What is the most likely diagnosis in a 70-year-old man with a resting tremor that improves with voluntary movement, masked facial expression, shuffling gait, and difficulty rising from a chair, with otherwise normal neurological examination?

Most Likely Diagnosis: Parkinson's Disease

The most likely diagnosis is Parkinson's disease (PD), based on the classic tetrad of resting tremor that improves with voluntary movement, masked facies (facial immobility), shuffling gait, and difficulty rising from a chair—all hallmark features of this neurodegenerative disorder 1.

Clinical Reasoning

This 70-year-old man presents with the cardinal motor features of PD:

• Resting tremor that improves with voluntary movement: This is pathognomonic for PD and distinguishes it from essential tremor (which worsens with action) 1, 2
• Masked facies (facial immobility): Reflects bradykinesia affecting facial muscles
• Shuffling gait: A highly specific finding with positive likelihood ratios ranging from 3.3 to 15 3
• Difficulty rising from a chair: Another characteristic feature with positive likelihood ratios of 1.9 to 5.2 3

The age of onset (70 years) falls within the typical peak incidence range of 60-70 years for PD 1. The clinical presentation aligns with the diagnostic criteria requiring bradykinesia plus at least one of: rest tremor, rigidity, or both 4.

Key Diagnostic Features

The combination of symptoms provides strong diagnostic evidence:

• Rigidity and bradykinesia together: Positive likelihood ratio of 4.5 3
• Shuffling gait: Highly specific for PD 3
• Resting tremor that improves with movement: Classic for PD, distinguishing it from action tremors 5, 2

The mention of "some of the scalp" (likely seborrheic dermatitis) is consistent with PD, as autonomic dysfunction commonly accompanies the motor syndrome 1.

Differential Considerations

While the presentation strongly suggests PD, be aware of:

• Atypical Parkinsonisms (PSP, MSA, CBD): These typically present with additional "red flag" features not mentioned here, such as early falls (PSP), prominent autonomic dysfunction (MSA), or asymmetric limb apraxia (CBD) 1
• Essential tremor: Would present with action/postural tremor, not resting tremor that improves with movement 5
• Drug-induced Parkinsonism: Always review medications, particularly antipsychotics and antiemetics 5

Clinical Pitfalls to Avoid

1. Don't rely solely on tremor: Up to 21% of PD patients may not have tremor initially 3
2. Assess glabellar tap reflex: Positive likelihood ratio of 4.5 for PD 3
3. Test heel-to-toe walking: Difficulty has a positive likelihood ratio of 2.9 3
4. Look for asymmetry: PD typically begins unilaterally before spreading 6

Next Steps

The diagnosis of PD remains primarily clinical 4. Structural imaging (MRI) is typically normal in PD but useful to exclude alternative diagnoses like vascular Parkinsonism or structural lesions 1. DaT-SPECT imaging can confirm dopaminergic denervation if the diagnosis remains uncertain, though it's not routinely necessary with classic presentation 7.

Initiate dopaminergic therapy (levodopa) as both diagnostic and therapeutic: Response to levodopa strongly supports the diagnosis and is the most effective symptomatic treatment 6.