Zone II is an annulus with the inner border defined by zone I and the outer border defined by the radius defined as the distance from the optic nerve to the nasal ora serrata. Zone III is the residual temporal crescent of the retina. The circumferential extent of the disease is described in segments as if the top of the eye were 12 on the face of an analog clock, e.g. stage 1 from 4:00 to 7:00. (The extent is a bit less important since the treatment indications from the Early Treatment for ROP. )The Stages describe the ophthalmoscopic findings at the junction between the vascularized and avascular retina. Stage 1 is a faint demarcation line. Stage 2 is an elevated ridge. Stage 3 is extraretinal fibrovascular tissue. Stage 4 is sub-total retinal detachment. Stage 5 is total retinal detachment. Plus disease
Plus disease can be present as a major complicating factor at any stage. It is characterised by:

Significant level of vascular dilation and tortuosity observed at the posterior retinal arterioles. This reflects the increase of blood flow through the retina. Vitreous haze and anterior chamber haze
Iris vascular engorgement
Persistent tunica vasculosa lentis or immature blood vessels growing over the lens which also restrict the dilatation of the pupils. Differential diagnosis
The most difficult aspect of the differential diagnosis may arise from the similarity of two other diseases:

Familial exudative vitreoretinopathy which is a genetic disorder that also disrupts the retinal vascularization in full-term infants.
Butalbital/acetaminophen, sold under the brand name Butapap among others, is a combination medication used to treat tension headaches and migraine headaches. It contains butalbital, a barbiturate and paracetamol (acetaminophen), an analgesic. Versions also containing caffeine are sold under the brand name Fioricet among others. It is taken by mouth. The combination is also sold with codeine.The most common side effects include sleepiness, dizziness, trouble breathing, and abdominal pain. Other severe side effects may include liver problems, confusion, addiction, and allergic reactions. Frequent use may result in medication overuse headache. Barbiturate withdrawal may occur if rapidly stopped following long term use. Use is not generally recommended during pregnancy or breastfeeding.The combination was approved for medical use in the United States in 1984. It is available as a generic medication. In 2019, the combination butalbital/acetaminophen/caffeine was the 228th most commonly prescribed medication in the United States, with more than 2 million prescriptions. In the United States it is a schedule III controlled substance in some states but not federally. It is banned in a number of European countries. Medical uses
Butalbital/acetaminophen is indicated for the treatment of tension headaches.Butalbital/acetaminophen/caffeine is indicated for the treatment of tension headaches. Side effects
Commonly reported side effects include euphoria, dizziness or lightheadedness, drowsiness or sedation, intoxication, nausea, vomiting, dependence, shortness of breath, and abdominal pain.Prolonged use can cause rebound headaches.Rarely, use of barbiturates can lead to Stevens–Johnson syndrome. Overdose
Butalbital exerts its toxicity through excessive sedation, resulting in respiratory depression and ultimately death via hypoxia. Nonlethal overdoses may also result in coma and death.