Classification of ROP

In the 1980's an international standard classification of retinopathy of prematurity (known as ICROP) was developed by a team of 23 experts from around the world.  It defines the disease by location relative to the optic nerve and macula, by extent of disease around the circumference of the eye, and by stages of progressive disease.

Location

Blood vessel development in the retina occurs from the optic nerve out towards the periphery, that is, from the back of the eye towards the front.  The location of the disease is referred to by the ICROP classification and is a measure of how far this normal progression of blood vessel development has progressed before the disease takes over.  Generally Zone II disease is more severe than Zone III disease  and Zone I disease is the most dangerous of all since progression to extensive scar tissue formation and total retinal detachment is most likely in this location.

From the "flattened" retina in the figure above you can see that:

• Zone I is a small area around the optic nerve and macula at the very back of the eye.

• Zone II extends from the edge of Zone I to the front of the retina on the nasal side of the eye (i.e. nose side) and part way to the front of the retina on the temporal side of the eye (i.e. temple side, or side of the head).

• Zone III is the remaining crescent of retina in front of Zone II on the temporal side of the eye.

Extent of Disease

Think of the eye as in time sections of a twelve hour clock.  The extent of ROP is defined by how many clock hours of the eye's circumference is diseased.  The numbers around the "flattened" retina in the figure show the hours of the clock for each eye.  For example, 3 o'clock is to the right, which is on the nasal side for the right eye and temporal side for the left eye.  Often the disease is not present around all twelve clock hours, so a description may often refer to "x" number of clock hours of disease (e.g. nine clock hours would mean that three quarters of the circumference of the retina is involved).

Stages of the Disease

Retinopathy of prematurity is a progressive disease.  It starts slowly, usually anywhere from the fourth to the tenth week of life, and may progress very fast or very slowly through successive stages, from Stage 1 through Stage 5.  Or it may stop at Stage 1, Stage 2 or mild Stage 3 and disappear completely.

Stage 3 ROP is characterized by the development of abnormal "new blood vessels" and fibrous tissue (like scar tissue) on the edge of the ridge seen in Stage 2 and extending into the vitreous (the back cavity of the eye).  Stage 3 is further divided into:

• "Mild" - with only a limited amount of abnormal tissue,

• "Moderate" - with significant amounts of abnormal tissue infiltrating into the vitreous, or

• "Severe" - massive amounts of abnormal tissue infiltrate into the vitreous.

Statistically, eyes which reach stage 3 moderate or severe have a 50% chance of proceeding to Stage 4 or Stage 5 and possible blindness.  Therefore, it is at Stage 3 that treatment is instituted.

Stage 4 is further categorized depending on the location of the partial detachment:

• Stage 4A is a partial detachment outside the macula - the area of central vision - in the preiphery of the retina.  Therefore, the chance for usable vision, if the retina reattaches, is relatively good.

• Stage 4B is partial detachment involving the macula, usually with a fold extending out through Zones I, II, and III.  The involvement of the macula severely limits the prospect for usable vision in this eye.

Stage 5 ROP involves a complete retinal detachment, with the retina assuming a closed or partially closed funnel, from the optic nerve to the front of the posterior cavity fo the eye, just behind the lens.  Infants with Stage 5 ROP have essentially no useful vision inthat eye.  Treatment at this stage involves surgery to relieve the traction, causing the detachment, in an attempt to reattach the retina.

Some vision may be recovered by this surgery but the infant will most likely be legally blind in the involved eye.

Plus Disease

Additional complications to those described in Stages 1 through 5 may involve abnormal blood vessels in the iris and engorgement and tortuosity of the normal blood vessels in the retina.  If these additional symptoms are particularly bad, the plus ("+") designation is added to the Stage number, e.g. Stage 2+.

If ROP is located in Zone I and there is plus disease present, then progression of the disease may be particularly rapid.  This form is sometimes called Rush Disease.