What are the diagnostic criteria and initial treatment for Parkinson's disease?

Diagnosis of Parkinson's Disease

Diagnostic Criteria

Parkinson's disease is diagnosed clinically when bradykinesia (slowness of movement) is present along with at least one of the following: resting tremor or rigidity. 1, 2, 3

Essential Clinical Features

• Bradykinesia is mandatory for diagnosis and must be accompanied by either resting tremor, rigidity, or both 1, 4
• Postural instability is a cardinal feature but typically appears later in disease progression and is not required for initial diagnosis 1
• Symptoms typically manifest only after approximately 40-50% of dopaminergic neurons in the substantia nigra have been lost, usually about 5 years after neurodegeneration begins 1, 5

Physical Examination Techniques

To assess rigidity properly: passively move the patient's limbs while instructing complete relaxation, testing resistance throughout the full range of motion at varying speeds in both upper and lower extremities 1

• Look for "lead-pipe rigidity" (constant resistance) or "cogwheel phenomenon" (ratchet-like jerky resistance when rigidity combines with tremor) 1
• Use activation maneuvers (e.g., opening/closing the contralateral hand) to enhance detection of subtle rigidity 1
• Compare sides for asymmetry, as this is characteristic of Parkinson's disease 1

Common pitfall: Failure to have the patient completely relax leads to false positives from voluntary muscle contraction; do not confuse spasticity (velocity-dependent) with rigidity (constant throughout movement) 1

Prodromal Features to Elicit in History

• REM sleep behavior disorder 2
• Hyposmia (reduced sense of smell) 2
• Constipation 2, 3
• Micrographia (small handwriting) - positive likelihood ratio 2.8 to 5.9 6
• Shuffling gait - positive likelihood ratio 3.3 to 15 6
• Difficulty with specific tasks: turning in bed (positive LR 13), opening jars (positive LR 6.1), rising from a chair (positive LR 1.9 to 5.2) 6

Useful Clinical Signs

• Glabella tap test (positive likelihood ratio 4.5, negative likelihood ratio 0.13) 6
• Difficulty walking heel-to-toe (positive likelihood ratio 2.9) 6
• Combined history of rigidity and bradykinesia (positive likelihood ratio 4.5) 6

Red Flags Suggesting Alternative Diagnoses

Watch for these features that indicate atypical parkinsonism rather than Parkinson's disease:

• Vertical gaze palsy, especially downward → suggests Progressive Supranuclear Palsy 1
• Asymmetric rigidity with alien hand phenomenon → suggests Corticobasal Syndrome 1
• Ataxia → suggests Multiple System Atrophy 1
• Early prominent postural instability, poor response to levodopa, or rapid progression → consider atypical parkinsonian syndromes 2

Diagnostic Imaging

When clinical presentation is unclear, I-123 ioflupane SPECT/CT (DaTscan) is the first-line imaging modality to differentiate Parkinsonian syndromes from essential tremor or drug-induced tremor 1, 7, 8

• A normal DaTscan essentially excludes Parkinsonian syndromes 1, 7, 8
• DaTscan shows decreased radiotracer uptake in the striatum (beginning in putamen, progressing to caudate) in Parkinson's disease 1
• Sensitivity for Parkinson's disease ranges from 77-97% and specificity from 74-96% across readers 8

Alternative Imaging

• MRI brain without contrast is optimal when structural causes need exclusion, though often normal in early Parkinson's disease 1, 7
• FDG-PET/CT can discriminate Progressive Supranuclear Palsy from Parkinson's disease based on metabolic patterns 7
• CT has limited utility due to poor soft tissue contrast 1

Histopathological Confirmation

• Parkinson's disease is a synucleinopathy characterized by Lewy bodies (intracellular alpha-synuclein inclusions in neurons) 5
• Neuropathologic confirmation is required for "Definite" Parkinson's disease diagnosis; clinical criteria alone establish "Probable" or "Possible" Parkinson's disease 4
• This distinguishes Parkinson's disease from Multiple System Atrophy (alpha-synuclein in oligodendroglia) and Alzheimer's disease (amyloid plaques and tau tangles) 5

Disease Assessment

Use the Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) as the standard clinical assessment tool for disease severity, which evaluates mentation, activities of daily living, motor examination, and treatment complications 1

• Monitor nutritional status throughout disease course, as 15% of community-dwelling patients are malnourished and 24% are at medium-high nutritional risk 1
• Assess body weight changes, vitamin status, dysphagia, and functional status regularly 1

Initial Treatment

Levodopa (combined with carbidopa) is the most effective first-line symptomatic treatment and should be initiated when motor symptoms cause functional disability 9, 2, 3, 10

Pharmacologic Approach

• Carbidopa-levodopa remains the gold standard: carbidopa inhibits peripheral decarboxylation of levodopa, reducing the required levodopa dose by approximately 75% and decreasing nausea/vomiting 9
• The plasma half-life of levodopa increases from 50 minutes to approximately 1.5 hours when combined with carbidopa 9
• There is no reason to postpone symptomatic treatment in patients developing disability 3
• Alternative first-line options include dopamine agonists, MAO-B inhibitors, or anticholinergics, though levodopa is most effective 2, 10

Disease Subtypes and Prognosis

• Mild motor-predominant subtype (49-53% of patients): mild symptoms, good response to dopaminergic medications, slower progression 2
• Diffuse malignant subtype (9-16% of patients): prominent early motor and nonmotor symptoms, poor medication response, faster progression 2
• Intermediate subtype: remaining patients fall between these extremes 2

Nonmotor Symptom Management

• Depression/anxiety: selective serotonin reuptake inhibitors 2
• Cognitive impairment: cholinesterase inhibitors 2
• Constipation, sleep disorders, and other nonmotor symptoms require nondopaminergic approaches 2

Nonpharmacologic Interventions

• Exercise and rehabilitative therapy (physical, occupational, speech) complement pharmacologic treatment and should be initiated at diagnosis 2, 3
• Multidisciplinary team approach is essential for optimal management 3

Advanced Therapies

For patients developing motor complications (off periods, medication-resistant tremor, dyskinesias):

• Deep brain stimulation 2
• Levodopa-carbidopa enteral suspension 2

Critical note: No disease-modifying treatments are currently available; all treatment is symptomatic 2, 3