NU 245 · NursingSection 3.3 · Neurological disorders

Section 15 of 18

Parkinson’s disease and spinal muscular atrophy

Explain how the loss of dopamine-producing nerve cells in the substantia nigra produces the movement signs of Parkinson’s disease, recognise those signs and the main treatment options, then explain how a faulty SMN1 gene kills motor neurons in spinal muscular atrophy, describe its types and complications, and distinguish a muscle-wasting disease from a brain disease of movement control.

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Why this matters

These two diseases look alike from the end of the bed — a weak or clumsy patient — and are taught together for that reason, but they are broken in opposite places. Parkinson’s disease is a disorder of the brain’s movement control: the motor neurons and muscles are intact, but the signal that starts and smooths movement is missing. Spinal muscular atrophy is a disorder of the motor neurons themselves: the brain is unaffected and intelligence is normal, but the muscles are cut off from their command. If you can say where the break is, you can predict what the patient will keep and what will be lost, and you will not offer a Parkinson’s patient the wrong reassurance or miss the breathing risk in a child with spinal muscular atrophy.

What you need to know first

You need only everyday ideas: that the brain sends commands down the spinal cord to make muscles move, that some brain cells make a chemical messenger while others receive it, and that a muscle not used because its nerve is damaged becomes small and weak. Every term used here — dopamine, substantia nigra, bradykinesia, atrophy — is defined below, so no previous neurology course is assumed.

Words to know (5)

Dopamine: A chemical messenger made by nerve cells in a small midbrain region; it carries the signals that start and smooth voluntary movement. When dopamine falls, movement becomes slow and small even though the muscles themselves are strong.
Substantia nigra: The dark pigmented band of the midbrain that contains the dopamine-producing nerve cells; it is the structure that degenerates in Parkinson’s disease. Parkinson’s symptoms usually appear only once about half of the nerve cell activity in the substantia nigra has been lost.
Bradykinesia: Abnormally slow movement that makes ordinary tasks, such as dressing or cutting food, take far longer than they should. A patient who needs twenty minutes to button a shirt has bradykinesia, not laziness.
Motor neuron: The nerve cell that runs from the spinal cord or brainstem to a muscle and delivers the command to contract; when it dies, that muscle loses its command. In spinal muscular atrophy the motor neurons die, so the muscle is not simply weak but disconnected.
Contracture: A fixed shortening of a muscle or tendon that holds a joint in one position and cannot be stretched back by exercise alone. Children with spinal muscular atrophy develop contractures, which is why daily stretching and positioning start early.

One idea at a timeIdea 1 / 6

In one sentence Parkinson’s disease is an age-related degenerative brain condition in which dopamine-producing nerve cells in the substantia nigra are slowly lost, so movement becomes slow, stiff and shaky.

Parkinson’s begins with the loss of dopamine nerve cells in the substantia nigra

Parkinson’s disease is an age-related degenerative brain condition: parts of the brain deteriorate over years. The deterioration starts in a small midbrain region called the substantia nigra, where nerve cells produce dopamine, the chemical messenger that links the parts of the brain and nervous system which control and coordinate body movement. When those cells die or are damaged, the amount of dopamine falls, and the movement control centre can no longer work normally, so movements become slow and abnormal. The loss is slow and silent for a long time: symptoms usually begin only when about half of the nerve cell activity in the substantia nigra has been lost, which is why a person can have the disease for years before the first tremor appears. The cause of the loss is not known, and research continues; the current explanation is a combination of genetic changes and environmental factors. A minority of cases run in families because of faulty genes passed from parent to child, but inheritance of this kind is rare. Pesticides and herbicides used in farming, and traffic or industrial pollution, have been suggested as environmental contributors, although the evidence is inconclusive. For the nurse this means that no exposure in a patient’s history explains the disease on its own, and that families should not be blamed for it.

For example

A 68-year-old farmer reports that his right hand has become slow and stiff over two years; his muscles are strong on testing, but the dopamine signal that would smooth the movement is running out.

Pause and check yourself

Which cells are lost in Parkinson’s disease, in which brain region, and what messenger do they produce?

Show the answer

Dopamine-producing nerve cells of the substantia nigra, a small region of the midbrain; their loss reduces dopamine, so movement control becomes slow and abnormal.

Read detailed explanations

Why losing dopamine freezes movement: the basal ganglia circuits

Dopamine is not the command to contract a muscle; it is the permission slip that lets a chosen movement start and stops unwanted ones. The substantia nigra delivers dopamine to a set of deep grey nuclei — the striatum, globus pallidus, subthalamic nucleus and thalamus — that together form the basal ganglia, a loop which does not create movement but selects, scales and releases it. The loop has two arms. The direct pathway facilitates the movement that has been chosen, and it is switched on by dopamine acting on excitatory receptors. The indirect pathway suppresses competing movements, and it is switched off by dopamine acting on inhibitory receptors. Both arms end at the thalamus, which then passes the selected programme up to the motor cortex.

When dopamine falls, the direct arm becomes underactive and the indirect arm becomes overactive. The net effect is excessive inhibition of the thalamus, so the cortex receives a weak and delayed go-ahead: the patient has to think the movement through, and it emerges small and slow. This single imbalance explains the whole motor picture. Bradykinesia and reduced movement amplitude are the weak go-ahead. Rigidity is the persistent muscle tone that remains when the competing programmes are no longer suppressed; it differs from spasticity, which is velocity-dependent tone from an upper motor neuron lesion and produces a catch-and-give feel.

Rest tremor is thought to arise when the overactive subthalamic nucleus and globus pallidus generate an oscillatory rhythm that spreads through the loop, which is why the tremor appears when the limb is unoccupied and quietens during a deliberate task. Loss of automatic movement — the vanished arm swing, the reduced blink and the expressionless face — follows because automatic sequences depend on the same loop that is now starved of dopamine. This circuit model has three practical consequences. First, medication must be timed, because the loop can only work when dopamine is present, so a late dose is a frozen patient. Second, it explains why exercise, which uses sensory feedback and repetition to drive cortical and cerebellar routes around the damaged loop, is a treatment and not a pastime.

Third, it explains deep brain stimulation, which does not replace dopamine but electrically interrupts the overactive nodes, most often the subthalamic nucleus, so the thalamus can be released again.

From a faulty gene to a wasted muscle: the molecular cause of SMA, and what treatment can now do

The SMN1 and SMN2 genes both carry instructions for making a protein called the survival motor neuron protein, usually shortened to SMN. Normally most functional protein comes from SMN1, with only a small amount from SMN2. The two genes differ in a way that decides the disease. A single base change in SMN2 makes the cell’s splicing machinery skip one segment of the message, so SMN2 produces a little full-length functional protein and several shortened versions that are quickly broken down. Most people with SMA are missing a piece of the SMN1 gene, so production is impaired; a shortage of the protein leads to motor neuron death, and because signals are then not transmitted between the brain and the muscles, many skeletal muscles become weak and waste away.

SMN protein belongs to a group of proteins called the SMN complex, which maintains motor neurons, and motor neurons are the cells that transmit the signals from brain and spinal cord that tell skeletal muscles to contract. Two features of this mechanism drive clinical practice. The first is the copy-number rule: people typically have two copies of SMN1 and one to two copies of SMN2, but SMN2 copy number varies up to eight, and several copies mean more full-length protein, so the phenotype is usually milder and starts later. That is why SMN2 copy number is reported with the genetic result and why it cannot be read as a guarantee. The second is that the protein shortage is a dose problem, not an absolute absence, and a dose problem can be treated.

Antisense therapy with nusinersen is injected into the fluid around the spinal canal and changes the splicing of SMN2 so that more full-length protein is made; risdiplam is taken by mouth once daily and works by the same principle; gene replacement with onasemnogene abeparvovec gives a working copy of SMN1 in a single intravenous infusion for the youngest children. Outcomes depend heavily on when treatment begins, because lost motor neurons are not replaced, which is why newborn screening and early diagnosis matter so much.

Diagnosis before treatment uses a blood test for creatine kinase, which leaks from deteriorating muscle; a genetic test that identifies SMN1 problems and is reported to be 95 per cent effective at finding the altered gene; a nerve conduction study and electromyogram that measure the electrical activity of muscles and nerves; and, rarely, a muscle biopsy, which shows atrophy. The nurse’s role in these therapies is practical and continuous: these patients need their immunisations, respiratory care and nutrition managed around an intensive treatment schedule, and families need honest information about what the treatment can and cannot restore.

Because the diagnosis is confirmed by genetic testing, the condition can also be identified before birth, by amniocentesis after fourteen weeks or chorionic villus sampling from ten weeks; both carry a slight increase in the risk of miscarriage or pregnancy loss, so they are offered with counselling rather than routinely.

What the nurse does with each diagnosis: the practical difference in daily care

Because the two diseases break different links, the nursing actions they demand are almost opposite, and holding both pictures side by side prevents the commonest care errors. In Parkinson’s disease the motor neurons are alive, so the aim is to keep the available movement usable: give antiparkinsonian drugs at the prescribed hour and never let a meal round or a transport delay shift them, because the effect of each dose wears off before the next is due and an off period can leave the patient unable to turn in bed or reach a call bell. Watch for the drug-related movement complications that appear after years of treatment, the involuntary writhing movements called dyskinesias and the on–off fluctuations, and report them rather than adjusting doses.

Assess blood pressure sitting and standing, because dizziness on standing is common and combines with postural instability to cause falls. Assess speech and swallowing, since a quiet voice and a weak cough are easily missed and can precede aspiration. Remove trip hazards, avoid sudden turns, and plan physiotherapy and exercise in the best-dose window so that training is possible. In spinal muscular atrophy the motor neurons are dying, so the aim is to protect what they still supply: the breathing muscles and the swallowing muscles. That means monitoring respiratory rate, effort, cough strength and oxygen saturation, teaching airway clearance and non-invasive ventilation, and vaccinating promptly against respiratory infection, since pneumonia and respiratory failure are the leading causes of death.

It means assessing feeding and swallowing, thickening fluids or planning a feeding tube when malnutrition and dehydration appear, and recording weight. It means preventing contractures and the progression of scoliosis with daily stretching, positioning and orthoses, and protecting fragile, poorly mobile bone from fracture and hip dislocation. It means anticipating the emotional dimension honestly: intelligence is normal, so the child understands everything, and genetic counselling and prenatal options matter to the whole family. Finally both groups need the same baseline nursing discipline — skin care, bowel and bladder care, pain relief, sleep, and a plan agreed with the patient and family — because in both diseases the difference between a good and a bad month is usually made by nursing rather than by the diagnosis.

Explore the diagram

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Two weak patients, two different broken links

Follow one command for movement from the movement centre of the brain down to the muscle. In Parkinson’s disease the break is high, above the motor neuron: dopamine-producing cells of the substantia nigra die, so the movement command is generated slowly and weakly although the nerve and muscle below are still intact. In spinal muscular atrophy the break is low, inside the motor neuron: a faulty SMN1 gene kills the motor neuron in the spinal cord, so the command never arrives and the muscle wastes away although the brain is normal. Same weakness at the end of the bed, opposite explanations.

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01 / 5Substantia nigra: the dopamine source

If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.

Substantia nigra: the dopamine source

The substantia nigra is the dark pigmented band of the midbrain whose nerve cells produce dopamine, the messenger that lets a chosen movement start and keeps unwanted ones from firing. In Parkinson’s disease these cells die slowly, and symptoms usually appear only once about half of the nerve cell activity there has been lost. Nothing below the lesion is destroyed, so the patient’s strength stays surprisingly good while movement becomes slow, stiff and shaky.

Schematic, not anatomical: the pathway is drawn as a single chain with two marked break points rather than as the real branching basal ganglia and corticospinal circuits, and no structure is drawn to scale. The aim is to show where each disease breaks the chain and what that changes for the patient, not to represent neuroanatomy.

Key points to remember

Parkinson’s disease is an age-related degeneration of dopamine-producing nerve cells in the substantia nigra; symptoms usually start after about half of that activity is lost.
Its four movement signs are tremor at rest, bradykinesia, rigidity and impaired posture and balance; automatic movements, voice volume and handwriting also fade.
Diagnosis is clinical; blood tests and imaging exclude other conditions such as essential tremor, drug-induced parkinsonism and vascular parkinsonism, so a normal scan excludes nothing.
There is no cure, but levodopa and other drugs, aerobic exercise, physiotherapy, speech therapy and deep brain stimulation in selected cases keep patients moving and safe.
Spinal muscular atrophy is inherited: SMN1 mutations kill motor neurons so muscles weaken and waste, SMN2 copy number sets the severity, and the four types are told apart by age at onset from before six months to adult life.

Go deeper

Worked example

Reading two weak patients differently at the end of the bed. Patient A is a 70-year-old man who has become slow and stiff, trembles in his right hand when it is resting on his lap, and shuffles with a stooped posture. Patient B is a 9-month-old girl who is floppy, cannot sit without support, and has a weak cry with a normal, alert face. Both are described in handover as “weak”. Work out where each break lies and what it means for care.

Locate the level of the lesion. In patient A the problem is above the motor neuron: the substantia nigra has lost its dopamine-producing cells, so the command to move is present but poorly generated and poorly smoothed. In patient B the problem is in the motor neuron itself, in the spinal cord and brainstem, so the command never reaches the muscle at all.
Read the signs against that level. Patient A has a rest tremor, rigidity, bradykinesia and a stooped, unsteady posture — an excess of stiffness and slowness together, with normal or near-normal strength on testing. Patient B has floppiness, no tendon reflexes, twitching muscles and no independent sitting — a loss of tone and of reflex, which is the signature of a lost lower motor neuron.
Ask what is intact. Patient A’s muscles and motor neurons are functional, so exercise, physiotherapy and correctly timed medication can still change his movement. Patient B’s intelligence is not affected and her brain is normal, so she understands and interacts normally while her body fails — which is why talking to her and to her parents about what she can do, not only about what she cannot, is part of nursing care.
Turn the difference into priorities. For patient A the immediate risks are falls from stiffness and postural instability, the loss of an effective dose if medication is late, and the quiet voice that may hide choking. For patient B the immediate risks are aspiration and respiratory infection from weak swallowing and breathing muscles, malnutrition and dehydration, contractures and scoliosis, and fractures from fragile, immobile bone.

Both patients are weak, and the handover word hides the whole assessment. Naming the broken link — dopamine-producing cells above the motor neuron in Parkinson’s disease, the motor neuron itself in spinal muscular atrophy — tells you which organs are at risk, which treatments can still work, and what the patient and family most need to hear. This is the same habit you built in the previous sections: cause, mechanism, consequence, then nursing priority.

Common misconception

Many beginners believe that Parkinson’s disease is mainly a tremor, and that a person without a tremor cannot have it. In fact the tremor is the most visible sign, not the most important one: slowness, rigidity and postural instability are what limit walking, dressing and safety, and some patients have little or no tremor at all while being severely slowed. The opposite error is equally common with spinal muscular atrophy: because the muscles waste, it is mistaken for a muscle disease such as muscular dystrophy, and the child’s normal intelligence is not expected. In SMA the muscle is healthy but disconnected from a dying motor neuron, and intelligence is unaffected, which is exactly why the assessment looks at reflexes, tone and breathing rather than at the muscle alone.

Check your reasoning

A family asks you why their grandfather, who has Parkinson’s disease, can still move his legs but cannot start walking, and why he sometimes freezes halfway across the room. Using the mechanism you learned, explain the difference between weakness and a failure of movement control, and say what you would assess.

Reveal an answer

His motor neurons and muscles are intact, which is why he can move his legs when he is sitting or lying down and why his strength tests normally. What is missing is the dopamine signal from the substantia nigra that starts and smooths movement, so the command to walk is generated too slowly and too weakly, and he cannot initiate the movement or carry it through. Freezing halfway is the same problem: the automatic pattern of walking is not being reinforced, and it can be helped by an external cue such as a line on the floor, a metronome or counting aloud, because a deliberate cue can bypass the failing automatic route. This is not a case for saying he is “not trying” or for calling it weakness. I would assess his ability to stand and turn safely, falls in the last months, how his dose timing relates to his best and worst hours, his blood pressure on standing, his swallow and voice, his mood and sleep, and the home layout with the family, because the risk that changes fastest is a fall.

Where this comes from

MUSCULOSKELETAL SYSTEM PATHOLOGIES lecture notes (supplied), Level 200 nursing · Lines 911–939: Parkinson’s disease as an age-related degenerative brain condition; loss of nerve cells in the substantia nigra and the dopamine messenger; the onset of symptoms after about 50 per cent loss of nerve cell activity; genetic and environmental factors with inconclusive pollution evidence; the symptom list (pill-rolling rest tremor, bradykinesia, rigidity, impaired posture and balance, loss of automatic movements, speech and writing changes); diagnosis by laboratory and imaging tests to rule out other conditions; treatment by medication (dopamine agonists, MAO-B and COMT inhibitors, amantadine, anticholinergics), deep brain stimulation, exercise, physiotherapy, speech-language therapy, diet, massage, yoga and tai chi.
MUSCULOSKELETAL SYSTEM PATHOLOGIES lecture notes (supplied), Level 200 nursing · Lines 940–978: spinal muscular atrophy as an inherited disease of motor neurons in brain and spinal cord; SMN1 and the survival motor neuron protein; SMN2 copy number modifying severity; symptoms (floppy limbs, twitching, scoliosis, swallowing and breathing difficulty) with intelligence unaffected; types I–IV with ages of onset, Werdnig-Hoffmann and Kugelberg-Welander names and outcomes; diagnosis by creatine kinase, genetic testing reported as 95 per cent effective, EMG/nerve conduction and muscle biopsy, with prenatal amniocentesis after 14 weeks and chorionic villus sampling from 10 weeks; management by nusinersen, risdiplam and onasemnogene abeparvovec with therapy and assistive devices; complications of fractures, hip dislocation, scoliosis, malnutrition, dehydration, pneumonia and respiratory failure.
Beyond the supplied notes: basal ganglia circuitry and dopamine replacement, and SMA therapy labelling · the direct (facilitatory) and indirect (suppressive) basal ganglia pathways and their dopamine receptor pharmacology, the resulting thalamic over-inhibition as the mechanism of bradykinesia and rigidity, the distinction between rigidity and velocity-dependent spasticity, the oscillatory origin of rest tremor, and cueing strategies for freezing of gait; levodopa as the dopamine precursor that crosses the blood–brain barrier, the wearing-off and on–off phenomena and dyskinesias of long-term treatment, and orthostatic hypotension in Parkinson’s disease; the SMN2 splicing mechanism that yields only a small amount of full-length protein, risdiplam taken orally once daily, nusinersen approved across a wider age range than the supplied notes state, and the principle that outcomes depend on how early treatment starts. Correction: the supplied notes describe dopamine agonists as stimulating “the production of dopamine”; they act at dopamine receptors and do not increase dopamine production, and levodopa is the drug converted into dopamine.
Beyond the supplied notes: standard neurological examination of the motor system · the distinction between upper and lower motor neuron signs used throughout this section — lower motor neuron lesions cause reduced tone, lost tendon reflexes, twitching and muscle wasting, whereas upper motor neuron lesions cause increased tone and exaggerated reflexes; the term atrophy as shrinkage of tissue from loss of innervation or disuse; and the standard look-alikes of Parkinson’s disease (essential tremor, drug-induced parkinsonism from dopamine-blocking medicines, and vascular parkinsonism) with their distinguishing features.

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About this course and its sources

The musculoskeletal and neurological lessons build on the supplied Level 200 nursing notes. Reproductive, respiratory, skin and nursing-response lessons use cited external references. Diagrams are original educational schematics. Sections that go beyond the supplied notes say so in their sources. Specialist clinical and French-language editorial review remain pending.