Parkinson disease

Definitions & Key takeaways

Parkinson's is a progressive movement disorder caused by degeneration of dopamine-producing neurons in the substantia nigra, specifically in the pars compacta, which leads to resting tremor, rigidity, problems initiating movement, and postural instability, and for which therapy primarily focuses on increasing brain dopamine.

There is no cure for this condition, but medications can increase dopamine levels in the brain and control tremors. There is also deep-brain stimulation, which involves an implantable device that directly sends electrical signals to the basal ganglia that counteract the abnormal signaling in Parkinson's.

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Introduction0:00–0:17

Parkinson's disease, named after Doctor James Parkinson, who first described it, is a progressive neurodegenerative condition characterized by the gradual loss of dopamine producing neurons in a brain region called the substantia nigra.

Causes and risk factors0:17–1:04

Long before the time of Doctor Parkinson, signs of this condition appeared in ancient tales. In Homer's Odyssey, King Nestor, once a fierce and fearless warrior, slowly changed with age.
His hands started to tremble. His steps became sluggish, and his body became stiff, so he could no longer join the battle.
His story sounds all too familiar to what we today recognize as Parkinson's disease. Now, to understand what was going on with King Nestor, we need to look deep within the brain, specifically at the basal ganglia.
The basal ganglia play a major role in carrying out learned motor plans such as riding a bike to university. Moreover, they help fine tune and shape movement while also controlling unwanted or unnecessary motor activity.

Pathology1:04–2:57

Basal ganglia do this by modulating signals from the motor part of the thalamus to the motor cortex. Once those signals reach the motor cortex, they travel down the spinal cord to activate muscles, so you can spin those wheels and make it to pathology class on time.
At the same time, basal ganglia help regulate muscle tone and help maintain posture. The basal ganglia include the striatum, which consists of the caudate nucleus and the putamen, the globus pallidus, the subthalamic nucleus, and the substantia nigra.
When it comes to Parkinson's disease, the most important player is the substantia nigra. This phrase literally means black substance and originates from its dark appearance, which comes from melanin-rich neurons.
You can picture it as a strong shot of espresso tucked into your midbrain, small, dark, and packed with power. Now, the substantia nigra consists of two parts.
The pars reticulata receives signals from the striatum and sends them down to the thalamus using the inhibitory calming neurotransmitter GABA.
This helps keep your movements steady and prevents unwanted motions from slipping through. On the other hand, the pars compacta contains dopamine producing neurons that project to the striatum.
This connection forms the nigrostriatal pathway, which supplies the striatum with dopamine and helps initiate movement. Without enough dopamine flowing through it, even simple actions like reaching for your favorite chocolate bar can become difficult.
In Parkinson's disease, the dopamine producing neurons in the pars compacta slowly die off. As the brain loses more of these neurons over time, it sends weaker signals through the negrostriatal pathway, which impairs the initiation and control of voluntary movements.

Clinical features2:57–5:16

While most of the time the underlying cause is unknown, we know that several genetic mutations are associated with Parkinson's disease.
Based on the type of mutation, we can classify Parkinson's disease into autosomal dominant and autosomal recessive types.
Autosomal dominant types are associated with mutations in the SCNA gene, which encodes alpha synuclein that enables neurons to communicate with each other.
When the SCNA gene mutates, the cell produces abnormal alpha synuclein that misfolds and accumulates within neurons, eventually forming Lewy bodies.
These round eosinophilic structures are the hallmark of Parkinson's disease. Next, there's the LRRK2 gene, which encodes leucine-rich repeat kinase 2, an enzyme that's important with intracellular signaling.
When this gene mutates, the enzyme stops working, causing various cellular functions to malfunction. On the flip side, in the autosomal recessive types, we have mutations of the PARC 7 gene, which encodes DJ1 protein, PINC1 gene, which encodes PINK 1 protein, and PARC2 gene, which encodes Parkin protein.
These proteins are essential for normal mitochondria functioning. When these proteins don't work as they should, mitochondrial dysfunction sets in, eventually leading to cell damage and death.
It's also important to remember that some people with Parkinson's disease carry mutations in the GBA1 gene. This gene makes an enzyme called glucocerebrosidase, which helps lysosomes break down and clear out cellular waste.
When the enzyme doesn't work properly, cellular waste accumulates, disrupting normal cell processes and eventually causing cell death.
These same mutations are known to cause a lysosomal storage disorder called Gaucher disease. However, we are still not sure how this mutation increases the risk of Parkinson's disease.
Regardless of the underlying genetic mutation and type, the loss of melanin-rich neurons causes substantia nigra to look much paler than usual.
It's like pouring milk into that cup of espresso tucked in the brain. As dopamine producing neurons die off, dopamine levels in the nirostriatal pathway gradually decline.

Treatment5:16–8:13

With less dopamine available, the brain struggles to initiate movement, which results in clinical features we call Parkinsonism.
Parkinsonism is a clinical syndrome that includes bradykinesia, rigidity, tremor, and postural instability. The term bradykinesia comes from the Greek words brady meaning slow, and kinesis meaning movement.
So bradykinesia means slowness of voluntary movement. Individuals with bradykinesia often say that their limbs feel heavy, as if they are moving through wet sand.
This makes everyday tasks such as writing, buttoning a shirt, and turning over in bed feel much more difficult. Since initiating and maintaining movement becomes more difficult, people naturally begin to move less and with reduced range.
This decrease in movement is known as hypokinesia. It can appear as a shuffling gait with short, hesitant steps or as micrographia, where handwriting becomes unusually small.
Hypokinesia might cause a reduction in facial expression known as hypomimia. People may blink less frequently or show little emotion.
This is often described as a masked face. As the condition progresses, bradykinesia and hypokinesia can progress to kinesia, which is the complete inability to initiate movement.
This can lead to frozen episodes where a person feels frozen in place, unable to move even though they want to. Next up is a tremor, specifically a resting tremor, because it's there at rest and disappears with movement.
It usually shows up in the hands as a steady rhythmic shaking that looks like rolling a tiny pill between the thumb and fingers.
Tremors can also affect the legs, jaw, or even the lips, and they often get worse when someone feels stressed or anxious.
Since the basal ganglia help regulate muscle tone and posture, we arrive at the final two key features of Parkinson's disease.
First, rigidity refers to muscle stiffness and resistance to passive movement. In Parkinson's disease, rigidity presents as either lead pipe rigidity or cogwheel rigidity.
In lead pipe rigidity, when attempting to passively move a limb, the limb resists movement evenly throughout the entire range, like trying to bend a solid lead pipe.
In cogwheel rigidity, a series of small jerks interrupt the resistance similar to the motion of a cog turning in a gear.
This jerky feel occurs when a tremor is superimposed on top of the underlying stiffness. As the disease progresses, the patient might experience postural instability, which often results in poor balance and even falls.
Besides motor symptoms, these individuals often face non-motor symptoms, including sleep disturbances, constipation, and erectile dysfunction.

Review8:13–10:12

As the condition progresses, it affects the cerebral cortex, causing more serious symptoms such as hallucinations and dementia.
If dementia appears within a year of the onset of motor symptoms, the condition is classified as dementia with Lewy bodies.
Diagnosis mainly relies on clinical findings, especially the classic triad of bradykinesia, tremor, and rigidity. At the same time, it's important to rule out other conditions that can mimic Parkinson's disease.
For example, antipsychotic medications like haloperidol block dopamine receptors, reducing dopamine activity in the brain and causing symptoms that resemble Parkinson's disease.
If the diagnosis is unclear, a few tools can help. The L-DOPA test involves giving a person the dopamine precursor called levodopa.
If symptoms improve within a few hours, it's a strong clue that Parkinson's disease is at play. It's like the brain is telling us, yes, this is what I've been missing.
Another tool is the DAT scan, which is a specialized brain imaging test that looks at dopamine activity in the striatum.
Low dopamine levels create a distinct pattern on the scan, helping confirm the diagnosis. Finally, treatment focuses on boosting dopamine levels using medications like levodopa and dopamine agonists.
When medications alone are not enough, consider deep brain stimulation, which involves placing a device that delivers electrical signals to the basal ganglia to help improve motor symptoms.
All right, as a quick recap, Parkinson's disease is a neurodegenerative condition caused by the progressive loss of dopaminergic neurons in the substantia nigra.
It is characterized by a classic triad of motor symptoms, which include bradykinesia, rigidity, and resting tremor. As the condition progresses, individuals may also develop postural instability.
While the exact cause remains a mystery, it's certain genetics play a role.