Parkinson's Disease

Populations and Risk Factors

• Age is the strongest risk factor — rare before age 50; incidence rises sharply after age 60; mean onset age is approximately 60–65 years
• Men affected approximately 1.5:1 over women
• Environmental exposures: pesticides (rotenone, paraquat), herbicides, heavy metals, rural living, and well-water consumption are associated with increased risk
• Genetic factors: mutations in LRRK2, PARK7, PINK1, and SNCA genes account for 5–10% of cases (familial PD); first-degree relatives have a 2–3 times increased risk
• History of traumatic brain injury is a recognized risk factor
• Inverse associations reported with caffeine intake and tobacco use (epidemiological; not protective recommendations)
• Occupational factors: welding (manganese exposure), farming, industrial solvent exposure

Causes and Pathophysiology

Dopaminergic Neurodegeneration

• Substantia nigra cell death: PD is caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) — a midbrain nucleus that manufactures dopamine and projects it to the striatum (caudate nucleus and putamen) via the nigrostriatal pathway. By the time motor symptoms emerge, approximately 60–80% of these neurons have already been lost.
• Lewy bodies: The cellular hallmark of PD is the accumulation of abnormal alpha-synuclein protein aggregates (Lewy bodies) within surviving neurons. These intracellular inclusions disrupt normal cell function and are believed to contribute to progressive neuronal death. Lewy body pathology spreads in a predictable pattern from the brainstem upward (Braak staging), which accounts for the sequential emergence of non-motor symptoms (anosmia, sleep disturbance, constipation) before motor symptoms appear.

Basal Ganglia Circuitry — Why Rigidity and Bradykinesia Develop

• Normal function: The basal ganglia regulate voluntary movement through two parallel pathways. The direct pathway facilitates movement (excitatory output to the motor cortex via the thalamus). The indirect pathway inhibits unwanted movement (inhibitory output). Dopamine from the substantia nigra activates the direct pathway and suppresses the indirect pathway, creating a net facilitation of intended movement.
• PD imbalance: When dopamine is depleted, the direct pathway loses its facilitation and the indirect pathway loses its suppression. The result is excessive inhibitory output from the basal ganglia to the thalamus, which suppresses voluntary movement initiation and execution. This explains two cardinal features:
• Bradykinesia: reduced thalamic excitation to the motor cortex means movement is slow to initiate (akinesia) and slow to execute (bradykinesia); repetitive movements progressively decrease in amplitude (decrement phenomenon)
• Rigidity: the loss of inhibitory modulation from the basal ganglia results in simultaneous co-contraction of agonist and antagonist muscles; resistance to passive movement is constant throughout the entire range and is independent of velocity — the defining distinction from UMN spasticity

Rigidity — Lead-Pipe vs. Cogwheel

• Lead-pipe rigidity: smooth, constant resistance to passive movement throughout the full range of motion; the examiner feels the same resistance at the beginning, middle, and end of the range; the resistance does not change with the speed of passive movement
• Cogwheel rigidity: the same constant baseline resistance but with superimposed ratchet-like catches — a rhythmic "give-and-resist" pattern as the limb is moved passively; cogwheel rigidity occurs when resting tremor is superimposed on lead-pipe rigidity; most readily palpated at the wrist during passive flexion-extension or at the elbow during passive pronation-supination
• Rigidity vs. spasticity — key exam differentiator: rigidity is constant regardless of velocity (move the limb slowly or quickly and the resistance is the same); spasticity is velocity-dependent (fast movement produces greater resistance, slow movement produces less). This single distinction separates basal ganglia pathology (PD) from upper motor neuron pathology (MS, stroke, spinal cord injury) and is a frequently tested clinical differentiation.

Postural Reflex Loss

• Normal postural adjustments rely on automatic righting reflexes that are modulated by the basal ganglia. Dopamine depletion impairs these reflexes, producing postural instability that worsens as the disease progresses. The patient cannot correct for perturbation — a small push backward results in retropulsion (stumbling backward) rather than a corrective step.
• Postural instability is a late feature (Hoehn and Yahr Stage III and beyond) and is the primary risk factor for falls in PD.

Freezing of Gait

• Mechanism: freezing episodes occur when the basal ganglia fail to generate the internal timing cues that initiate and sustain gait cycles. The feet appear "glued to the floor" despite the patient's intention to move. Freezing is triggered by narrow doorways, turns, transitions (starting to walk, changing direction), and dual-tasking (walking while talking).
• External cueing bypasses the deficit: auditory cues (metronome, rhythmic counting) and visual cues (floor lines to step over) can temporarily restore gait because they engage cerebellar and cortical pathways that bypass the basal ganglia timing circuit. This is the physiological basis for rhythmic initiation techniques in rehabilitation and MT remedial exercise.

Autonomic Dysfunction

• Lewy body pathology extends beyond the basal ganglia into autonomic nervous system nuclei (brainstem, peripheral autonomic ganglia), producing: orthostatic hypotension (blood pressure drop on position change), constipation, urinary urgency, excessive or reduced sweating, and sialorrhea (excessive salivation). Orthostatic hypotension is directly relevant to MT — position changes during treatment can provoke dizziness, lightheadedness, or syncope.

Signs and Symptoms

Early Disease (Hoehn and Yahr Stages I–II)

• Resting tremor: typically unilateral at onset; "pill-rolling" tremor of the hand (4–6 Hz); present at rest, decreases with intentional movement; often the first symptom noticed by the patient
• Bradykinesia: slowness of voluntary movement; difficulty with fine motor tasks (buttoning, writing); micrographia (progressively smaller handwriting); reduced facial expression (hypomimia / masklike face); reduced blink rate; decreased arm swing during walking
• Rigidity: initially unilateral; stiffness and reduced range perceived by the patient; may present as shoulder pain mimicking rotator cuff pathology or frozen shoulder
• Non-motor symptoms: anosmia (loss of smell — often precedes motor symptoms by years), sleep disturbances (REM sleep behavior disorder), constipation, depression, anxiety
• Symptoms typically begin unilaterally and remain asymmetric throughout the disease course

Advanced Disease (Hoehn and Yahr Stages III–V)

• Bilateral involvement: rigidity and bradykinesia affect both sides; the initially affected side remains more severely involved
• Postural instability: impaired balance; retropulsion on perturbation; unable to recover from a push or stumble; increased fall risk — the most dangerous feature of advanced PD
• Gait changes: festinating gait (short, shuffling, accelerating steps with the trunk leaning forward as if chasing the center of gravity); freezing of gait at doorways, turns, and transitions; reduced step height; reduced arm swing bilaterally
• Stooped posture: progressive anterior trunk flexion (camptocormia); rounded shoulders; hip and knee flexion tendency; center of gravity displaced anteriorly
• Autonomic dysfunction: orthostatic hypotension (may be exacerbated by PD medications), excessive sweating or anhidrosis, constipation, urinary urgency
• Cognitive decline: executive function impairment, bradyphrenia (slowness of thought), and in late stages, Parkinson's disease dementia (distinct from Lewy body dementia, though both involve alpha-synuclein pathology)

Assessment Profile

Subjective Presentation

• Chief complaint: stiffness ("I feel stiff all the time — like I'm moving through mud"), difficulty initiating movement ("it takes me a long time to get going"), tremor at rest, balance problems, or falls; may present initially with unilateral shoulder pain or stiffness that has not responded to conventional treatment (rigidity misdiagnosed as MSK)
• Pain quality: deep muscular aching from sustained rigidity and co-contraction; cramping or dystonic pain (especially in the feet and calves); rigidity-related pain may be bilateral but is typically worse on the more affected side; pain is NOT radicular or dermatomal
• Onset: insidious and gradual; motor symptoms develop over months to years; often preceded by non-motor symptoms (sleep disturbance, constipation, anosmia) by years; tremor or unilateral stiffness is often the first motor complaint
• Aggravating factors: stress and anxiety worsen tremor; cold temperatures increase rigidity; prolonged immobility worsens stiffness (morning stiffness is common); medication "off" periods produce marked symptom worsening; dual-tasking (walking while talking) may trigger freezing episodes
• Easing factors: movement and warm-up reduce rigidity (the "warming up" phenomenon); PD medications (levodopa, dopamine agonists) substantially improve symptoms during "on" periods; warm bathing may temporarily reduce stiffness; external rhythmic cues help initiate and sustain movement
• Red flags: Sudden onset of symptoms or rapid progression (weeks to months rather than years) → not typical PD; consider drug-induced parkinsonism, progressive supranuclear palsy, or MSA — refer to neurology; falls with loss of consciousness → cardiovascular or autonomic evaluation; new cognitive changes → reassessment of diagnosis and medication review

Observation

• Local inspection: masklike face (hypomimia — reduced facial expression, reduced blink rate); resting tremor visible in the hands, sometimes the jaw or chin; micrographia if the patient writes; no muscle atrophy in early-to-mid disease (distinct from LMN conditions); may use assistive devices in advanced stages (walker, wheelchair)
• Posture: characteristic stooped posture — forward head, rounded thoracic kyphosis, flexion at the hips and knees, anterior trunk lean; shoulders protracted; lateral trunk flexion (Pisa sign) may be present; posture worsens progressively with disease stage
• Gait: festinating gait — short shuffling steps that accelerate, reduced step height, reduced or absent bilateral arm swing, en-bloc turning (the entire body turns as a unit rather than segmentally); start hesitation (difficulty initiating the first step); freezing at doorways or turns; retropulsion if perturbed

Palpation

• Tone: Rigidity — constant resistance throughout the full range of passive movement, independent of velocity. This is NOT spasticity. Move the limb slowly — resistance is present. Move it quickly — resistance is the same. Lead-pipe rigidity is smooth and uniform. Cogwheel rigidity is the same constant resistance with superimposed ratchet-like catches (tremor component). Most readily assessed at the wrist (passive flexion-extension) and elbow (passive pronation-supination). Bilateral but asymmetric — the initially affected side is typically more rigid. Activation maneuver: have the patient perform a repetitive movement with the contralateral hand (e.g., open and close the fist) while you assess tone in the target limb — this may unmask subtle rigidity.
• Tenderness: deep muscular tenderness in chronically rigid muscles — cervical extensors, upper trapezius, thoracic and lumbar paraspinals, hip flexors, hamstrings, and calf muscles from sustained co-contraction; tenderness is bilateral but typically worse on the more affected side; NOT dermatomal or following a referred path (no radicular component); no trigger point referral pattern characteristic — pain is from sustained contraction and ischemia, not from myofascial trigger points per se
• Temperature: typically normal; affected limbs may feel slightly cool from reduced movement and autonomic dysfunction (vasomotor dysregulation); no warmth indicative of active inflammation; warm hydrotherapy is generally well tolerated and may reduce rigidity (contrast with MS where heat is contraindicated)
• Tissue quality: rigid muscles palpate as dense and poorly extensible but NOT fibrotic — the resistance is neurological (sustained contraction), not structural (scar tissue); tissue quality may improve temporarily with sustained pressure, rhythmic mobilization, or post-medication during "on" periods; reduced fascial mobility in chronically rigid segments; no muscle wasting in early-to-mid disease (normal bulk with increased tone)

Motion Assessment

• AROM: bradykinesia is the dominant finding — movements are slow to initiate and slow to execute; range may be near-normal in early disease but performed at reduced speed with reduced amplitude; repetitive movements show progressive decrement (each repetition smaller and slower than the last — finger tapping test demonstrates this); facial AROM reduced (hypomimia); trunk rotation markedly reduced; arm swing absent or diminished during gait
• PROM / end-feel: constant resistance throughout the entire range (rigidity) — end-feel is muscular but abnormal; resistance does NOT increase with velocity (critical velocity test: perform passive movement slowly, then quickly — if resistance is the same, this is rigidity; if resistance increases with speed, this is spasticity). PROM range may be near-normal despite significant rigidity — the limb CAN be moved through full range with steady pressure, but the resistance never releases. Cogwheel ratcheting is most evident during slow passive movement at the wrist and elbow.
• Resisted testing: strength is typically normal or near-normal in early disease — PD is not a weakness-producing condition; bradykinesia may mimic weakness (slow voluntary contraction mistaken for reduced force production); in advanced disease, deconditioning may reduce strength secondarily; no myotomal pattern of weakness (distinguishes from radiculopathy or LMN conditions)

Special Test Cluster

Rigidity vs. spasticity velocity test — critical clinical reasoning:

| Feature | Rigidity (PD) | Spasticity (UMN — MS, stroke) | |––––-|–––––––-|–––––––––––––––-| | Resistance | Constant throughout range | Velocity-dependent — increases with speed | | Type | Lead-pipe (smooth) or cogwheel (ratcheting) | Clasp-knife (initial catch, then release) | | DTRs | Normal (2+) | Hyperreflexia (3+/4+), clonus | | Babinski | Negative | Positive | | Pathology | Basal ganglia (extrapyramidal) | Corticospinal tract (pyramidal) |

Differential Diagnoses