The visual system Macleod’s 14th edition, chapter 8, pages 151-169

Rahaf Hasanein

Anatomy and physiology

Eye is situated the in the Bony orbit. Covered by: Eyelid (protection from injury / maintain tear film/ facial expression)

*Opening: elevated by: 1- levator palpebrae superiosis.(Cranial Nerve lll) 2-Müller’s muscle (smpathetic autonomic system) *Closure : mediated by orbicularis oculi(cranial nerve VII) The orbit houses the :

1- lacrimal gland 2-blood vessels 3-autonomic nerve fibers 4-cranial nerves (ll , lll , lV , Vl) The contents are cushioned by orbital FAT ( enclosed anteriorly by the orbital

septum and eyelids).

The conjunctiva: is a thin mucous membrane lining the posterior aspects of the eyelid, reflected at the superior and inferior fornices on the surface of the globe.

The conjunctiva is coated in a tear film that protects and nourishes the ocular surface.

• Tear film:

1. Oil from Meibomian gland (Eyelids)

2. Aqueous Humor from:

-Lacrimal gland (upper lateral region of each orbit)

-Accessory Lacrimal gland (Conjunctiva)

3. Mucin from goblet cells (Conjunctiva)

Function:

Carries nutrients and immune proteins to cornea,

wash away foreign bodies and express emotions.

• Tears drain through lacrimal canaliculus via the pucta at medial edge of each eyelid Lacrimal sac Nasolacrimal duct opening in inf meatus.

• The lacrimal ducts drain into the superior fornix.

Eye:

The eyeball length (25mm).

3 layers:

1. Outer fibrous layer: this includes the sclera and the

clear cornea. The cornea accounts for two-thirds of

the refractive power of the eye, focusing incident

light on to the retina.

2. Middle vascular layer (uveal tract): anteriorly this consists of the ciliary body and the iris, and posteriorly the choroid.

3. Inner neurosensory layer (retina): the retina is the structure responsible for converting light to neurological signals.

EXTRAOCLEAR MUSCLES :

• 6 extraoclear muscles: Responsible of eye movement - superior/medial/ inferior rectus muscle and inferior oblique muscle (Cranial nerve lll) -Superior oblique mucle (Cranial nerve lV) -Lateral rectus muscle (cranial nerve Vl)

Cranial nerves originate in the midbrain and pons and then pass through the cavernous sinus.

Refractive elements of the eye :

• refractive power: The degree to which a transparent body deflects a ray of light from a straight path.

• the major refractory elements:

1-Tear film 2-cornea 3-crystalline lens

• cornea posses The greatest refractive power and the main refracting element of the eye

• Lens (biconvex):provides additional controllable refraction, causing the light to focus on to the retina.

When light is :

1. Precisely focused on the retina … emmetropia

2. Focus point falls behind the retina .. Hypermetropia (long slightness) .. Eye is too short in infants, youngs can compensate by contracting ciliary muscle

3. Focus in front of the retina .. Myopia (short sightedness) .. Eye is too long start in childhood and worsen during growing age.

These errors (2 and 3 ) can be corrected with

1) Lenses.. For point num 2 use a convex (plus) lens & for point num 3 use a concave (minus) lense

2) Pinhole

-Presbyopia: impaired accomodation, >45 -Astigmatism: cornea irregularly curved

https://www.youtube.com/watch?v=VDehC_Txa1U

Visual pathway :

• Consists of: Retina, optic nerve, optic chiasm, optic tracts, lateral geniculate bodies, optic radiations and visual cortex

• Any deficits in this pathway leads to specific field defects.

Pupillary pathway

• Pupil Control the amount of light entering the eye

• Intensity of the light determines the aperture of the pupil Via autonomic reflexes :

1) Pupillary constriction : by parasympathetic nerves (travels with CN3).

2) Pupillary dilation : by sympathetic nerves.

• Bear in mind the anatomy of the eye and the visual pathway.

• allow the patient to describe the condition in their own words to Provide clues for more directed questions later.

History

COMMON PRESENTING SYMPTOMS: 1) Altered vision 2) Pain 3) Red eye 4) Double vision 5) Discharge 6) Swollen eye

Intraocular diseases:

1. change in the optical or refractive properties of the eye.

2. Prevent the incident light rays from being clearly focused on the retina.

Extraocular factors:

damage to the visual pathway , which runs from the optic nerve to the optical lobe.

1) Altered vision

vision may be not just reduced but also distorted,

this results from disruption to the normal structure of the macula , the central part of the retina .. The most common cause is macular degeneration. but also may frequently stem from an epiretinal membrane ,vitreous traction or central serous retinopathy What to ask about when you have a patient with altered vision :

1) Onset: sudden, gradually 2) How the vision is affected (loss/cloudy/floaters / distortion)? 3) One or both eyes that are affected ? 4) The whole or only part of the visual field affected ? 5) If partial . Which part of the visual field is affected

Blurry vision

Distorted Vision (Metamorphopsia)

• Haloes : are colored lights seen around bright light Occurs

with corneal edema and mostly associated with angle-closure glaucoma.

This results from disturbance of the vitreous and the retina , occurring most commonly in posterior vitreous detachment Usually found in older patient as the vitreous gradually degenerates and liquefies , causing it to peels from the retina. The vitreous is attached to retina in certain regions , in these regions the vitreous either : 1)detaches with traction … resulting in flashing light 2) Detaches by tearing the retina .. Releasing retinal pigment cells, Patient will see either of these as floaters

Ask: 1) when it began ? 2) Anything started the pain 3) Character of the pain 4) severity? 5) Exacerbation and relieving factors 6) Associated symptoms

*Pain on eye movement usually indicates either optic neuritis (white eye) or scleritis (red eye) • Cornea is one of the highly innervated regions of the body

When corneal nerves are activated it leads to 1)Pain 2)sensation of a foreign body 3)reflex watering 4) photophobia

2) Pain

The eye is covered with a network of vessels in the conjunctiva ,episclera and sclera . Ciliary vessels are also found around cornea causes leads to red eye :

1) Dilation or hemorrhage in these vessels 2) uveitis , in acute angle closure glaucoma and corneal irritation: the ciliary vessels around the cornea become more prominent ( ciliary flush) The appearance is distinct from conjunctivitis , in which there is blanching of vessels towards the cornea

• Ask:

1)If the eye is painful or photophobic? 2)Vision is affected? 3)Recent trauma? 4)Eye is itchy? 5)Any discharge? 6)Any recent contact lens wear or foreign body exposure? There are causes will be mentioned later

3) Red eye

Clinical pictures in different types of congestion:

Whether it is : 1)Monocular 2)Binocular There are causes will be mentioned later Ask: 1)Occurs in one or both eyes? 2)Character ? Whether Images seen side by side ? One above the other? Or at angle ? 3)Associated with trauma

• Test the eye movement and use your knowledge of the functions to of the

extraoclear muscles to work out which cranial nerve is affected in binocular diplopia

4) Double vision (diplopia)

Increase in discharge from the eye results from : 1)Increase in production 2) Decrease in drainage from the ocular surface Irritation of corneal nerves activates cranial nerve V1 and results in reflex tearing

response. Tears normally drain through the punctum at the medial end of the lower eyelid

into the nasolacrimal duct, which opens below the inferior turbinate into the nasal cavity. Blockage of tear drainage or abnormal lid position can also result in excessive discharge.

Ask: 1)Discharge clear? opaque? 2)Associated pain ?foreign body sensation? Itchiness? 3)Any noticed abnormalities ? Such as red eye ? There are causes will be mentioned later

5) Discharge

Orbit is an enclosed structure except anteriorly, so any swelling inside the orbit will results in : Anterior displacement of the globe (Proptosis) Ask 1. unilateral or bilateral 2. Acute ? gradual ? 3. Painful? 4. Itchiness? Irritation ? 5. Associated with double vision? There are causes will be mentioned later.

6) Swollen eyes

Past ocular history Ask

1. Any known opthalmic conditions.

2. Ask specificcaly about having amblyopia (reduction in vision in one eye from childhood)

3. If the patient wears glasses or contact lenses.

4. Last time they had their eyes checked for refractive correction.

5. Any previous eye operation that may affect vision.

Past medical history : Focus on systematic diseases that could affect the eyes ( directly / side effect of treatment ) :

1) diabetis or HTN .. Especially in the context of visual loss or double vision

2) Thyroid disease in the context of red, swollen eyes and double vision

Drug and allergy history The eyes may be affected by medications for other conditions .. Like glaucoma exacerbation by conjunctival absorption of nebulised anticholinergic drugs in COPD.

Medications for the eyes (like beta blocker eye drops) can aggrevate other conditions like asthma

Ask History of fever or allergy .. If the patient has itchy eyes

Family history Several eye diseases have an inherted predisposition

Ask:

1)glaucoma in first order relatives

2)Genetic diseases Includes retinitis pigmentosa

3)Thyroid eye disease

Social histoty: Visual impairment has a wide range of effects on daily life

Ask:

1) Daily activities require good vision: reading, Tv, sport, hobbies, …

2) Driving

3) Occupation .. Certain professions including drivers of heavy good vehicles / pilots require specific visual acuity criteria

4) Smoking and alcohol use: this may affect vascular and optic nerve function within the eyes

• General Examination

• Visual Acuity

• Orbital and Periorbital Exam

• Pupils

• Visual Field

• Opthalmoscopy

Physical examination

Carefully and systematically examine:

1) Posture and gait

2) Head position

3) Facial asymmetry and dysmorphic features

4) Eyelid position and periocular skin

5) Position and asymmetry of gaze (any squint/strabismus)

General examination:

Visual aquity:

Mandetory to assess in all patients.

Each eye should be assessed separately .. The most common method is using snellen chart (displays a random selection of letters at diminishing font size in successive lines )

Ask:

1) To wear their distance spectacles if they require them .. Near/ reading spectacles should be worn only when testing reading vesion

https://www.youtube.com/watch?v=jW7ADI7rLMc

Examination sequence • Use a backlit Snellen chart positioned at 6 metres and dim the room lighting.

• Cover one eye and ask the patient to read the chart from the top down until they cannot read any further. Repeat for the other eye.

• If the patient cannot see the largest font, reduce the test distance to 3 metres, then to 1 metre if necessary.

• If they still cannot see the largest font, document instead whether they can count fingers, see hand movement or just perceive the difference between light and dark.

• On the Snellen chart, lines of decreasing font size are numbered according to the distance in metres that a person with normal vision could read them. Express visual acuity as the distance at which text is read (usually 6 metres) over the number of the smallest font line read correctly on the chart. For example, 6/60 means that the patient sees at 6 metres the font size that is seen at 60 metres by a person with normal vision.

• If the patient cannot read down to line 6 (6/6), place a pinhole directly in front of the eye (with the patient keeping their usual spectacles on, if they wear them) to correct any residual refractive error

• if the visual acuity is not improved with a pinhole, this indicates the presence of eye disease not related to the refractive apparatus alone, such as retinal or optic nerve pathology.

• Note that 6/6 is regarded as normal vision; in the UK, 6/12 or better with both eyes is the requirement for driving.

• Assess near vision with a similar test using text of reducing font size held at a comfortable reading

distance. It is important to consider the need for reading spectacles in patients over the age of 40 years because of presbyopia (age-related deterioration in near vision).

Orbit and periorbital examination

• Observe the face and orbit for asymmetry and any obvious abnormality, including swelling, erythema or any other skin changes.

• Look for any abnormality in the position of the lids and ptosis

• Look for any asymmetry in the position of the eyeballs. Eyeball protrusion (proptosis) is best detected by looking down on the head from above.

• Palpate around the orbital rim and orbit, looking for any masses.

• Check eye movements .

• Use an ophthalmoscope to look for optic disc swelling from compression.

Pupils

Examine 1) inspect generally for squint and ptosis 2)Examine pupil shape and symmetry . - Physiological anisocoria (unequal pupil size ) is seen in 20% of

population .

Direct and consensual light reflex Examination sequence • With the patient fixating on a point in the distance and in ambient lighting, shine

a bright light from the temporal side into one eye and look for constriction of the ipsilateral pupil.

• • To test the consensual reflex, assess the pupil response in the contralateral pupil when light is directed towards the ipsilateral pupil. Repeat for the other pupil.

https://www.youtube.com/watch?v=IwBEjEbU-Yw

Relative afferent pupillary defect (RAPD)

is an important clinical sign that occurs when disease of the retina or optic nerve reduces the response of the eye to a light stimulus Testing for RAPD is an extention of the direct and consensual light

responses

Examination sequence • Use a bright light source • Move the light briskly from one eye to the other but place it on each eye

for a minimum of 3 seconds. • In normal patients, this results in symmetrical constriction of both

pupils. In RAPD, light in the affected eye causes weaker constriction (apparent dilatation) compared to light shone in the normal eye

Accommodation Examination sequence

• Ask the patient to look at a close fixation target (do not use a light source) after fixating on a distant target.

• There should be constriction of the pupil on near gaze.

• Failure to constrict to light but constriction on near gaze is referred to as light-near dissociation.

Anisocoria: The eyes should be assessed to determine which is the abnormal pupil. Examination sequence: • With the patient fixating at a point in the distance, increase and decrease the illumination and look for any change in the degree of anisocoria If the degree of anisocoria is greater in brighter lighting, then it is the larger pupil that is abnormal; if it is more pronounced in dim lighting, the smaller pupil is the abnormal one.

An equal degree of anisocoria in all levels of lighting indicates physiological anisocoria.

Visual fields • The normal visual field extends 160 degrees horizontally and 130 degrees vertically.

• Fixation is the very centre of the patient’s visual field. • The physiological blind spot is located 15 degrees temporal to the

point of visual fixation and represents the entry of the optic nerve head into the eye.

• The aim of the visual field examination is to test the patient’s visual

fields against your own (making the assumption that you have normal visual fields). The visual field can be tested using the fingers for gross examination. Finer examination can be performed using a small hatpin.

https://www.youtube.com/watch?v=IwBEjEbU-Yw

Check visual acuity and ensure that the patient has at least enough vision to count fingers.

Sit directly facing the patient, about 1 metre away. Ask the patient to keep looking straight at your face. Test each eye

separately. Ask the patient to close or cover one eye and look directly across to your opposite eye; you should also close your other eye.

Hold your hands out and bring an extended finger in from the periphery towards the centre of the visual field. For an accurate assessment of the patient’s fields, it is vital that the testing finger is always kept in the plane exactly halfway between yourself and the patient.

Wiggle your fingertip and ask the patient to point to it when they first see it

If the patient fails to notice your finger when it is clearly visible to you, their field is reduced in that area.

Test all four quadrants separately.

More subtle visual field defects can be elicited using a small white hatpin or a white Neurotip. With the patient looking directly at your eye, bring the white target in from the periphery to the centre (again always in the plane halfway between you and the patient). Ask the patient to say when they first see the target.

Undertake this for all four quadrants, testing each eye separately.

To assess very early visual field loss, repeat the same test using a red hatpin or a red Neurotip

It is important to show the patient the red target and ask them to report what colour they see. A dull or pale red suggests colour desaturation, which may indicate optic nerve dysfunction.

When testing each quadrant with a red target, be sure to explain to the patient that they should say when they first see that the target is red and not when they first see it. The target may be visualised before they appreciate the red colour.

To test the blind spot, place a red-tipped target equidistant between the patient and yourself at the visual fixation point.

Move the target temporally until it disappears.

Then move the target slowly up and down, as well as from side to side, until it reappears. This allows you to compare the patient’s blind spot with yours

Ocular alignment and eye movements

• The eyes are normally parallel in all positions of gaze except for convergence.

• Any misalignment is referred to as a squint (strabismus).

• Squints are described as manifest (tropia) if present with both eyes open, or latent (phoria) if revealed only by covering one eye.

• In addition, they can be concomitant (where the angle of squint remains the same in all positions of gaze) or incomitant (where the angle of squint deviation is greatest in a single position of gaze).

• The latter is commonly the result of paralysis of particular extraocular muscles

Detection of squint

Sit directly facing the patient, approximately 1 metre away and at a similar height.

Check visual acuity as part of the examination.

Look for any abnormal head posture such as head tilts (seen in cranial nerve IV palsy) or head turns (cranial nerve VI palsy).

Hold a pen torch directly in front of the patient and instruct them to look at the light. Observe the reflection of the light on the cornea in relation to the pupil. The reflections should be symmetrical between the two eyes.

Ask the patient if they see a single or double light.

If they see double, this may indicate the presence of a squint, but not seeing double does not exclude a squint. If the reflection is on the nasal aspect of the pupil in one eye, this suggests that the eye is deviated outwards and is described as an exotropia

https://www.youtube.com/watch?v=D_-Qwgk0kQ0

To confirm the presence of a squint, perform the cover/ uncover test: • Ask the patient to look at the pen torch at all times and then cover one eye. • Look at the uncovered eye for any movement. It may be helpful to repeat this

several times. • Inward movement of the uncovered eye suggests that it was positioned

abnormally outwards and is described as an exotropia (divergent manifest squint).

• Conversely, if the eye moves outwards when the contralateral eye is covered, this suggests that it was abnormally positioned inwards and is described as an esotropia (convergent manifest squint).

• Repeat the cover/uncover test for the other eye. • Failure of an eye to move despite an obvious corneal light reflex may indicate

that the eye has such poor vision that it cannot take up fixation or else it is restricted from moving.

• The alternating cover test involves covering the eyes alternately and quickly while the patient is fixated on the pen torch. Leave the cover on each eye for about 2 seconds but move between the eyes in less than 1 second. The movement is repeated multiple times. This test will help to elicit latent squint.

Ocular movements

• In the same seating position, ask the patient to look at a target or pen-torch light about 50 cm away.

• Ask them to say if and when they experience diplopia.

• Starting from the primary position, move the target in the six positions of gaze and up and down.

• If diplopia is present, ask whether this is horizontal, vertical or a combination of the two.

• Determine where the image separation is most pronounced.

• Look for nystagmus and determine whether the eye movement is smooth. Interpretation of any limitation of excursion is made by reference to the functions of the extraocular muscles

Oculocephalic (doll’s-eye) reflex

• This reflex is the ability of the eyes to remain fixated while the head is turned in the horizontal plan

• An impaired reflex indicates a brainstem abnormality.

Examination sequence With the patient supine, ask them to look at your face. Gently turn their head from side to side, noting the movement of the eyes.

Ophthalmoscopy

• The direct ophthalmoscope is a useful tool for assessing both the anterior and the posterior segments of the eye.

• Pharmacological pupil dilatation is essential, though the optic disc can be examined sufficiently without dilatation.

Examination sequence: • Ask the patient to sit upright and look at a distant target. • When using the direct ophthalmoscope to examine the patient’s right eye,

hold it in your right hand and use your right eye to examine. Hold it in your left hand and use your left eye to examine the patient’s left eye.

• Place your free hand on the patient’s forehead and brow, as this will steady the head and improve your proprioception when moving closer to the patient with the ophthalmoscope.

• Rotate the ophthalmoscope lens to +10. This will allow a magnified view of the anterior segment. You will be able to examine the eyelid margins, conjunctiva, cornea and iris. If epithelial defects are suspected, fluorescein can be administered and a cobalt blue filter used to reveal these.

• To examine the fundus, dial the lens back to 0.

• With your hand on the forehead and the brow, use the ophthalmoscope to see the red reflex (red light reflected off the retina) at a distance of about 10 cm. When the red reflex is in focus, look for opacities and determine whether they are static or mobile. Static opacities are usually due to cataract changes, while mobile opacities indicate vitreous opacities.

• Slowly move the ophthalmoscope closer to the patient almost to the point that your forehead touches your thumb, which is resting on the patient’s forehead and brow

• Turn the lens dial until the optic disc comes into focus; if it does not, focus on a blood vessel.

• The optic disc can usually be located easily; if not, follow a blood vessel centrally (in the direction opposite to its branches) to locate it.

• Examine the optic disc, paying particular attention to its shape, colour, edges and cup size.

• Follow each blood-vessel arcade and examine each of the retinal quadrants.

• To examine the macula, ask the patient to look directly at the light.

• The normal retina looks different in Asian and Caucasian patients.

• Swelling of the optic disc is a very important clinical sign.

• A variety of diseases that can damage the optic nerve cause an abnormally pale optic disc.