Parkinson disease is a neurodegenerative disorder that mostly presents in later life with generalized slowing of movements (bradykinesia) and at least one other symptom of resting tremor or rigidity. Other associated features are a loss of smell, sleep dysfunction, mood disorders, excess salivation, constipation, and excessive periodic limb movements in sleep (REM behavior disorder). 
Over the past century, our understanding of the etiology of PD has evolved immensely. In 1919, it was first recognized that loss of pigmentation in the substantia nigra of the midbrain is a feature of the post-mortem brain examination of PD patients. In the 1950’s, it was further understood that the pigmented neurons that are lost in the substantia nigra are dopaminergic, and it is the loss of dopamine in subcortical motor circuitry that is implicated in the mechanism of the movement disorder in PD.
PD affects 1 to 2 people per 1000 at any time; the prevalence increases with age to affect 1% of the population above 60 years. 5% to 10% of patients have a genetic predisposition.
In 1999, several members of an Italian family in the New Jersey area were identified as suffering from PD, and the search for the genetic basis of this illness gained momentum. Studies on families with the rare autosomal dominant form of this disease led to a further breakthrough: That the primary etiology appears to be the accumulation of alpha-synuclein in various parts of the brain, primarily the substantia nigra, leading to degeneration and subsequent loss of dopamine in the basal ganglia that control muscle tone and movement. The accumulation of the alpha-synuclein protein may be secondary to a genetic predisposition, such as with the PARK-1 mutation identified in the Contursi kindred of New Jersey, or triggered by an unknown environmental agent. There has been some recent interest in establishing an infectious etiology that triggers off this alpha-synuclein accumulation, after reports that the earliest degenerative change in PD appears in the myenteric plexus on the GIT, from where it progresses up to involve the dorsal motor nucleus of the vagus nerve, the sleep centers in the pons, and then the midbrain. This interesting theory also addresses why PD patients typically suffer from gastrointestinal motility issues and REM sleep disorder, both of which can precede the motor manifestations of PD for many years.
Therefore, as our knowledge about the etiology of PD evolves, it is not enough to regard this disorder as primarily caused by a lack of dopamine in the substantia nigra. There appears to be a darker, more extensive pathophysiology behind this, and the non-genetic causes of alpha-synuclein deposition in the brain are a subject of active research.
Loss of dopamine pigment in the substantia nigra is visible to the naked eye in cases of PD.
Histologically, PD is characterized by neuronal inclusions of alpha-synuclein in neuronal cell bodies (Lewy bodies) and within neuronal cell processes (Lewy neurites). The combination of Lewy bodies and Lewy neurites is sometimes referred to as Lewy-related pathology. The hallmark of any neurodegenerative disease is a selective neuronal loss, and loss in PD is most marked in the substantia nigra pars compacta. It has been known for many years, however, that Lewy bodies in PD extend well beyond the substantia nigra. Based on the distribution of alpha-synuclein pathology, Braak and co-workers have proposed a staging scheme for PD. In this scheme, neuronal pathology occurs early in the dorsal motor nucleus of the vagus in the medulla and the anterior olfactory nucleus in the olfactory bulb. As the disease progresses, locus ceruleus neurons in the pons and then dopaminergic neurons in the substantia nigra are affected. In later stages, pathology extends to the basal forebrain, amygdala, and the medial temporal lobe structures, with convexity cortical areas affected in the last stages. Subsequently, there have been proposals that autonomic neurons in the peripheral nervous system may be affected by the involvement of the central nervous system (CNS) which has prompted recognition that PD is a multiorgan disease process, not merely a disorder of the CNS. Moreover, it has fed the debate on cell-to-cell transmission of unknown transmissible agents, from the gut onwards to the brain through retrograde transmission in the vagus nerve.
An earlier feature of PD is tremor, typically unilateral and present at rest, which is usually the reason for seeking help at the Neurology Clinic. After using the hands, such as to pick up a book, the tremor may vanish for some minutes, only to return when the patient is distracted and resting once again. This is the so-called re-emerging tremor that is typical of PD.
Although tremor is a prominent and early symptom of PD, it is not always present and is not a necessary feature for diagnosis.
Slowness, or bradykinesia, on the other hand, is a core feature of PD. Patients will notice it takes them longer to do simple tasks, their walking is slower, and their ability to respond to threats is compromised. In the clinic, patients demonstrate an inability to tap their index finger and thumb rapidly, tap their foot rhythmically on the floor, or walk steadily.
Rigidity is the third prominent feature on examination. Patients appear stiff, and they find it difficult to rise out of a chair without support. While walking there is reduced arm swing, more so on one side than the other, as PD typically is asymmetric at onset. On checking muscle tone, lead pipe and cogwheel rigidity can be appreciated.
A combination of bradykinesia and rigidity leads to some other characteristic features of PD, such as micrographia. Interestingly, the diminishing size of Hitler’s handwriting with the passage of time leads to a retrospective diagnosis that he may have suffered with PD.
The fourth prominent feature of PD is gait disturbance, although this is typically a late manifestation. Flexed posture, reduced arm swing, festination, march-a-petits- pas, camptocormia, retropulsion, and turning enbloc are popular terms to describe the gait in PD. In advanced PD, patients will have trouble rising from a chair without support, they take small, slow steps, they are unable to stop themselves from falling if pushed lightly, they cannot turn around without taking several small steps, and they tend to freeze when faced with certain stimuli such as a doorframe or a passer-by. With experience, a neurologist can diagnose a long-suffering PD patient from their progress to the office chair from the waiting room, although it has to be said that some other neurologic conditions such as normal pressure hydrocephalus can produce similar gait. Gait disorder is not an early feature of PD but is frequently described as it is easy to recognize and cinches the diagnosis in later stages.
Apart from the above four prominent features of PD in the clinic, patients are usually asked about constipation, drooling, mood disorder and depression, REM sleep disorder, and anosmia. These subtle symptoms frequently accompany the tremor/rigidity paradox of PD patients and reflect the underlying alpha-synuclein deposition- mediated neurodegeneration in parts of the brain other than the substantia nigra.
Evaluation of PD patients usually starts with history and physical examination geared to picking up the signs described above. Movement disorders clinics may employ the Unified Parkinson Disease Rating Scale to quantify patient mentation, behavior, mood, activities of daily living, tremor, motor examination, and complications of therapy.
An essential part of the evaluation of PD is to exclude the effect of medications that can lead to extrapyramidal side effects and motor manifestations that are, at times, indistinguishable from classical PD. Typically, it is traditional antipsychotics that are implied. In these cases, the nonmotor manifestations of PD, such as anosmia, are not found.
It is also important to exclude other neurodegenerative conditions. Other alpha-synucleinopathies such as Lewy body disease and multiple system atrophy can present with PD but have subtle differences that distinguish them as separate disease entities. Similarly, tauopathies such as progressive supranuclear palsy can present with bradykinesia, rigidity, and gait disturbance.
For example, all the diseases below may have the same motor manifestations of PD, but:
In terms of investigations, PD remains one of the few medical conditions where clinical examination remains sufficient for formal diagnosis. MRI is useful in narrowing the differential and excluding other conditions that may present with a similar examination, such as normal pressure hydrocephalus or subcortical stroke. In certain situations, a DAT scan may be used to identify loss of dopaminergic uptake in the basal ganglia. However, the interpretation of this test can be challenging, and the routine use of this test is to be discouraged.
Practically, one of the best ways of establishing a diagnosis of PD in a patient with suggestive symptoms is a clear response to Levodopa treatment. This is either short-lived or not well seen with overlapping syndromes. 
Pharmacologically, this is typically Levodopa (combined with Carbidopa, which decreases side effects and improves CNS bioavailability). A dopamine agonist (Pramipexole, Ropinirole) may be started in younger patients; it may not be as effective as Levodopa but will have fewer side effects. Anticholinergics or Amantadine may be used if the predominant symptom to be controlled is a tremor.
A multidisciplinary approach to the management of PD is essential. Patients do much better when a structured physical therapy program geared to PD is employed; they can be taught to improve their balance and gait, improve their stability, and maintain an active life. Some unique features of this movement disorder have been exploited to advantage; patients do well with music therapy in physical fitness programs, cycling, and boxing. Patients who cannot walk may find themselves able to dance. Depression, carer fatigue, constipation, REM sleep disorder, paranoia and psychosis as side effects of the medication or part of the primary disease, and all need to be addressed.
Patients responding well to pharmacological treatment may grow resistant with time, develop abruptly on/off motor symptoms, or suffer from disabling dyskinesia as a side effect. Varying the pharmacokinetics of Levodopa has sometimes helped, such as newer formulations of delayed release Levodopa or a continuous GI pump infusion form of Levodopa. The addition of other medications and balancing the timing of the medication can have a huge effect on the patient.
For some patients who do not respond to all measures, there is increasing availability of deep brain stimulation techniques to stabilize the balance of excitatory and inhibitory signals to the subthalamic nucleus or the globus pallidus. Although not entirely understood why this has such a positive effect on treatment failures in PD, it is now a new hope for advanced PD patients and an area of active progress.
The rate of progression of the disease may be predicted based on the following:
All patients diagnosed with PD need long-term follow up because of the progressive behavior and motor abnormalities. Slight adjustments in medications are needed in most patients. In addition, many patients develop erratic behavior like impulsivity, psychosis, paranoia, and somnolence.
PD is the most common motor disorder in the U.S. The disorder has no cure and is progressive. The condition can present with motor abnormalities and a variety of psychiatric and autonomic problems. Almost every organ is affected by this disorder, and as the disease progresses, management can be difficult. Besides physicians, nurses, pharmacists, social workers, and physical therapists play a vital role in the daily management of these patients. Nurses have many roles when it comes to looking after PD patients. The first is the education of the patient and family and encouraging them to participate in decisions regarding the treatment. In addition, nurses are often the first to spot difficulties in daily living and hence should make the appropriate referrals or recommendations. Nurses also provide psychosocial support and referral to a therapist, if required. Finally, the nurse plays a key role in assessing the risk of falls and making referrals to a physical therapist for an ambulatory device. Other issues commonly encountered in these patients that require attention include dysphagia, cognitive decline, depression, and somnolence. Finally, the nurse should provide information on end of life care, financial planning, disability application and referral to a nursing home.  (Level V)
Since most PD patients are on multiple medications, the pharmacist has to closely monitor the drugs to ensure that they are safe and not causing any adverse reactions.
PD is a progressive disease with no cure. The lifespan is reduced compared to the general population. However, the rate of disease progression is not always easy to predict. In the later stages of the disease, falls, gait difficulties and dementia are common. The quality of life for most patients with PD is poor.  (Level V)