Physiology Practical 2: Vision

Question 1

What are the 5 sections of this practical?

Answer 1

The purpose of this session is to introduce you to a range of tests and explain their physiological basis. This activity is split into 5 sections as follows: 
The ophthalmoscope and the Fundus Oculi
Eye reflexes
Visual fields
Convergence and accommodation
Visual acuity

Question 2

What is the fundus of the eye and what does it include?

Answer 2

The fundus of the eye is the interior surface of the eye, opposite the lens, and includes the retina, optic disc, macula (which includes the fovea, the region of the retina with the greatest visual acuity, at its center).

Question 3

What is the clinical relevance of the fundus?

Answer 3

Observation of eye fundus (generally by fundoscopy) is used to detect abnormalities for example exudates, and blood vessel abnormalities. The eye’s fundus is the only part of the human body where the microcirculation can be observed directly.

Question 4

How does you use an ophthalmoscope to view the interior/back of the eyeball?

Answer 4

There is a very clear technique to using an ophthalmoscope for fundoscopy - as outlined in these videos.

The top video summarise everything you need to know about fundoscopy in 5 mins and is a short practical guide.

www.youtube.com/watch?v=AzxNGz1cjgI

The bottom video is much more detailed and provides a more in depth analysis of the subject.

https://www.youtube.com/watch?v=7lhvhKvK_iM

Question 5

For a fundoscopy examination which light setting on the opthalmoscope is used?

Answer 5

Large plain disc

Question 6

What is the response of a red circle in the eye when using an ophthalmoscope called?

Answer 6

Red reflex

Question 7

What is the corneal/blink reflex?

Answer 7

The corneal is one of the blink reflexes, is an involuntary blinking of the eyelids elicited by stimulation of the cornea. Stimulation should elicit both a direct and indirect or consensual response (opposite eye).

Procedure:
With a wisp of clean cotton wool, touch the conjunctiva and then the cornea of the subject, approaching from outside the field of vision. Note the results.
More details can be found at: http://teachneuro.blogspot.co.uk/2013/01/the-corneal-or-blink-reflex.html

Question 8

What is the neuroanatomical pathway for the corneal/blink reflex?

Answer 8

Information goes in (input) on cranial nerve V (ophthalmic) and comes out (output) on cranial nerve VII (facial).

Question 9

What is the clinical relevance of the corneal/blink reflex?

Answer 9

An absent corneal reflex can be due to sensory loss in Vi (e.g. neuropathy or ganglionpathy), weakness or paralysis of the facial muscles (myopathy) or facial nerve (facial palsy, for example Bell’s palsy) or brain stem disease.

Question 10

What is the pupillary light reflex?

Answer 10

The pupillary light reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye. The reflex is a response in adaptation to various levels of darkness and light, in addition to retinal sensitivity. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.

Question 11

What is the procedure to check the pupillary light reflex?

Answer 11

First, record the pupil size and shape at rest. Next, note the direct response, meaning constriction of the illuminated pupil, as well as the consensual response, meaning constriction of the opposite pupil when illuminated by torch/flashlight. Check both eyes.
Abnormal = reflex is absent or particularly sluggish

Question 12

What is the neuroanatomical pathway of the pupillary light reflex?

Answer 12

Information goes in on cranial nerve II (optic) and comes out on cranial nerve III (oculomotor)
This is a consensual (both eyes involved) parasympathetic reflex.

Question 13

What is the clinical relevance of the pupillary light reflex?

Answer 13

If only the eye illuminated constricts then there is damage to the crossing fibres i.e. damage in the midbrain.

Question 14

When testing the reflexes, what is it important to observe?

Answer 14

When testing the reflexes, it is important to observe both the direct response of the stimulated eye as well as the indirect/consensual in the opposite (unstimulated) eye.

Question 15

In the neuroanatomical pathway for the corneal/blink reflex, information goes in and out on which cranial nerves?

Answer 15

In the neuroanatomical pathway for the corneal/blink reflex: information goes in on cranial nerve V and comes out on cranial nerve VII

Question 16

In the neuroanatomical pathway for the pupillary light reflex, information goes in and out on which cranial nerves?

Answer 16

In the neuroanatomical pathway of the pupillary light reflex: Information goes in on cranial nerve II and comes out on cranial nerve III

Question 17

What is a confrontation visual field test? How is it carried out?

Answer 17

A confrontation visual field test is a quick and easy way to measure the overall field of vision.

Procedure:
The patient occludes the vision in one eye by placing a hand over the eye. The “confronter” sits or stands 2-3 feet directly in front of the patient who gazes straight ahead at a fixed point such as the “doctor’s” nose. It is essential to keep looking at this fixation point. The “doctor” then slowly moves a finger from the outer edge of your visual field toward the centre and from the centre toward the edge through all areas of your visual field. This done from the NE, NW, SE and SW directions (reference points on a compass). The patient tells the doctor when they first see the finger. You should always keep your eye focused on the health professional’s nose, not on the hand or finger. The test is then repeated for the other eye.

Question 18

What is the clinical relevance of the confrontation visual field test?

Answer 18

Visual field tests assess the potential presence of blind spots (scotomas), which could indicate eye diseases. A blind spot in the field of vision can be linked to a variety of specific eye diseases, depending on the size and shape of the scotoma.

Many eye and brain disorders can cause peripheral vision loss and visual field abnormalities. For example, optic nerve damage caused by glaucoma creates a very specific visual field defect.

Other eye problems associated with blind spots and other visual field defects include optic nerve damage (optic neuropathy) from disease or damage to the light-sensitive inner lining of the eye (retina).

Brain abnormalities such as those caused by strokes or tumors can affect the visual field. In fact, the location of the stroke or tumor in the brain can frequently be determined by the size, shape and site of the visual field defect.

Question 19

Why is there a physiological blind spot?

Answer 19

About 4mm from the fovea towards the nose lies the
optic disc
- this is where the optic nerve fibes and retinal blood vessels passes through the retina. There are no
photoreceptors
here.

Question 20

What is the name for a blind spot in any part of the visual field?

Answer 20

Scotoma

Question 21

What is the term for a group of eye diseases characterized by damage to the optic nerve usually due to excessively high intraocular pressure (IOP)?

Answer 21

Glaucoma

Question 22

What is convergence?

Answer 22

Convergence is the ability to bend the angle of the eyes inwards so that both can simultaneously fixate a near object.

Question 23

What is accommodation?

Answer 23

Accommodation is the ability to alter the focus of the eye so that a near object is in focus.

Question 24

How are convergence and accomodation linked?

Answer 24

These are normally linked and ideally (and usually) when a person focuses on a near object, the degree of accommodation needed causes their eyes to converge just the right amount (as shown in the video). The effort of accommodation is measured in diopters, and the effort of convergence is measured in degrees.

Question 25

What is presbyopia?

Answer 25

Presbyopia is a condition associated with aging in which the eye exhibits a progressively diminished ability to focus on near objects. Presbyopia’s exact mechanisms are not fully understood; research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens’ curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause.

Question 26

How is light focused for close objects?

Answer 26

For close objects, light rays are diverging as they enter the eye and greater refraction, required to bring them to focus at the fovea, is achieved by the accommodation reflex.

Question 27

What is the condition associated with aging in which the eye exhibits a progressively diminished ability to focus on near objects?

Answer 27

Presbyopia

Question 28

What is visual acuity and how is it measured?

Answer 28

Visual acuity can be defined as a measure of the spatial resolution of the visual processing system. Visual acuity is measured by reading from an eye chart - often known as a Snellen test. Watch the video on this page for an explanation.

The letters of the familiar eye chart (Snellen Chart) are constructed with limbs 1 minute wide and 5 minutes in overall height when viewed from the specified distance. With normal vision people read from 6 meters the letters they ought to see from 6 meters and have 6/6 vision. In some countries they still measure in feet hence “20/20 vision”. Some people only read from 6 meters, letters they ought to read from 12. They have 6/12 vision. Some people cannot read from 6 meters the top of the letter constructed for 60 meters i.e. they are less than 6/60. However, if they walk towards the chart to e.g. 4 meters they can read it, so they have 4/60 vision.

Question 29

What is myopia and why does it occur?

Answer 29

Myopia is the term used to define short sightedness. Light from a distant object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens is too strong.

A myopic person has clear vision when looking at objects close to them, but distant objects will appear blurred.

Question 30

What is hypermetropia and why does it occur?

Answer 30

Hypermetropia means long sight and is where the image of a nearby object is formed behind the retina. This could be because the eye is too short, or the cornea or crystalline lens is too weak.

A hypermetropic person may have blurred vision when looking at objects close to them, and clearer vision when looking at objects in the distance.

Question 31

How is myopia corrected?

Answer 31

Myopia is easily corrected using prescription glasses or contact lenses specifically designed to counteract the effect. A concave lens (minus powered) is placed in front of a myopic eye, moving the image back to the retina and clarifying the image.

Question 32

How is hypermetropia corrected?

Answer 32

Hypermetropia is corrected by placing a convex (plus powered) lens in front of a hypermetropic eye, the image is moved forward and focuses correctly on the retina.

Question 33

How do we define the strength of a lens?

What is the refractory power of the human eye?

Answer 33

The refraction power of a lens is in dioptres.
f = focal length (m)
Refractory power = 1/f dioptre

60 Dioptre