Diagnosing Trachoma

Diagnosing Trachoma

The diagnosis of trachoma is done clinically. Although an individual may complain about sticky eyes or itchy, painful eyes, often the disease is subclinical. After repeated infections, when scarring of the conjunctiva has occurred, the patient may complain about a feeling of sand or insects in the eyes.

Clinically the diagnosis of trachoma can be done by using magnifiers (loupe) and a flashlight.

The WHO has developed a simplified trachoma grading system, with 5 grades:

  1. Trachomatous Inflammation – Follicular (TF);
  2. Trachomatous Inflammation – Intense (TI);
  3. Trachomatous Scarring (TS);
  4. Trachomatous Trichiasis (TT);
  5. and Corneal Opacity (CO)

 

Basic Anatomy of the Eye

Knowledge of basic eye anatomy is essential to understanding trachoma. In this section only the most important parts of the eye are described, including those affected by trachoma. 
Basic Anatomy of the Eye (pdf)

Cornea:This is the clear part in front of the eye, through which we can see the iris (the colored part of the eye). Normally there are no blood vessels in the cornea, as corneal transparency is absolutely necessary for good vision. The cornea is very rich in sensory nerves, which makes even an eyelash touching the cornea very painful. The black hole in the middle of the iris is the pupil, an aperture, which regulates the amount of light entering the eye.

Sclera:The cornea continues into the sclera, or the white of the eye. It is a firm layer to protect the eye and to keep its shape.

Eyelids:The eyelid contains the orbicularis muscle, which closes the eye, the Meibomian glands, which add a small oil part to the tear film (to reduce the evaporation and inhibit overflow on the cheeks), and a firm, fibrous plate, called the tarsal plate (larger in size in the upper eyelid). This tarsal plate provides stability to the eyelid.

Conjunctiva:The conjunctiva, normally a smooth, thin, transparent layer, covers the front part of the sclera and folds forward to cover the inside of the eyelids. Prominent deep conjunctival blood vessels run vertically over the superior tarsal conjunctiva.  With intense trachomatous inflammation (TI) more than half of these blood vessels are obscure.

 

Chlamydia trachomatis

Trachoma is caused by Chlamydia trachomatis. It is an intracellular energy parasite. Although it has many characteristics similar to a virus, it technically is a bacteria, and is closely related to Gram negative bacteria. 

The growth cycle of C. trachomatis is unique, characterized by elementary bodies, which are contagious, relatively resistant to the extra cellular environment, metabolically inactive, and able to enter a cell. Inside the cell, these elementary bodies develop into the larger, metabolically active, and noncontagious initial bodies. Through reproduction by binary partition, new elementary bodies are formed, which form an inclusion body in the cytoplasm of the cell. After 48-72 hours, the infected cell breaks open and the contagious elementary bodies are released, ready to infect new cells.

Trachoma is a chronic bilateral (both eyes) follicular conjunctivitis. Both the upper and lower conjunctivae have a follicular hypertrophy—the follicular reaction is most apparent in the upper conjunctiva, where the conjunctiva is bound down tightly to the underlying tarsus.  The corneal limbus (edge) also characteristically develops scarring and Herbert’s pits (depressions in the scarring in the area of previous follicles). Infections can either be self-limiting or become progressive (going from a suppurative state, i.e. pus forming state, to a stage involving deeper tissue as well).

The repeated infections (there is only partial immunity against C. trachomatis) of the conjunctiva of the upper eyelid result in scarring of this tissue, which leads to an inward rotation of the eyelid (entropion). It is unclear why some people have more scarring than others, even with apparently similar conditions.  Existing research suggests an important role of the human immune system. Intracellular survival of the C. trachomatis is influenced by the cellular immune response. Damage to the epithelium might be caused by a hypersensitivity reaction to the continual presence of the chlamydia antigen. Another factor might be the anatomical differences in strength of the orbicularis oculi muscle and the thickening of the tarsal plate. Once the eyelashes start rubbing the eyeball (trichiasis), the eye becomes vulnerable to infectious corneal ulcers, corneal scarring, and blindness.