Flashcards in Ophthalmology - Refractive errors Deck (18):
What are refractive errors?
These arise from disorders of the size and shape of the eye. Correct refraction depends upon the distance between the cornea and the retina, and the curvatures of the lens and cornea.
What is myopia?
Patients who are myopic (short sighted) tend to have an eye with an axial length that is LONGER than normal. Light from distant objects is focused in front of the retina, which produces a blurred image. Myopes usually have reasonable unaided near vision. Optical correction with glasses or contact lenses requires a concave ("minus") lens. When someone wears a myopic spectacle correction their eyes and face appear reduced in size to an observer looking at them.
What are the complications of myopia??
Patients with moderate and high myopia have an increased risk of developing a retinal detachment and open angle glaucoma. Progressive pathological myopia is a slightly different form but is also associated with macular disease and blindness.
What is the accommodation reflex?
This is a reflex action of the eye that causes changes in vergance (simultaneous movement of both eyes), lens shape and pupil size in response to looking at objects at different distances. The change in the shape of the lens is controlled by the ciliary muscles inside the eye and the whole reflex is under the control of the parasympathetic nervous system.
What are the three components of the accomodation reflex?
1) Pupil accomodation
2) Lens accomodation
How does the lens change shape when we want to focus on a nearby object?
A nearby object will appear large in the field of vision and the eye receives light from wide angles. When moving focus from a distant to a near object, the eyes converge. The ciliary muscles contract making the lens more convex, shortening its focal length. The pupil constricts in order to prevent diverging light rays from hitting the periphery of the retina and resulting in a blurred image.
What is the pathway for the accomodation reflex?
Information from light in each retina is transmitted to the occipital lobe via the optic nerve and the optic radiation (after a synapse in the lateral geniculate nucleus of the posterior thalamus), where it is interpreted as vision. The peristriate area 19 interprets accomodation and sends signals via the Erdinger-Westphal nucleus and the 3rd cranial nerve nucleus to the ciliary muscles, the medial rectus muscle and (via the parasympathetic system) to the sphincter pupillae muscles.
Why does the pupil constrict during accomodation?
The pupil constricts to increase the depth of focus of the eye by blocking the light that is scattered by the periphery of the cornea. The lens increases its curvature to become more convex which increases its refractive power. The ciliary muscles are responsible for the lens accomodation response.
Outline how the eye changes when focusing on a nearby object
In order to focus on a nearby object, the ciliary muscles contract around the lens to decrease its size. The suspensory ligaments relax and the radial tension around the lens is increased. This causes the lens to become more spherical achieving a higher level of refractive power.
Outline how the eye changes when focusing on a distant object
When focusing on a distant object, the lens holds itself in a flattened shape due to traction from the suspensory ligaments. Ligaments pull the edges of the elastic lens capsule towards the surrounding ciliary body and by opposing the internal pressure within the lens, keeps it flattened. In this case the ciliary muscles relax and the diameter of the lens increases to increase the lens' size. Tension along the suspensory ligaments is increased to flatten the lens, decrease the curvature and increase the refractive power.
What causes myopia?
Very close work in the early decades (not just in school) may lead to changes in the synthesis of mRNA and the concentration of matrix metalloproteinase resulting in myopia
What is the natural history of myopia?
In normal growth, changes in eyeball and lens curvature compensate for the eye getting longer as it grows, but in myopic children this does not happen, so myopia worsens with age. Most children do not become myopic until the age of 6 years (a few are born myopic). Myopia will then usually continue to worsen until the late teens, when changes stop below 6 dioptres in most people. It is important for children with myopia to have their eyes regularly checked, as spectacle changes are to be expected.
What is pathological myopia?
Rarely (>3%), myopia progresses above 6 dioptres. This has serious consequences later in life because secondary degeneration of the vitreous and retina can lead to retinal detachment, choroidoretinal atrophy and macular bleeding.
How is myopia managed?
Spectacles, contact lenses or LASIK (laser eye surgery)
What is hypermetropia? What type of lens do they need for correction and what are these patients at risk of?
Patients who are hypermetropic (long sighted) tend to have an eye with an axial length that is shorter than normal. Light from distant objects is focused behind the retina, but changing the shape of the lens, known as accommodation, younger patients can bring the image into focus. As the power of accomodation declines with advancing age, the hypermetrope finds it increasingly difficult to focus and requires a convex ("plus") lens for optical correction.
When someone wears hypermetropic spectacle correction their eyes and face appear magnified to an observer looking at them. Patients with moderate and high hypermetropia have an increased risk of developing both acute and chronic angle closure glaucoma.
What is presbyopia?
When someone with a normal (emmetropic) eye needs to focus on a near object, the shape of the lens is altered to make it more convex and increasing its refractive power. The ability to accomodate starts to significantly decline in middle age. This is known as presbyopia and patients notice that they find it increasingly difficult to read. Patients with early presbyopia notice that they need to hold books and newspapers further away from their face in order to read clearly. The use of reading glasses with a convex ("plus") lens can compensate for the reduction in accomodation. Alternatively, the patient may have bifocal lenses fitted to their glasses where the INFERIOR part of the lens corrects the presbyopia.
What is an astigmatism?
In a normal eye the cornea is the major refractive element involved in producing a focused image. The surface of the cornea should approximate to part of a sphere producing a clear retinal image. When an eye has an astigmatic error the surface of the cornea is not spherical, but rather part of a rugby ball shape where one meridian has a greater refractive power than the other, thereby resulting in the production of a blurred retinal image.