How the Eye Muscles Work 

The eye muscles work in pairs and movement of the eye muscle is often only a very small movement, a fraction of a degree. The muscles need to be coordinated for vision to be stereoscopic (three-dimensional vision). For example, if you look to the left, the lateral rectus muscle on the left side of your left eye contracts. At the same time, on the right side of the same eye, the medial rectus relaxes.  

Which Muscles Control Which Movements 

• Lateral rectus and medial rectus control left and right movements  

• Superior rectus and inferior rectus control up and down and medial movements  

• Superior oblique and inferior oblique control up and down and outward movements 

The four rectus muscles are attached at one end to a fibrous ring that encircles the optic nerve, and the other end of these muscles attaches to the sclera, midline, or widest part of the eyeball. The superior oblique muscle attaches at one end to the optic foreman (the passage through the orbit of the eye), through the trochlea; a pulley-like 23 structure attached to the frontal bone (the upper part of each orbit) and the other end to the top part of the eyeball. 

These muscles form a ‘cone’ within the orbit. The inferior oblique muscle, however, passes under the eye, near the floor of the orbit, and is attached to the eyeball (sclera) in between the superior rectus and lateral rectus. 

Common causes of visual impairment and blindness

• Cataract

• Trachoma

• Glaucoma

• Onchocerciasis

• Conjunctivitis

• Ophthalmia neonatorum

• Refractive errors

• Xerophthalmia due poor nutrition (Vitamin A deficiency)

• Infectious diseases e.g Measles, leprosy e.t.c

• Other medical condition e.g Diabetes Myelitis 

• Harmful traditional practices

• Trauma/accident (leads to corneal opacity)

• Congenital abnormalities

• Old age 

Blindness: According to W.H.O blindness refers to any vision that is less than 3/60 to NPL with all possible correction in the better eye.

Visual acuity: This refers to the procedure rendered when measuring or testing to know how well an eye can see both at far and near distance. Distance vision is been measured at 6 meters away from patient while near vision at 33 cm. 

Trachoma

Definition: Trachoma is a serious chronic infection of the conjunctiva (a contagious condition) caused by bacterium Chlamydia Trachomatis. It is the greatest single cause of preventable blindness world over.  

Initial Sign and Symptoms:

• Inflamed conjunctiva

• Reddened/pink eye

• Irritation and gritty sensation

• Swelling of lymph nodes in front of the ears

• Turn in/misdirected eyelashes  

• Slight yellowish discharge

• Excessive lacrimation

• Sensitivity to bright light

Incubation period: 5 to 12 days after which the infected person begins to experiences its symptoms.

Features of its grading: Several stages can occur simultaneously:

• Stage I: Trachomatous inflammation follicles (TF) this is the presence of 5 or more follicles in the upper tarsal conjunctiva, the follicles are whitish, grey or yellow elevation and paler than the surrounding conjunctiva.

• Stage II: Trachomatous inflammation intense (TI) the upper tarsal conjunctiva is red, rough and thickened. The blood vessels normally visible, are masked by a diffuse inflammatory infiltration or follicles.

• Stage III: Trachomatous scarring (TS) follicles disappear, leaving scars; the scars are white lines bands or patches in the tarsal conjunctiva. 

• Stage IV: Trachoma Trachiasis (TT) due to multiple scars, the margin of the eyelids turns inwards (entropion); the eyelashes rub on the cornea and cause ulceration and chronic inflammation.

• Stage V: Trachoma or Corneal opacity (TO or CO) this is situation were by cornea gradually loses its transparency and becomes opaque resulting to visual impairment and subsequent blindness.

Trachoma is common in the areas with:

• Lack/poor water supply and sanitation

• Overcrowding, poor personal hygiene and excess flies

• Lack/inadequate health/eye care services

• Trachiasis in adults and active infection in children

• Poverty and low level of education

Mode of transmission:

• Through direct contact with eye, nose and throat secretion of the infected person

• Contact with fomites agents e.g towels, hand kerchief, eye kajal e.t.c 

• Mechanical transmission e.g flies

• Cross infection e.g infected mother to child

Prevention and control measures: SAFE strategy is recommended by World Health Organisation (W.H.O):

S – Surgery for (TT) to correct advanced stages of the disease.

A – Antibiotic for (TF and TI) e.g Azithromycin to treat the infection.

F – Facial cleanliness reduces disease transmission.

E – Environmental and water sanitation, to control flies and improved personal hygiene

Note: Successful intervention programme requires active participation of communities; they are to be involved from planning phase up to the implementation stage.

Treatment:

• Stage I and II: 

  • Cleans eye and face several time per day

  • Antibiotic therapy e.g Azithromycin per oral (PO) children over 6months or over 6Kg 20mg/Kg a single dose while Adults 1g start then apply occ. TCN

•  Stage III: No treatment

• Stage IV: Surgical treatment for correction of trachiasis

• Stage V: No treatment