Anatomy - EYE

Treatments

The Cornea is the transparent front part of the eye which covers the iris (the colored part of the eye), pupil (the opening at the center of the eye), and anterior chamber (the fluid-filled inside of the eye). Collectively with the lens, the cornea refracts light, accounting roughly around 2/3rd of the total optical power of eye. In humans, the refractive power of the cornea was around 43 dioptres. The corneas core and major function is to refract, or bend, light. The cornea is mostly responsible for focusing most of the light which enters the eye.

Corneal Transplant Surgery
The cornea was composed of proteins & cells. It does not consist of any blood vessels, unlike most of the tissues in the human body. Blood vessels cloud the cornea will prevent it from the refracting light properly & adversely, affect the vision.

Because there were no nutrient-supplying blood vessels in the cornea, tears & the aqueous humor (a watery fluid) in the anterior chamber will provide the cornea with the nutrients.

Corneal Layers
There are five major layers in cornea they are:

  • Corneal epithelium
  • Bowmans layer
  • Corneal stroma
  • Descemets membrane
  • Corneal endothelium


The outer layer, is the epithelium, it is a layer of the cells which covers the cornea. It will absorb the nutrients & oxygen from tears & convey it to the rest of the cornea. It also contain free nerve endings and also prevents foreign matter entering the eye.

Various refractive eye surgery procedures change the shape of the cornea to reduce the need for corrective lenses or otherwise improvement of the refractive state of eye.

The cornea tends to repair by itself rapidly from minor abrasions. Deeper abrasions cause scars to form on the cornea, which causes the cornea to lose its transparency , thereby it leads to visual impairment.

 

Lens replacement surgery is normally used to refer to one of three similar types of treatments: Refractive Lens Exchange (RLE), Implantable Contact Lenses (ICL) and Cataract Surgery with each having alternative names and acronyms so it can be a little confusing!  Hopefully this article will clear up any confusion and give some clarity on the type of lens replacement surgery that may be suitable for you as well as the costs involved.



Types Of Lens Replacement Surgery

1. Refractive Lens Exchange (RLE)

In general when people talk about lens replacement surgery they are referring to some type of Refractive Lens Exchange (RLE) which is also known as Refractive Lens Replacement, Clear Lens Exchange (CLE) or Clear Lens Surgery. This is a treatment for patients suffering from presbyopia (long-sightedness normally occurring in middle and old age), hyperopia (farsightedness where objects nearby are not seen as clearly as objects in the distance) or those with a considerably thin cornea.

It is commonly used for patients over 40 who don’t qualify for either LASIK or PRK laser eye surgery but are not willing to continue using glasses or use contact lenses. It can also can correct myopia (nearsightedness) but it is not normally recommended when LASIK surgery or Phakic intraocular lens (IOL) can be used.

The procedure involves removing the eye’s natural lens and replacing it with an artificial intraocular lens. As well as removing the need to continue wearing glasses or use contact lenses, the new artificial intraocular lens will also mean that the patient will not suffer from cataracts in the future as a cataract can not form on an artificial lens.

2. Cataract Surgery

Cataract surgery involves the same procedure as refractive lens surgery described above except that the lens that is removed is not clear but cloudy due to existence of cataracts. Patients have the same choice of monofocal, multifocal or trifocal lenses (see lens types below) giving them the option to also remove the need for reading glasses as well as correct their cataracts.

3. Implantable Contact Lenses (ICL)

This type of lens surgery involves implanting contact lenses rather than removing and replacing the natural lens which occurs in lens replacement surgery.  This new phakic intraocular lens is placed on top of the natural lens and behind the iris (the exact positioning will depend on the lens chosen). As the natural lens is not removed this procedure can be reversed at a later date.


The pupil is the black center part of the eye. Pupils get larger (dilate) in dim light and smaller (constrict) in bright light. Usually both pupils are about the same size and respond to light equally. Unequal pupil size is called anisocoria.
 

If pupil sizes are very unequal, a person may notice the discrepancy. More often, unequal pupils are noticed only during a doctor's examination. Unequal pupils themselves usually cause no symptoms, but occasionally a person may have trouble focusing on near objects. Also, the underlying disorder sometimes causes other symptoms such as eye pain and redness, loss of vision, drooping eyelid, double vision, or headache. These more noticeable symptoms are often the reason people seek medical care rather than the unequal pupils.




 

Causes

The most common cause of unequal pupils is

  • Physiologic anisocoria

Physiologic anisocoria is pupils that are naturally different in size. No disorder is present. About 20% of people have this lifelong condition, which is considered a normal variation. In such people, both pupils react normally to light and darkness and there are no symptoms.

Less commonly, people have unequal pupils because of

  • Eye disorders

  • Nervous system disorders

Either the larger or the smaller pupil may be the abnormal one depending on the cause. Often, the larger pupil is unable to constrict normally. However, sometimes, as in Horner syndrome, the smaller pupil is unable to widen. If the larger pupil is abnormal, the difference between pupil sizes is greater in bright light. If the smaller pupil is abnormal, the difference is greater in the dark.

Eye disorders that cause unequal pupils include birth defects and eye injury. Also, certain drugs that get into the eye may affect the pupil. Such drugs may be drops intended to treat eye disorders (for example, homatropine used for certain inflammatory disorders or injuries or pilocarpine used for glaucoma), or they may be drugs or other substances that accidentally get into the eye (for example, scopolamine used as a patch for motion sickness, plants such as jimsonweed, or certain insecticides). Inflammation of the iris (iritis) and certain types of glaucoma cause unequal pupils, but this finding is usually overshadowed by severe eye pain.

Nervous system disorders that cause unequal pupils are those that affect the 3rd cranial nerve or certain parts of the sympathetic or parasympathetic nervous system (the autonomic nervous system). These pathways carry nerve impulses to the pupil and to the muscles that control the eye and eyelid. Thus, people with nervous system disorders that affect the pupil often also have a drooping eyelid, double vision, and/or visibly misaligned eyes. Brain disorders that can affect these pathways include strokes, brain hemorrhage (spontaneous or due to head injury), and, less commonly, certain tumors or infections. Disorders outside the brain that affect the sympathetic nervous system include tumors and injuries that involve the neck or upper part of the chest. Horner syndrome refers to the combination of a constricted pupil, drooping eyelid, and loss of sweating around the affected eye. Horner syndrome is caused by interruption of the sympathetic nerves to an eye due from any cause.

Evaluation

Doctors' first goal is to determine whether the pupils have always been unequal or whether there is another cause such as a drug or disorder. Then the goal is to decide whether the larger or the smaller pupil represents the problem. The following information can help people decide when a doctor's evaluation is needed and help them know what to expect during the evaluation.

Warning signs

In people with unequal pupils, certain symptoms and characteristics are cause for concern. They include

  • Drooping eyelid (ptosis)

  • Double vision

  • Loss of vision

  • Headache or neck pain

  • Eye pain

  • Recent head or eye injury

 

The term "immune reconstitution inflammatory syndrome" (IRIS) describes a collection of inflammatory disorders associated with paradoxical worsening of preexisting infectious processes following the initiation of antiretroviral therapy (ART) in HIV-infected individuals . Preexisting infections in individuals with IRIS may have been previously diagnosed and treated or they may be subclinical and unmasked by the host's regained capacity to mount an inflammatory response .

If immune function improves rapidly following the commencement of ART, systemic or local inflammatory reactions may occur at the site or sites of the preexisting infection. This inflammatory reaction is usually self-limited, especially if the preexisting infection is effectively treated. However, long-term sequelae and fatal outcomes may rarely occur, particularly when neurologic structures are involved.

The vitreous chamber

In the most basic of anatomical terms, the eye is divided into three sections: the anterior chamber, the vitreous chamber and the posterior chamber. The vitreous chamber is positioned at the back of the eyeball. It is the largest of the chambers and takes up around 80% of the eye.

What is the vitreous humour?

The vitreous humour (also known simply as the vitreous) is a clear, colourless fluid that fills the space between the lens and the retina of your eye. 99% of it consists of water and the rest is a mixture of collagen, proteins, salts and sugars. Despite the water-to-collagen ratio, the vitreous has a firm jelly-like consistency.

What does the vitreous humour do?

The vitreous performs a vital role in protecting your eye. Most importantly, it helps it to hold its ‘spherical’ shape. The vitreous also comes in contact with the retina (the light-sensitive tissue at the back of the eye that acts like the film of a camera). The pressure of the vitreous humour helps to keep the retina in place.

What happens with the vitreous over time?

As you age, the vitreous liquefies and shrinks, much like a bowl of gelatin shrinks with time. Portions of the mixture of collagen and proteins also become ‘stringy’, like the white of an egg does as it ages. These stringy entities float around the rest of the vitreous, casting a shadow on the retina. These are called ‘floaters’ – you may notice them as specks, strings or other shapes that you see just out of the corner of your eye.

Vitreous detachment

The vitreous itself does not adhere to the retina. However, on the surface of your retina there are literally millions of fine fibres. Because the vitreous shrinks as you age, these fibres can ‘tug’ on the retinal surface. If this shrinkage causes a section of the fibres to pull away all at once, it is called a vitreous detachment. These are common in people over the age of 65 and extremely common in people over 85. A vitreous detachment is more likely to happen if you have suffered from myopia (short-sightedness), have had an eye injury or if you have experienced eye inflammation (uveitis). In itself, a vitreous detachment should cause no problems with your vision other than an increase in floaters. While this can be annoying, you should find that they don’t affect your day-to-day life. The only risk factor you would face is the slightly increased chance of a retinal tear or the more serious, retinal detachment.

The retina is a thin layer of the tissue which lines the back of the eye on the inside. It is situated near the optic nerve. The purpose of the retina is to receive the light which the lens has been focused, converts the light into neural signals, & also send these signals on to the brain for the visual recognition. The retina is a light-sensitive layer positioned at the back of the eye which covers nearly 65 % of its interior surface.

The only neurons which are directly sensitive to the light are the photoreceptor cells. Photosensitive cells known as rods & cones in the retina converts incident light energy into the signals which are passed to the brain by optic nerve. Rods function mostly in dim light & also provide black-and-white vision, while cones support daytime vision & the perception of the color. In the middle of the retina there is a small dimple known as the fovea or fovea centralis. It is the center of the eyes sharpest vision & the location of the most color perception. A third, much rarer type of the photoreceptor, the intrinsically photosensitive ganglion cell, is significant for reflexive responses to bright daylight. The retina is a layered structure with many layers of neurons which are interconnected by the synapses.

The retina, processes light by a layer of photoreceptor cells. These were essentially light-sensitive cells, which are responsible for the detecting qualities like as color & light-intensity. The retina processes the information collected by the photoreceptor cells & also sends this information to brain from the optic nerve. Generally, the retina will process a picture from the focused light, & the brain is left to decide what the picture is. Because of the retinas vital role in the vision, damage to it may cause permanent blindness. Conditions like retinal detachment, where the retina abnormally detach from its original location, and obstructs the retina from receiving or processing the light. This prevents the brain from receiving the information, therefore leading to the blindness.
The optic nerve is located in the back of the eye. It is also known as the second cranial nerve or cranial nerve II. It is the second of many pairs of the cranial nerves. The main role of the optic nerve is to transfer the visual information from the retina to vision centers of the brain through electrical impulses. The optic nerve is made of nerve cells or ganglionic cells. It consists of over 1 million nerve fibers. Blind spot is absent of photosensitive (light-sensitive) , or photoreceptor cells.

The optic nerve will transmit all the visual information that includes brightness perception, color perception & contrast (visual acuity). It also conducts the visual impulses which are responsible for 2 important and main neurological reflexes. The light reflex & The accommodation reflex. The light reflex refers to the constriction of both the pupils which occurs when the light is shone into either eye. The accommodation reflex refers to the swelling of lens of the eye which occurs when a person looks at a near object while reading (lens adjusts to near vision). Based on this anatomy, the optic nerve can be divided into 4 parts:

» The optic head (where it begins in the eyeball (globe) along with the fibers from the retina)
» Orbital part (which is the part of the orbit).
» Intrancanalicular part (which is the part within a bony canal called the optic canal)
» Intracranial part (part within the cranial cavity, which ends at the optic chiasma)

Glaucoma is one of the major disease which affects the optic nerve. Glaucoma is caused by a high intraocular pressure, or high pressure in the fluid which is inside the eye (vitreous fluid). This high pressure will compress the optic nerve & cause the cells to die. It is referred as the atrophy of the optic nerve. Even though the optic nerve is a part of the eye, it is considered as the part of central nervous system.
Sclera is a part of the eye & generally known as the "white" It will form a supporting wall of the eyeball, & is continuous with the clear cornea. In humans, the entire sclera is white contrast with coloured iris, but in the other mammals the visible part of the sclera will equivalent to the colour of the iris, by that white part wont generally seen. In the embryo development, the sclera is derived from the neural crest. In children, it is thinner & shows some of the underlying pigment, which will appear slightly blue. In elder persons, the fatty deposits on the sclera will make it appear as slightly yellow.

The sclera is covered by conjunctiva, a clear mucus membrane which help to lubricate the eye. It is thickest in the area around the optic nerve.



The sclera is made up of three divisions:
  • Episclera - Loose connective tissue, right away beneath the conjunctiva.
  • Sclera proper - The dense white tissue which gives the area its color
  • Lamina fusca - The innermost zone made of elastic fibers.


There are a number of abnormalities which are related with the sclera. Some are genetically related abnormalities that may includes:
  • Melanosis - Excess deposits of the melanin pigment on the surface of sclera, which may become inflamed & uncomfortable.
  • Scleral Coloboma - Missing tissue which will results in the notching & bulging of the sclera (lesions)
  • Ectasia - A thinning & bulging of the sclera

Acquired abnormalities of sclera which may includes:
  • Ectasia - That may be brought on as a side effect from traumas or inflammations
  • Episcleritis - A hypersensitivity reaction that will be anterior or posterior, which was characterized by engorged blood vessels, & may also affect the cornea.

 

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