Parkinson’s Disease

Parkinson’s disease (also known as Parkinson disease or PD) is a degenerative disorder of the central nervous system that often impairs the sufferer’s motor skills and speech.

Parkinson’s disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of excessive muscle contraction, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

PD is the most common cause of parkinsonism, a group of similar symptoms. PD is also called “primary parkinsonism” or “idiopathic PD” (“idiopathic” meaning of no known cause). While most forms of parkinsonism are idiopathic, there are some cases where the symptoms may result from toxicity, drugs, genetic mutation, head trauma, or other medical disorders.

History

Symptoms of Parkinson’s disease have been known and treated since ancient times. However, it was not formally recognized and its symptoms were not documented until 1817 in An Essay on the Shaking Palsy by the British physician James Parkinson. Parkinson’s disease was then known as paralysis agitans, the term “Parkinson’s disease” being coined later by Jean-Martin Charcot. The underlying biochemical changes in the brain were identified in the 1950s, due largely to the work of Swedish scientist Arvid Carlsson who later went on to win a Nobel Prize. L-dopa entered clinical practice in 1967, and the first study reporting improvements in patients with Parkinson’s disease resulting from treatment with L-dopa was published in 1968.


Symptoms

Parkinson disease affects movement (motor symptoms). Typical other symptoms include disorders of mood, behavior, thinking, and sensation (non-motor symptoms). Individual patients’ symptoms may be quite dissimilar; progression is also distinctly individual.

Motor Symptoms

The cardinal symptoms are:

  • tremor: normally 4-7Hz tremor, maximal when the limb is at rest, and decreased with voluntary movement. It is typically unilateral at onset. This is the most apparent and well-known symptom, though an estimated 30% of patients have little perceptible tremor; these are classified as akinetic-rigid.
  • rigidity: stiffness; increased muscle tone. In combination with a resting tremor, this produces a ratchety, “cogwheel” rigidity when the limb is passively moved.
  • bradykinesia/akinesia: respectively, slowness or absence of movement. Rapid, repetitive movements produce a dysrhythmic and decremental loss of amplitude. Also “dysdiadokinesia”, which is the loss of ability to perform rapid alternating movements.
  • postural instability: failure of postural reflexes, which leads to impaired balance and falls.

Other motor symptoms include:

  • Gait and posture disturbances:
    • Shuffling: gait is characterized by short steps, with feet barely leaving the ground, producing an audible shuffling noise. Small obstacles tend to trip the patient.
    • Decreased arm swing: a form of bradykinesia.
    • Turning “en bloc”: rather than the usual twisting of the neck and trunk and pivoting on the toes, PD patients keep their neck and trunk rigid, requiring multiple small steps to accomplish a turn.
    • Stooped, forward-flexed posture. In severe forms, the head and upper shoulders may be bent at a right angle relative to the trunk (camptocormia).
    • Festination: a combination of stooped posture, imbalance, and short steps. It leads to a gait that gets progressively faster and faster, often ending in a fall.
    • Gait freezing: “freezing” is another word for akinesia, the inability to move. Gait freezing is characterized by inability to move the feet, especially in tight, cluttered spaces or when initiating gait.
    • Dystonia (in about 20% of cases): abnormal, sustained, painful twisting muscle contractions, usually affecting the foot and ankle, characterized by toe flexion and foot inversion, interfering with gait. However, dystonia can be quite generalized, involving a majority of skeletal muscles; such episodes are acutely painful and completely disabling.
  • Speech and swallowing disturbances
    • Hypophonia: soft speech. Speech quality tends to be soft, hoarse, and monotonous. Some people with Parkinson’s disease claim that their tongue is “heavy”.
    • Festinating speech: excessively rapid, soft, poorly-intelligible speech.
    • Drooling: most likely caused by a weak, infrequent swallow and stooped posture.
    • Non-motor causes of speech/language disturbance in both expressive and receptive language: these include decreased verbal fluency and cognitive disturbance especially related to comprehension of emotional content of speech and of facial expression.
    • Dysphagia: impaired ability to swallow. Can lead to aspiration, pneumonia, and ultimately death.
  • Other motor symptoms:
    • fatigue (up to 50% of cases);
    • masked faces (a mask-like face also known as hypomimia), with infrequent blinking;
    • difficulty rolling in bed or rising from a seated position;
    • micrographia (small, cramped handwriting);
    • impaired fine motor dexterity and coordination;
    • impaired gross motor coordination;
    • Poverty of movement: overall loss of accessory movements, such as decreased arm swing when walking, as well as spontaneous movement.

Non-motor Symptoms

Mood Disturbances

  • Estimated prevalence rates of depression vary widely according to the population sampled and methodology used. Reviews of depression estimate its occurrence in anywhere from 20-80% of cases. Estimates from community samples tend to find lower rates than from specialist centres. Most studies use self-report questionnaires such as the Beck Depression Inventory which may overinflate scores due to physical symptoms. Studies using diagnostic interviews by trained psychiatrists also report lower rates of depression.
  • More generally, there is an increased risk for any individual with depression to go on to develop Parkinson’s disease at a later date.
  • Seventy percent of individuals with Parkinson’s disease diagnosed with pre-existing depression go on to develop anxiety. Ninety percent of Parkinson’s disease patients with pre-existing anxiety subsequently develop depression; apathy or abulia.

Cognitive Disturbances

  • slowed reaction time; both voluntary and involuntary motor responses are significantly slowed.
  • executive dysfunction, characterized by difficulties in: differential allocation of attention, impulse control, set shifting, prioritizing, evaluating the salience of ambient data, interpreting social cues, and subjective time awareness. This complex is present to some degree in most Parkinson’s patients; it may progress to:
  • dementia: a later development in approximately 20-40% of all patients, typically starting with slowing of thought and progressing to difficulties with abstract thought, memory, and behavioral regulation. Hallucinations, delusions and paranoia may develop.
  • memory loss; procedural memory is more impaired than declarative memory. Prompting elicits improved recall.
  • medication effects: some of the above cognitive disturbances are improved by dopaminergic medications, while others are actually worsened.

Sleep Disturbances

  • Excessive daytime somnolence;
  • Initial, intermediate, and terminal insomnia;
  • Disturbances in REM sleep: disturbingly vivid dreams, and REM Sleep Disorder, characterized by acting out of dream content;

Sensation Disturbances

  • impaired visual contrast sensitivity, spatial reasoning, colour discrimination, convergence insufficiency (characterized by double vision) and oculomotor control.
  • dizziness and fainting; usually attributable orthostatic hypotension, a failure of the autonomous nervous system to adjust blood pressure in response to changes in body position.
  • impaired proprioception (the awareness of bodily position in three-dimensional space).
  • reduction or loss of sense of smell (microsmia or anosmia),
  • pain: neuropathic, muscle, joints, and tendons, attributable to tension, dystonia, rigidity, joint stiffness, and injuries associated with attempts at accommodation.

Autonomic Disturbances

  • oily skin and seborrheic dermatitis.
  • urinary incontinence, typically in later disease progression.
  • constipation and gastric dysmotility that is severe enough to endanger comfort and even health.
  • altered sexual function: characterized by profound impairment of sexual arousal, behavior, orgasm, and drive is found in mid and late Parkinson disease. Current data addresses male sexual function almost exclusively.
  • weight loss, which is significant over a period of ten years – 8% of body weight lost compared with 1% in a control group.

Diagnosis

There are currently no blood or laboratory tests that have been proven to help in diagnosing PD. Therefore the diagnosis is based on medical history and a neurological examination. The disease can be difficult to diagnose accurately. The Unified Parkinson’s Disease Rating Scale is the primary clinical tool used to assist in diagnosis and determine severity of PD. Indeed, only 75% of clinical diagnoses of PD are confirmed at autopsy. Early signs and symptoms of PD may sometimes be dismissed as the effects of normal aging. The physician may need to observe the person for some time until it is apparent that the symptoms are consistently present. Usually Doctors look for shuffling of feet and lack of swing in the arms. Doctors may sometimes request brain scans or laboratory tests in order to rule out other diseases. However, CT and MRI brain scans of people with PD usually appear normal.


Causes of Parkinson’s Disease

Most people with Parkinson’s disease are described as having idiopathic Parkinson’s disease (having no specific cause). There are far less common causes of Parkinson’s disease including genetic, toxins, head trauma, and drug-induced Parkinson’s disease.

Genetic

In recent years, a number of specific genetic mutations causing Parkinson’s disease have been discovered, including in certain populations (Contursi, Italy). These account for a small minority of cases of Parkinson’s disease. Somebody who has Parkinson’s disease is more likely to have relatives that also have Parkinson’s disease. However, this does not mean that the disorder has been passed on genetically.

Genetic forms that have been identified include:

  • PARK1 (OMIM #168601), caused by mutations in the SNCA gene, which codes for the protein alpha-synuclein. PARK1 causes autosomal dominant Parkinson disease. So-called PARK4 is probably caused by triplication of SNCA.
  • PARK2 (OMIM *602544), caused by mutations in protein parkin. Parkin mutations may be one of the most common known genetic causes of early-onset Parkinson disease. In one study, of patients with onset of Parkinson disease prior to age 40 (10% of all PD patients), 18% had parkin mutations, with 5% homozygous mutations. Patients with an autosomal recessive family history of parkinsonism are much more likely to carry parkin mutations if age at onset is less than 20 (80% vs. 28% with onset over age 40). Patients with parkin mutations (PARK2) do not have Lewy bodies. Such patients develop a syndrome that closely resembles the sporadic form of PD; however, they tend to develop symptoms at a much younger age.
  • PARK3 (OMIM %602404), mapped to 2p, autosomal dominant, only described in a few kindreds.
  • PARK5, caused by mutations in the UCHL1 gene (OMIM +191342) which codes for the protein ubiquitin carboxy-terminal hydrolase L1.
  • PARK6 (OMIM #605909), caused by mutations in PINK1 (OMIM *608309) which codes for the protein PTEN-induced putative kinase 1.
  • PARK7 (OMIM #606324), caused by mutations in DJ-1 (OMIM 602533).
  • PARK8 (OMIM #607060), caused by mutations in LRRK2 which codes for the protein dardarin. In vitro, mutant LRRK2 causes protein aggregation and cell death, possibly through an interaction with parkin. LRRK2 mutations, of which the most common is G2019S, cause autosomal dominant Parkinson disease, with a penetrance of nearly 100% by age 80. G2019S is the most common known genetic cause of Parkinson disease, found in 1-6% of U.S. and European PD patients. It is especially common in Ashkenazi Jewish patients, with a prevalence of 29.7% in familial cases and 13.3% in sporadic.
  • PARK12 (OMIM %300557), maps to the X chromosome.

Toxins

One theory holds that the disease may result in many or even most cases from the combination of a genetically determined vulnerability to environmental toxins along with exposure to those toxins. This hypothesis is consistent with the fact that Parkinson’s disease is not distributed homogenously throughout the population: rather, its incidence varies geographically. It would appear that incidence varies by time as well, for although the later stages of untreated PD are distinct and readily recognizable, the disease was not remarked upon until the beginnings of the Industrial Revolution, and not long thereafter become a common observation in clinical practice. The toxins most strongly suspected at present are certain pesticides and transition-series metals such as manganese or iron, especially those that generate reactive oxygen species, and or bind to neuromelanin, as originally suggested by G.C. Cotzias. In the Cancer Prevention Study II Nutrition Cohort, a longitudinal investigation, individuals who were exposed to pesticides had a 70% higher incidence of PD than individuals who were not exposed.

MPTP is used as a model for Parkinson’s as it can rapidly induce parkinsonian symptoms in human beings and other animals, of any age. MPTP was notorious for a string of Parkinson’s disease cases in California in 1982 when it contaminated the illicit production of the synthetic opiate MPPP. Its toxicity likely comes from generation of reactive oxygen species through tyrosine hydroxylation.

Other toxin-based models employ PCBs, paraquat (a herbicide) in combination with maneb (a fungicide) rotenone (an insecticide), and specific organochlorine pesticides including dieldrin and lindane. Numerous studies have found an increase in Parkinson disease in persons who consume rural well water; researchers theorize that water consumption is a proxy measure of pesticide exposure. In agreement with this hypothesis are studies which have found a dose-dependent an increase in PD in persons exposed to agricultural chemicals.

Head Trauma

Past episodes of head trauma are reported more frequently by sufferers than by others in the population. A methodologically strong recent study found that those who have experienced a head injury are four times more likely to develop Parkinson’s disease than those who have never suffered a head injury. The risk of developing Parkinson’s increases eightfold for patients who have had head trauma requiring hospitalization, and it increases 11-fold for patients who have experienced severe head injury. The authors comment that since head trauma is a rare event, the contribution to PD incidence is slight. They express further concern that their results may be biased by recall, i.e., the PD patients because they reflect upon the causes of their illness, may remember head trauma better than the non-ill control subjects.

These limitations were overcome recently by Tanner and colleagues, who found a similar risk of 3.8, with increasing risk associated with more severe injury and hospitalization.

Drug-induced

Antipsychotics, which are used to treat schizophrenia and psychosis, can induce the symptoms of Parkinson’s disease (or parkinsonism) by lowering dopaminergic activity. Due to feedback inhibition, L-dopa can also eventually cause the symptoms of Parkinson’s disease that it initially relieves. Dopamine agonists can also eventually contribute to Parkinson’s disease symptoms by increasing the sensitivity of dopamine receptors.


Treatment

Parkinson’s disease is a chronic disorder that requires broad-based management including patient and family education, support group services, general wellness maintenance, exercise, and nutrition. At present, there is no cure for PD, but medications or surgery can provide relief from the symptoms.

Levodopa

The most widely used form of treatment is L-dopa in various forms. L-dopa is transfomed into dopamine in the dopaminergic neurons by L-aromatic amino acid decarboxylase (often known by its former name dopa-decarboxylase). However, only 1-5% of L-DOPA enters the dopaminergic neurons. The remaining L-DOPA is often metabolised to dopamine elsewhere, causing a wide variety of side effects. Due to feedback inhibition, L-dopa results in a reduction in the endogenous formation of L-dopa, and so eventually becomes counterproductive.

Carbidopa and benserazide are dopa decarboxylase inhibitors. They help to prevent the metabolism of L-dopa before it reaches the dopaminergic neurons and are generally given as combination preparations of carbidopa/levodopa (co-careldopa) (e.g. Sinemet, Parcopa) and benserazide/levodopa (co-beneldopa) (e.g. Madopar). There are also controlled release versions of Sinemet and Madopar that spread out the effect of the L-dopa. Duodopa is a combination of levodopa and carbidopa, dispersed as a viscous gel. Using a patient-operated portable pump, the drug is continuously delivered via a tube directly into the upper small intestine, where it is rapidly absorbed.

Talcopone inhibits the COMT enzyme, thereby prolonging the effects of L-dopa, and so has been used to complement L-dopa. However, due to its possible side effects such as liver failure, it’s limited in its availability.

A similar drug, entacapone, has similar efficacy and has not been shown to cause significant alterations of liver function. A recent follow-up study by Cilia and colleagues looked at the clinical effects of long-term administration of entacapone on motor performance and pharmacological compensation is advanced PD patients with motor fluctuations. 47 patients with advanced PD and motor fluctuations were followed for six years from the first prescription of entacapone and showed a stabilization of motor conditions, reflecting entacapone can maintain adequate inhibition of COMT over time.

Mucuna pruriens, is a natural source of therapeutic quantities of L-dopa.

Dopamine Agonists

The dopamine-agonists bromocriptine, pergolide, pramipexole, ropinirole , cabergoline, apomorphine, and lisuride, are moderately effective. These have their own side effects including those listed above in addition to somnolence, hallucinations and /or insomnia. Dopamine agonists initially act by stimulating some of the dopamine receptors. However, they cause the dopamine receptors to become progressively less sensitive, thereby eventually increasing the symptoms.

Dopamine agonists can be useful for patients experiencing on-off fluctuations and dyskinesias as a result of high doses of L-dopa. Apomorphine can be administered via subcutaneous injection using a small pump which is carried by the patient. A low dose is automatically administered throughout the day, reducing the fluctuations of motor symptoms by providing a steady dose of dopaminergic stimulation. After an initial “apomorphine challenge” in hospital to test its effectiveness and brief patient and caregiver, the primary caregiver (often a spouse or partner) takes over maintenance of the pump. The injection site must be changed daily and rotated around the body to avoid the formation of nodules. Apomorphine is also available in a more acute dose as an autoinjector pen for emergency doses such as after a fall or first thing in the morning.

MAO-B Inhibitors

Selegiline and rasagiline reduce the symptoms by inhibiting monoamine oxidase-B (MAO-B), which inhibits the breakdown of dopamine secreted by the dopaminergic neurons. By-products of selegiline include amphetamine and methamphetamine, which can cause side effects such as insomnia. Use of L-dopa in conjunction with selegiline has increased mortality rates that have not been effectively explained. Another side effect of the combination can be stomatitis. One report raised concern about increased mortality when MAO-B inhibitors were combined with L-dopa; however subsequent studies have not confirmed this finding. Unlike other non selective monoamine oxidase inhibitors, tyramine-containing foods do not cause a hypertensive crisis.

Surgical Interventions

Treating Parkinson’s disease with surgery was once a common practice. But after the discovery of levodopa, surgery was restricted to only a few cases. Studies in the past few decades have led to great improvements in surgical techniques, and surgery is again being used in people with advanced PD for whom drug therapy is no longer sufficient. Deep brain stimulation is presently the most used surgical means of treatment, but other surgical therapies that have shown promise include surgical lesion of the subthalamic nucleus and of the internal segment of the globus pallidus, a procedure known as pallidotomy. The engineer who designed and built the equipment shown in the image is Rex Whitby of Hertfordshire.

Speech Therapies

The most widely practiced treatment for the speech disorders associated with Parkinson’s disease is Lee Silverman Voice Treatment (LSVT). LSVT focuses on increasing vocal loudness.

A study found that an electronic device providing frequency-shifted auditory feedback (FAF) improved the clarity of Parkinson’s patients’ speech.

Physical Exercise

Regular physical exercise and/or therapy, including in forms such as yoga, tai chi, and dance can be beneficial to the patient for maintaining and improving mobility, flexibility, balance and a range of motion.