells above the striola, the kinocilia point upwards, whereas for hair cells below, the kinocilia point downwards.
This allows the macula to sense linear acceleration in the vertical axis
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LITTLE BITCH
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RARE INNER EAR CONDITIONS YOU HAVE TO RULE OUT?
Ménière's disease/syndrome:
- Ménière's disease is a rare inner ear condition that can affect your balance and hearing
- Ménière's disease symptoms come and go, last for a few minutes to 24 hrs
SYMPTOMS:
- Vertigo - feeling like you or everything around you is spinning
- losing your balance
- tinnitus - ringing or buzzing sound in one or both ears
- hearing loss
- feeling pressure discomfort or pain in one or both ears
- feeling or being sick
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THE MOST COMMON CAUSE OF VIRTIGO?
Benign paroxysmal positional vertigo (BPPV):
- BPPV is a problem in the inner ear.
- It is the most common cause of vertigo, which is a false sensation of spinning or movement.
- can affect all ages but more commonly affects people over 60.
- Caused by calcium crystals, otoconia, becomes dislodged from the otolith organs, utricle, disctrupting the flow of endolymph in the semicircular canals.
- Symptoms are felt when changing head position e.g. when looking up or down, rolling over in bed, going from a lying to seated position or vice versa.
The symptoms can be very distressing. People can fall out of bed or lose their balance when they get up from bed and try to walk. If they tilt their head back or forward while walking, they may even fall, risking injury.
Vertigo can cause the person to feel quite ill with nausea and vomiting.
TREATMENT:
- BPPV with the most common variant (crystals in the posterior SCC) can be treated successfully — with no tests, pills, surgery or special equipment — by using the EPLEY MANEUVER or SERMONT MANEUVER
EPLEY MANEUVER: This simple, effective approach to addressing BPPV involves sequentially turning the head in a way that helps remove the crystals and help them float out of the semicircular canal
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MAV - Migraine associated vertigo
Migraine can sometimes trigger attacks of sensation of spinning and movement affecting you or your surroundings.
This is known as vertigo.
Up to 40% of migraine sufferers will experience spinning or balance problems as part of their migraine symptoms. This is called Vestibular Migraine.
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ANATOMY OF THE EYE - SAGGITAL CUT
- Looks like two spheres fused together
- Eyeball is made up of dense connective tissue, the outer layer (smaller sphere) is called the CORNEA and the outer layer of the big sphere is called the SCLERA
- CORNEA is transparent and lets light through into interior of eyeball
- SCLERA is opaque and makes up the white shells of the eye
- It occupies the majority of the posterior eyeball and serves for attachment of the extrinsic muscles of the eye.
- Also, the sclera is pierced by the optic nerve at the posterior end of the eyeball.
- The place where the cornea and the sclera meet, is called the CORNEOSCLERAL JUNCTION
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CORNEAL REFLEX
- When something touches or irritates the cornea
- it is sensed by the OPTHALMIC NERVE, a branch of cranial nerve V.
-This sensory signal then reaches the brain stem, and signals the facial nerve, cranial nerve VII, to contract the ORBICULARIS OCULI to close our eyes.
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SCLERA
So, the bigger posterior sphere is made of another two layers of tissue:
1. the vascular layer that provides blood supply, located beneath the sclera
2. the internal layer called the retina, that contains the photoreceptors and supporting cells.
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VASCULAR LAYER (aka UVEA)
- The choroid
- The ciliary body
- the iris
The choroid:
The choroid is made of two layers of blood vessels that provide blood supply to both the loosely attached sclera above, and the firmly attached retina beneath
The Ciliary body:
The ciliary body is a thickening of the vascular layer, made of blood vessels and smooth muscle, and attaches the choroid layer to the circumference of the iris.
The internal surface of the ciliary body has projections called ciliary processes, that serve as attachments for the suspensory ligament of the lens which keeps the lens in place, while also secreting aqueous humor that fills the anterior chamber or “smaller sphere” of the eyeball.
Iris:
- gives eye color
- regulates the amount of light that passes through the pupil
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Lacrimal Apparatus
- Secretes tears which lubricate the eyes
- The bone around the eye is called the orbit
- We have a fat pad behind the eye to prevent friction and protect the eye
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RETINA
Has two different parts:
- Optic Part: right below the choroid layer, where the light impacts and is sensed by the photoreceptors
- Non-visual retina: found underneath and covering the ciliary body and the posterior iris
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RETINA
The retina has an inner neural layer that contains ganglion cells which synapse with bipolar cells, which then synapse with the photoreceptors - both rods and cones.
RODS:
- The rods are more numerous, about 120 million of them in each eye.
- Rods are highly sensitive to light and even a single photon can cause them to activate, making them great for seeing in low light conditions
- they only offer black and white vision
CONES:
- 6 million cones in each eye
- most are located in the macula which is an oval spot in the middle of the posterior retina
- Cones are less sensitive than rods to light, and each cone can detect either a red, green, or blue wavelength of light.
- So when you see a red apple, only the red cones are activated, whereas when you see a purple flower, both the red and blue cones are activated.
FOVEA:
- At the center of the macula is the fovea which contains the highest concentration of cones
- is the part of the retina that offers the highest visual acuity.
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How do we see ?
When rods and cones are activated, they hyperpolarize and create an action potential, which triggers depolarization of the bipolar cells, which then triggers the depolarization of the ganglion cells.
The action potential in the ganglion cells travels through their axons to the posterior portion of the retina to form the optic nerve, which leaves the eye through the optic disc medial to the macula.
The optic disc doesn’t have any photoreceptors so it’s known as the blind spot.
The optic nerve then carries the visual information to the brain where we process and recognize the visual information.
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EXAMINATION OF EYE
Now, the optic part can be clinically analyzed in order to detect abnormalities that could be affecting the vision.
This can be done by looking through an instrument called ophthalmoscope at the fundus of the eyeball, formally known as the ocular fundus, which is defined as the internal aspect of the posterior part of the eyeball where light enters and is focused.
The retina of the fundus contains the optic disc, which is a circular area that corresponds to the exit of the optic nerve and the central retinal vein from the posterior eyeball, and the entrance of the central retinal artery, a branch of the ophthalmic artery which supplies the retina.
This circular area contains no photoreceptors, so it’s a veritable ‘blind spot’ in our vision.
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EXAMINATION
- On a fundoscopic examination, you can also see the retinal arterioles and venules diverging from the optic disc, and to differentiate the two, remember that the arterioles are narrower.
MACULA of the retina:
- Another important area of the fundus is the macula of the retina, lateral to the optic disc, which contains only photoreceptor cones.
- This area is important because it specializes in acuity of vision - so without it, everything would be blurry.
- Actually, the most acute vision is sensed at the central area of the macula, called the fovea centralis, which is more depressed than the rest of the retina.
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How does light reach the eye?
There are a number of transparent elements that light passes through in order to reach the retina:
- CORNEA
- AQUEOUS HUMOUR
- LENS
- VITREOUS HUMOUR
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What is the function of the lens of the eye?
In contrast to the cornea, which is the structure that primarily bends light, the lens, found behind the iris, is in charge of FINE TUNING our focus, just like the lens of a camera
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- The cornea and the lens help us delineate the two segments of the eye, which are the anterior and posterior segment.
- The anterior segment is between the cornea and the lens
- The posterior segment is found between the lens and the retina.
- The anterior segment contains the aqueous humor, which is fluid and provides nutrients for the cornea and the lens.
-The anterior segment is further divided into an anterior chamber, between the cornea and the iris; and a posterior chamber, between the iris and the lens.
- The aqueous humor is produced by the ciliary processes in the posterior chamber, and it flows through the pupil and into the anterior chamber.
- In the anterior chamber, the aqueous humor is reabsorbed by the scleral venous sinus, or the canal of Schlemm, at the angle where the iris meets the cornea, called the iridocorneal angle.
- On the other side, the posterior segment, also called the VITREOUS CHAMBER, contains a more dense fluid called vitreous humor, which is also transparent, and it provides support to the lens and holds the retina in place.
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SAGGITAL VIEW OF EYE LABELLED
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How do we modify the path of light as it enters the eye?
CORNEA: The cornea, being the first and the most powerful refractive medium, makes the light rays converge to pass through the pupil. It does this in a constant manner, which means it always changes the direction of light at the same angle because the curvature of the cornea remains the same.
LENS: is an elastic and biconvex structure, which means both its anterior and posterior surfaces are convex. This means the lens can be stretched or bulged to modify its curvature, and by doing this, it fine tunes our ability to focus.
- Unfortunately, the lens can’t stretch or bulge by itself, so it needs a partner: the smooth muscle within the ciliary body, called the ciliary muscle
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CILIARY MUSCLE
- The ciliary muscle is under autonomic control in order to contract and relax quickly, which in turn causes the ciliary body to shrink or dilate, allowing our lens to change shape so we can rapidly change our vision from near to distant objects and vice versa
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CILIARY MUSCLES RELAX
- no parasympathetic innervation and inhibition of sympathetic innervation - causes the ciliary muscles to relax, increasing the diameter of the ciliary body which tenses the suspensory ligament so as to pull the lens in each direction. This results in thinning of the lens, which allows for more distant objects to come into focus.
- YES parasympathetic innervation - contraction of the ciliary muscles - via cranial nerve III (oculomotor). Less tension over lens, its now more bulged in order to better focus on closer objects. THIS PROCESS IS CALLED ACCOMODATION.
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IRIS
- Iris is another intrinsic muscle of the eye like the ciliary muscle.
- The iris allows us to see well either in a dark or light environment because it REGULATES THE AMOOUNT OF LIGHT that passes through the pupil.
- It can do this because it’s actually made of two rings of muscle. If you look at it from the front, you’ll see the one that immediately surrounds the pupil is circularly arranged, called the SPHINCTER PUPILLAE.
- The second muscle, called the DILATOR PUPILLAE is radially arranged around the sphincter pupillae
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What happens when you shine light into eye?
Your pupil gets smaller
- Your parasympathetic system sends a signal to the sphincter pupillae to contract immediately, therefore contracting the pupil and preventing excessive light from entering your retina.
MIOSIS - is when the pupil gets smaller
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What happens when you go into the dark ?
- Your sympathetic system immediately sends a signal to the dilator pupillae to contract.
- This is because you are now used to having an abundance of light entering the retina, so in the new low light environment your body reacts by trying to dilate the pupil to allow more light into focus
- MYDRIASIS - dilation of the pupil
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IRIS vs CILIARY MUSCLE
The ciliary muscle which allows us to focus on distant and closer objects.
The iris allows us to see well either in a dark or light environment because it regulates the amount of light that passes through the pupil.
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ACUTE ANGLE CLOSURE GLAUCOMA
Glaucoma: optic nerve damage that is caused by a significant increase in intraocular pressure.
It is an ophthalmological emergency requiring rapid treatment to prevent permanent vision loss.
Cause:
- BLOCKAGE OF AQUEOUS HUMOUR
- iris bulges forward and seals off the trabecular meshwork from the anterior chamber, preventing aqueous humour from draining and leading to a continual increase in intraocular pressure.
- The pressure builds in the posterior chamber, pushing the iris forward and exacerbating the angle closure.
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RISK FACTORS FOR ACG
- Increasing age
- Family history
- Female (four times more likely than males)
- Chinese and East Asian ethnic origin
- Shallow anterior chamber
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Presentation of Acute Angle Glaucoma
A typical patient with acute angle-closure glaucoma presents appearing generally unwell, with a short history of:
-Severely painful red eye
- Blurred vision
- Halos around lights
-Associated headache, nausea and vomiting
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Signs of Acute Angle Glaucoma
- Red eye
- Hazy cornea
- Decreased visual acuity
- Mid-dilated pupil
- Fixed-size pupil
-Hard eyeball on gentle palpation
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Initial Management of Acute angle-closure glaucoma
Acute angle-closure glaucoma requires immediate admission. Measures while waiting for an ambulance are:
-Lying the patient on their back without a pillow
-Pilocarpine eye drops (2% for blue and 4% for brown eyes)
-Acetazolamide 500 mg orally
-Analgesia and an antiemetic, if required
Pilocarpine acts on the muscarinic receptors in the sphincter muscles in the iris and causes pupil constriction (it is a miotic agent - shrinking of pupil).
It also causes ciliary muscle contraction.
These two effects open up the pathway for the flow of aqueous humor from the ciliary body, around the iris and into the trabecular meshwork - which allows better drainage of the aqueous humor.
Acetazolamide is a carbonic anhydrase inhibitor that reduces the production of aqueous humour.
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SECONDARY CARE MANAGEMENT
- Pilocarpine eye drops
- Acetazolamide (oral or intravenous)
- Hyperosmotic agents (e.g., intravenous mannitol) increase the osmotic gradient between the blood and the eye
- Timolol is a beta blocker that reduces the production of aqueous humour
- Dorzolamide is a carbonic anhydrase inhibitor that reduces the production of aqueous humour
- Brimonidine is a sympathomimetics that reduces aqueous humour production and increases uveoscleral outflow
LASER IRIDOTOMY is usually required as a definitive treatment. This involves making a hold in the iris using a laser, which allows the aqueous humour to flow directly from the posterior chamber to the anterior chamber. This relieves the pressure pushing the iris forward against the cornea and opens the pathway for the aqueous humour to drain.
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CONJUCTIVITIS
What is it:
- Conjunctivitis is inflammation of the conjunctiva. The conjunctiva is a thin layer of tissue that covers the inside of the eyelids and the sclera.
- Conjunctivitis may be bacterial, viral or allergic.
- It may be unilateral or bilateral.
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Conjunctivitis presentation - DOES NOT CAUSE PAIN
- Red, bloodshot eye
- Itchy or gritty sensation
- Discharge
Conjunctivitis does not cause pain, photophobia or reduced visual acuity.
Discharge covering the eye may cause blurry vision, but this should return to normal when the discharge is cleared.
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BACTERIAL CONJUCTIVITS
- Bacterial conjunctivitis presents with a purulent discharge ( milky look and texture)
- It is typically worse in the morning when the eyes may be stuck together. It usually starts in one eye and can spread to the other.
- It is highly contagious.
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VIRAL CONJUCTIVITIS
- Viral conjunctivitis is common and usually presents with a clear discharge.
- It is often associated with other symptoms of a viral infection, such as a dry cough, sore throat and blocked nose.
-There may be tender pre-auricular lymph nodes (in front of the ears).
- It is also contagious.
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DIFFERENT THINGS CAN CAUSE RED EYE
Causes of an acute painful red eye include:
-Acute angle-closure glaucoma
-Anterior uveitis
-Scleritis
-Corneal abrasions or ulceration
-Keratitis
-Foreign body
-Traumatic or chemical injury
Causes of an acute painless red eye include:
-Conjunctivitis
-Episcleritis
-Subconjunctival haemorrhage
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MANAGEMENT OF CONJUCTIVITIS
- Conjunctivitis usually resolves in 1-2 weeks without needing treatment (even bacterial cases).
- Hygiene measures can reduce spreading (e.g., avoiding towel sharing and close contact, and regular hand washing). Cleaning the eyes with cooled boiled water and cotton wool can help clear the discharge.
- Chloramphenicol or fusidic acid eye drops are options for bacterial conjunctivitis if necessary.
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ALLERGIC CONJUCTIVITS
Allergic conjunctivitis is caused by contact with allergens. It causes swelling of the conjunctival sac and eyelid with itching and a watery discharge.
Antihistamines (oral or topical) can help symptoms.
Topical mast-cell stabilisers can be used in patients with chronic seasonal symptoms. They work by preventing mast cells from releasing histamine. These require several weeks of use before they show any benefit.
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VISUAL FIELDS
- A visual field is everything that can be seen with a single eye, so we have a left visual field for the left eye, and a right visual field for the right eye.
- These visual fields overlap to produce a BINOCULAR VISUAL FIELD
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LEFT VISUAL FIELD
The left visual field can be divided into two halves.
- NASAL VISUAL FIELD
- TEMPORAL VISUAL FIELD
When light enters the eye, the temporal field of vision is projected to the opposite side, onto the medial nasal retina, and the nasal field of vision gets projected to the opposite side, onto the lateral temporal retina.
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What happens when light hits the eye?
- Light hits the retina and triggers rods and cones, to send an electrical signal
- rods and cones -> bipolar cells -> ganglion cells
- ganglion cell have a long axons that travels through the retina layer to form a single optic nerve aka CN II, that exits the retina from the optic disc.
- This area in the retina where the optic nerve exits doesn’t have any receptor cells, so it’s also called the blind spot.
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Optic nerve journey
-The optic nerve exits the eye through the optic disc and travels back towards the brain.
- reaches the optic chiasm
- Axons from the NASAL PORTION of the retina crossover to the opposite side
- Whereas the axons from the TEMPORAL retina do not cross over.
- The purpose for this crossover is to organize the visual information coming from both eyes.
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Where does information for the left visual field come from ?
- Nasal retina of left eye
- Temporal retina of the right eye
These go to the right cerebral hemisphere
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Where does information for the right visual field come from ?
- right nasal retina
- left temporal retina
The information goes to the left cerebral hemisphere
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BINOCUALR VISUAL FIELD
- NASAL visual field of both eyes overlap
- This is seen by the temporal retina
-This means if you lose you left eye, you can still see the left nasal visual field using the right eyes nasal visual field because they overlap
- Whereas the temporal/peripheral visual field is only seen by the each eyes separate nasal retina and do not overlap
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How do we see an image?
- Optic chiasm -> Optic tract synapses with cells in the LATERAL GENICULATE NUCLEUS of each thalamus on both sides of the brain
- LATERAL GENICULATE NUCLEI: is a processing center for visual information that sharpens contrast and enhances depth perception.
- Neurons in the lateral geniculate nucleus send out nerve fibers that form optic radiations that travel to the PRIMARY VISUAL CORTEX in the brain which is located in the occipital lobe.
- The optic radiations include Meyer’s loop, which carries information from the inferior retina, and Baum’s loop, which carries information from the superior retina.
- The visual cortex is where the electrical signal is processed in terms of form, color, and motion which allows the brain to finally interpreted the signal as a recognizable image.
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OVERVIEW OF THE OPTIC PATHWAY?
Alright, as a quick recap of the optic pathway: light entering the eye from the nasal half of the visual field falls on the temporal retina and light from the temporal half of the visual field falls on the nasal retina.
The retina sends a signal via the optic nerve to the optic chiasm where axons from the nasal retina crossover and the axons from the temporal retina continue on the same side - forming the optic tract.
The optic tract then synapses at the lateral geniculate nucleus of the thalamus, which send out optic radiations to the primary visual cortex of the brain where the electrical signal is interpreted as an image.
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CORNEA
- outer layer is made up of stratified squamous epithelial cells they help to heal after a corneal injury or abrasion
- cornea has no blood vessels so no immune cells get to it - it is immune privileged
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PHYSIOLOGY OF THE EYE
- When light enters the eye, it hits a light sensitive neural layer of tissue near the back of the eye called the retina.
- That’s where phototransduction occurs, which is the process by which light waves are converted into electrical signals.
- Afterwards, these electrical signals are sent to the brain for visual processing.
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EXTRAOCULAR MUSCLES
- Extrinsic muscles of the eye are separate from the eyeball.
They control the movements of the:
- Eyeball
- Superior Eyelid
There are 7 extraocular muscles:
- the levator palpebrae superioris
- superior rectus
- inferior rectus
- medial rectus
- lateral rectus
- inferior oblique
- superior oblique
Functionally they can be divided into two:
Responsible for eye movement – Recti and oblique muscles.
Responsible for superior eyelid movement – Levator palpebrae superioris.
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Levator Palpebrae Superioris
- Its the only muscle involved with moving the superior eyelid
- A small portion of this muscle contains a collection of smooth muscle fibres – known as the superior tarsal muscle
Action: Elevates the upper eyelid.
Innervation:
-The levator palpebrae superioris is innervated by the oculomotor nerve (CN III).
-The superior tarsal muscle (located within the LPS) is innervated by the sympathetic nervous system.
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Recti muscles
The name recti is derived from the latin for ‘straight’ – this represents the fact that the recti muscles have a direct path from origin to attachment.
This is in contrast with the oblique eye muscles, which have an angular approach to the eyeball.
- All the muscles originate from the COMMON TENDINIOUS RING (This is a ring of fibrous tissue, which surrounds the optic canal at the back of the orbit)
Superior rectus
Action: Elevation, medial rotation and adduction
Innervated: Oculomotor nerve (CNIII)
Inferior rectus
Action: Depression, lateral rotation, adduction
Innervated: Oculomotor nerve (CN III)
Medial Rectus
Action: Adducts the eyeball
Innervated: Oculomotor nerve (CNIII)
Lateral rectus
Action: Abducts the eye
Innervated: Abducens nerve ( CN 6)
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Oblique Muscles
- Do not originate from the common tendinous ring
Superior Oblique
Actions: Depresses, abducts and medially rotates the eyeball
Innervation: Trochlea nerve (4)
Inferior Oblique:
Actions: Elevates, abducts and laterally rotates the eyeball.
Innervated: Oculomotor nerve (CN III)
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Oculomotor nerve palsy (weakness or problems with suing muscle)
The extraocular muscles are innervated by three cranial nerves:
- Oculomotor (3)
- Trochlear (4)
- Abducens (6)
a lesion of each cranial nerve has its own characteristic appearance
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HORNERS SYNDROME
Horner’s syndrome refers to a triad of symptoms produced by damage to the sympathetic trunk in the neck:
- Partial ptosis (drooping of the upper eyelid) – Due to denervation of the superior tarsal muscle.
- Miosis (pupillary constriction) – Due to denervation of the dilator pupillae muscle.
- Anhidrosis (absence of sweating) on the ipsilateral side of the face – Due to denervation of the sweat glands.
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OPEN ANGLE GLAUCOMA
- The aqueous humour supplies nutrients to the cornea. It is produced by the ciliary body. It flows through the posterior chamber and around the iris to the anterior chamber.
- It drains through the trabecular meshwork to the canal of Schlemm at the angle between the cornea and the iris. From the canal of Schlemm, it eventually enters the general circulation.
- Normal intraocular pressure is 10-21 mmHg, created by the resistance to flow through the trabecular meshwork.
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OPEN ANGLE GLAUCOMA
With open-angle glaucoma, there is a gradual increase in resistance to flow through the trabecular meshwork. The pressure slowly builds within the eye.
Risk factors for open-angle glaucoma include:
-Increasing age
-Family history
-Black ethnic origin
-Myopia (nearsightedness)
SYMPTOMS:
Glaucoma affects the peripheral vision first, resulting in a gradual onset of peripheral vision loss (tunnel vision). It can also cause:
Fluctuating pain
Headaches
Blurred vision
Halos around lights, particularly at night
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DIAGNOSING OPEN ANGLE GLAUCOMA
- Non-contact tonometry is the commonly used machine for estimating intraocular pressure by opticians. It involves shooting a “puff of air” at the cornea and measuring the corneal response. It is less accurate but gives an estimate for general screening purposes.
-Goldmann applanation tonometry is the gold-standard way to measure intraocular pressure. It involves a device mounted on a slip lamp that makes contact with the cornea and applies various pressures.
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MANAGEMENT
Treatment is typically started at an intraocular pressure of 24 mmHg or above.
- Beta-blockers (e.g., timolol) reduce the production of aqueous humour
- Carbonic anhydrase inhibitors (e.g., dorzolamide) reduce the production of aqueous humour
- Prostaglandin analogue eye drops (e.g., latanoprost) are the first-line medical treatment. They increase uveoscleral outflow. Notable side effects are eyelash growth, eyelid pigmentation and iris pigmentation (browning).
- 360° selective laser trabeculoplasty is recommended in the NICE guidelines (updated 2022) for all patients needing treatment. During the procedure, a laser is directed at the trabecular meshwork, improving drainage. It may delay or prevent the need for eye drops. A second procedure may be necessary at a later date.
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BITEMPORAL HEMAINOPIA
- Is lateral vision loss
- Caused by compression of optic chiasm
- Information from the temporal visual fields carried by the nasal retina cannot cross over
- Often by a pituitary tumor
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HEMANOPIA
- Individual loses half of visual field
- Hemianopia is a loss of vision or blindness in half the visual field, to either the right or left side, which can occur after stroke, brain injury, or a brain tumour.
- You get hemianopia after a stroke
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HOMONYMOUS HEMANOPSIA
- Vision loss in each eye (corresponding halves of the visual field)
- is a field loss deficit in the same halves of the visual field of each eye
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