Blindness

The eyeball is the organ of sight. It is made up of several structures (layers) that work together to ensure the person sees properly. For a person to see, light has to pass through these structures (mainly the cornea, pupil and lens) to focus on the retina. The retina is a layer of special nerves that recognises light signals and then transmits them to the brain. To maintain the structure of the eye, it is filled with a fluid known as humour that prevents the collapse of the eyeball by maintaining constant pressure. Any abnormality in any of these structures would result in a compromise of vision.

Blindness is defined as the inability to see anything, including light. However, in present times it is a commonly used term to signify visual impairment that affects activities of daily life even after corrective measures. Blindness can be congenital (present since birth) or acquired later in life. Blindness can be classified in many ways such as:

• Unilateral/Bilateral: either one eye is affected or both eyes are affected
• Temporary or permanent blindness
• Based on the type of blindness: this includes
  • Night blindness: the individual is unable to see in the dark or at night
  • Colour blindness: the individual is not able to perceive various shades of colour, most commonly green and red

If a person is completely blind they see nothing. However, in partial blindness the following symptoms may be experienced:

• Cloudy or foggy vision
• Poor night vision
• Selective blindness to colours (colour blindness)
• Inability to see or determine shapes or see only shadows of objects
• Tunnel vision

In an infant, eye and vision development occur till the age of 2 years. Since the infants aren't vocal about any visual impairment symptoms, these are often unnoticed by the caregivers. However, certain habits or complaints in children can indicate a visual problem. These include:

• Eye pain
• Redness in the eyes
• Watering of the eyes
• Constant eye rubbing
• Extreme sensitivity to light
• Squinty eyes, i.e. abnormal gaze
• Poor visual tracking or the child keeps bumping into objects or tripping because they can't see clearly

There are many causes of blindness such as:

• Congenital causes: these are present since birth. This could be due to structural issues. Examples include congenital glaucoma and retinopathy of prematurity.
• Developmental causes: in such cases, the child has normal vision at birth but may develop visual impairment as the child grows due to various causes such as crossed eyes (squint), lazy eye (amblyopia)
• Genetic causes: the most common example is colour blindness
• Acquired causes: these can be further divided into:
  • Refractive errors: due to structural defects in the cornea and lens there can be gross vision loss
  • Cataracts: opacity of the lens of the eyes is a very common cause of blindness in the elderly.
  • Infections: infections of various structures of the eyes can spread to the entire eye and can cause blindness
  • Inflammation: Inflammation of the eye due to autoimmune conditions such as uveitis can cause blindness
  • Trauma: traumatic injuries or any foreign body injury can lead to loss of vision
  • Neurological causes: any infection or degeneration in the neurological pathway to vision can lead to blindness as seen in the case of optic neuritis, retinal detachment, macular degeneration
  • Tumour: any abnormal growth of the eye tissue can cause blindness. Retinoblastoma occurring in children is one such example
  • Nutritional causes: deficiency of vitamin A is a leading cause of night blindness in children
  • Systemic causes: such as hypertension and diabetes can lead to abnormal growth of blood vessels in the retina and can cause vision loss
  • Vascular injury or blockage to the blood supply to the eye, as in the case of stroke or haemorrhage will lead to vision loss.

Blindness is prevented by early detection. Hence, regular eye examinations are necessary. If diagnosed with conditions that may lead to blindness, such as glaucoma, medications are prescribed which keep the condition under control. Adequate nutrition can prevent night blindness. Using protective eye equipment in harsh work environments reduces the risk of traumatic injury to the eye.

However, it is to be noted that genetic conditions which induce blindness or tumours of the eyes cannot be prevented and, hence, in such cases, blindness is an eventuality.

Blindness is evaluated by an ophthalmologist (eye specialist). The doctor takes a detailed history that includes asking about symptoms related to vision. They include onset, duration of symptoms, progression of symptoms, whether they are aggravated at night, whether specific colours are not seen clearly, and whether the symptoms are through one eye or both. Associated medical conditions such as endocrine disorders (diabetes, thyroid disorders), hypertension, etc., if present, are also noted.

After a general physical examination, a detailed ocular examination is done. This involves observing each eye under an instrument called an ophthalmoscope. The examination helps identify external structural abnormalities (for example, the opacity of the eye lens, aka cataract). The internal structures of the eye are also visualised by increasing the size of the pupil. This is done by administering medicated drops that temporarily paralyse the pupil and increase its size so that the doctor can see the inside contents of the eyeball for vascular abnormalities. This procedure is known as indirect ophthalmoscopy.

Certain investigations are also done to help narrow the diagnosis. These include:

• Blood tests: routine complete blood counts (CBC), thyroid hormone levels, inflammatory markers or C-reactive protein (CRP), Erythrocyte sedimentation rate (ESR), and blood sugar levels
• Eye tests: several eye tests are conducted to determine the cause and degree of blindness. Some of the tests are
  • Snellen’s eye test: this test is used to determine the degree of vision loss
  • Slit-lamp examination: this identifies any structural aberrations of the eye
  • Intraocular pressure measurement: it measures the pressure of the liquid in the eyeball. Excessive pressure (as in the case of glaucoma) can result in blindness

After the history, examination and investigations, the doctor can reach a definitive diagnosis of the cause of blindness.

In children, the child doctor (paediatrician) would first diagnose any visual problems. During regular health check-ups, the doctor would notice and assess the child’s ability to focus and follow light or colourful objects. Usually, this is achieved by two to three months of age. However, if the child is still not able to focus or respond to light then the child is referred to an ophthalmologist for further evaluation.

The treatment for blindness depends on the cause and how early the disease has been detected.

• The majority of the causes of poor vision are due to refractive errors of the eye. This can be corrected by prescribing glasses or contact lenses. A more permanent solution would involve surgical techniques that alter the curvature of the cornea.
• Night blindness is due to a deficiency of Vitamin A. When detected in the early stages, treatment with vitamin A is able to reverse the condition completely.
• Cataracts are a major cause of blindness in the elderly population. It can be age-induced or associated with other conditions such as diabetes. Surgical removal of the cataract and implanting of a lens restores normal vision.
• Inflammatory and infectious conditions of the eye can be treated with steroid and antibiotic drops, respectively, which restore vision by treating the cause.
• In cases of retinal damage, certain laser procedures are helpful to regain vision. However, it is to be noted that complete recovery of normal vision is not possible in such conditions.

However, some causes of blindness are permanent and cannot be cured. These individuals are termed as legally blind and are issued a certificate of disability. These individuals will need to undergo rehabilitation and learn new skills to perform daily activities. Some of the measures involve learning Braille and using a guide dog.

The prognosis depends on the cause of blindness. Treatable and temporary causes of blindness have an excellent prognosis. However, in conditions that cannot be treated or have progressed to such a stage where recovery is not possible and blindness is permanent, rehabilitative measures are necessary to help the person lead a near-normal life. Often, people with unilateral blindness can still function normally since the normal eye compensates for the vision loss. This makes it even more important to keep the functioning eye healthy and, hence, regular eye check-ups are important.

Blindness is a very common medical ailment worldwide. The issue is identifying the cause in its initial stages and treating it before irreversible blindness occurs. The majority of the causes are treatable and reversible. Regular eye visits ensure early diagnosis and thereby quicker treatment and better management. In people who are permanently blind, rehabilitation is given so that their quality of life is not compromised.