In 1967, Drs. Margaret Hoehn and Melvin Yahr devised a 6-point scale from 0 to 5, in order to classify the stages of PD. When this scale was first created, levodopa had not yet changed the course of PD treatment, nor were dopamine agonists or MAO-B inhibitors prescribed. At the time, doctors might differ over which of two close stages a patient was in (Stage 1 vs. Stage 2), but not over a greater disparity, such as Stage 1 vs. Stage 3. Reason-able predictions could be made and thus a patient diagnosed as Stage 1 or Stage 2 might expect to progress to Stage 3 in 5-10 years, and Stage 4 in another 5-10 years.
After 10-20 years most patients were significantly dis-abled and few survived 20 or more years. Today, to rate the true stage of PD, patients would have to stop taking their PD drugs for at least one month. With the drugs “washed out” of their system, the underlying stage of PD would appear. However, for most patients such a “wash-out” or “drug holiday” is not practical or feasible. In the past, when there were few ways to counteract the adverse effects of levodopa, “drug holidays” were recommended to give patients a break from taking their medication.
These drug holidays were poorly tolerated by patients: they became immobile; markedly depressed or anxious; developed difficulty swallowing and breathing; developed pneu-monia; and occasionally developed a life-threatening disorder called neuroleptic malignant syndrome.
Aside from the impracticality of patients taking a drug holiday to reveal the underlying PD stage, making pre-dictions about the progression of PD based on the Hoehn and Yahr Scale is further complicated by fluc-tuations in patient medication response. After 2–5 years, many patients fluctuate such that their days consist of being “on” (levodopa is working) followed by being “off ” (levodopa not working). When this occurs, the patient is rated during both the “on” and “off ” states, with the off state being a better measure of the underlying PD stage.
Additionally, the Hoehn and Yahr Scale rates mobility, but does not rate anxiety, aberrant behavior, depression, dyskinesia, memory loss, difficulty thinking, or difficulty swallowing, all of which may overshadow a patient’s lack of mobility. The Hoehn and Yahr Scale is not a cancer rating scale—it is not a guide to treatment and it is an imperfect guide to prognosis. When a person has cancer, a surgeon can stage a patient based on the type of cancer diagnosed and how far it has spread.
This data, coupled with historical data on thousands of similar cancers, is then used to predict prognosis and survival. Similar data is not available for PD. Progression of PD is related to how long you have had it, which is not the same as how long you have been diagnosed with it. The process of PD may have begun years earlier with the appearance of symptoms such as anxiety, depression, or anosmia (loss of smell).
About 5% of patients will recall that 3–5 years prior to diagno-sis of PD, even before initial symptoms became apparent, a mild symptom, such as a hand or foot turning in or a tremor of one hand appeared during a particularly stressful event. In addition to the traditional symptoms of PD (tremor, rigidity, slowness of movement, difficulty walking), many PD patients develop a curved spine. Initially their chin touches their chest, then their shoulders stoop, and eventually their spine curves—either bending forward or to one side.
A curved spine may, in fact, be the earliest symptom of PD. The curved spine of PD differs from the curved spine of osteoporosis in that with osteoporosis the curvature results from softening of the bones, while in PD the curve may result from an unequal pull of muscles; the muscles in front pulling harder than those in the back.
The progression of PD is related to a loss of cells in a region of the brain called the substantia nigra. Everyone is born with approximately 400,000 of these cells (neurons); 200,000 on each side of the brain. It is estimated that each year every one of us loses 2,000 of these neurons. When a person has cumulatively lost approxi-mately 240,000 neurons, or 60%, the first symptoms of PD appear. Thus, if we all lived for 120 years, we would all likely have PD.
It is thought that the process of PD (which begins before the first symptoms appear) is char-acterized by an increased rate in the destruction of neurons, perhaps up to 5, 10, or even 20 thousand a year. When PD begins in an older person—70 years or older—they have already lost at least 140,000 neurons simply through age-related attrition. The additional accelerated loss of neurons in PD results in a more rapid rate of progression of PD in older people.
The response of a patient to PD drugs probably affects the perception of the rate of PD progression. There-fore, a person whose medication successfully alleviates the symptoms of PD after several weeks generally has a better prognosis and outlook than a patient who either continues to have symptoms of PD while on medication or is troubled by its side effects.
While a number of other rating scales are presently available, the Hoehn and Yahr scale continues to be the simplest and most widely used (see Table 2). The scale classifies PD into five categories, from early, one-sided effects, to full disability. In staging the patient, a doctor notes whether the patient’s Sinemet is working (and thus the patient is “on”) or if it is not working and the patient is considered “off.”