What could be causing internal tremor feelings, decreased endurance, and blurred vision?

Differential Diagnosis: Orthostatic Intolerance vs. Parkinson's Disease

Your symptom triad of internal tremors, decreased endurance, and blurred vision most likely represents orthostatic intolerance, specifically Postural Orthostatic Tachycardia Syndrome (POTS), though early Parkinson's disease or essential tremor with autonomic dysfunction must also be considered.

Primary Diagnostic Consideration: Orthostatic Intolerance/POTS

Key Clinical Features

• Internal tremors with blurred vision and decreased endurance strongly suggest orthostatic intolerance, which characteristically presents with tremulousness, generalized weakness, blurred vision, exercise intolerance, and fatigue upon standing 1
• POTS specifically manifests with lightheadedness, palpitations, tremor, generalized weakness, and blurred vision that develop upon standing and are relieved by sitting or lying 1
• These symptoms are typically worse in the morning, with heat exposure, and after meals or exertion 1

Immediate Diagnostic Steps

• Perform an active standing test immediately: Measure heart rate and blood pressure supine, then at 1,3,5, and 10 minutes of standing 2
  • POTS diagnosis requires heart rate increase ≥40 bpm within 10 minutes (if age 12-19) or ≥30 bpm (if age ≥20) without significant blood pressure drop 1, 2
  • Classic orthostatic hypotension shows systolic BP drop ≥20 mmHg or diastolic BP drop ≥10 mmHg within 3 minutes 1
  • Critical pitfall: Don't measure standing vitals for only 1-2 minutes, as delayed orthostatic hypotension may be missed 2

Essential Laboratory Workup

• Complete blood count to evaluate for anemia contributing to decreased endurance 2
• Comprehensive metabolic panel for electrolytes, renal function, and glucose 2
• Thyroid function tests (TSH, free T4) to rule out thyroid dysfunction 2
• Iron studies (ferritin, serum iron, TIBC, transferrin saturation) as iron deficiency commonly contributes to orthostatic intolerance 2
• Inflammatory markers (ESR, CRP) if systemic illness suspected 2

Cardiovascular Assessment

• 12-lead ECG to evaluate for cardiac conduction abnormalities 2
• Consider 24-hour Holter monitoring if palpitations or tachycardia episodes are prominent 2
• Tilt-table testing if active standing test is inconclusive but symptoms strongly suggest orthostatic intolerance 1, 2

Secondary Consideration: Early Parkinson's Disease

Distinguishing Features

• Parkinson's disease presents with resting tremor (not internal tremors), bradykinesia, and rigidity as cardinal features 1, 3
• Symptoms manifest after approximately 40-50% of dopaminergic neurons have been lost, typically with a 5-year interval between initial neuronal loss and symptom appearance 1, 3
• Blurred vision in Parkinson's relates to slow saccades and ocular motor dysfunction, not orthostatic changes 1
• Decreased endurance in Parkinson's results from increased energy expenditure due to dyskinesias and rigidity, not orthostatic intolerance 1, 3

Red Flags for Parkinsonism

• Progressive symptoms over months to years 1
• Asymmetric onset of motor symptoms 1
• Response to levodopa trial 1
• Associated autonomic dysfunction, behavioral changes, or cognitive impairment 1

Tertiary Consideration: Essential Tremor

Clinical Characteristics

• Essential tremor presents as action or postural tremor, not internal tremors 4, 5, 6
• Tremor involves rhythmic oscillations of agonist and antagonist muscles, visible externally 6, 7
• Does not typically cause blurred vision or decreased endurance unless severe 5, 7
• May be temporarily alleviated by alcohol consumption 6

Special Consideration: Post-Viral/Long COVID Syndrome

• Internal tremors and vibrations have been described in long COVID, associated with worse quality of life and higher rates of new-onset neurologic conditions 8
• Consider this diagnosis if symptoms began after COVID-19 infection 8
• These patients often have multiple associated symptoms and worse health status 8

Critical Clinical Pitfalls to Avoid

• Don't dismiss these symptoms as "just anxiety" or "just dehydration" without objective testing 2
• Don't confuse internal tremors with visible tremor disorders like essential tremor or Parkinson's disease 4, 8, 7
• Don't stop standing vital signs at 2 minutes—delayed orthostatic changes require 10-minute monitoring 2
• Don't overlook medication review—diuretics, vasodilators, and sedatives commonly cause orthostatic symptoms 1

Immediate Management Considerations

If Orthostatic Intolerance Confirmed

• Increase salt and fluid intake (8-10 grams sodium daily, 2-3 liters fluid) unless contraindicated by cardiac dysfunction, hypertension, or kidney disease 1
• Review and reduce medications that may cause hypotension (diuretics, vasodilators, sedatives) 1
• Physical countermaneuvers: leg crossing, muscle tensing, compression garments 1

If Neuroimaging Indicated

• Brain MRI if headaches have orthostatic features to rule out intracranial hypotension 2
• Consider MRI if atypical features suggest Parkinsonism or other neurodegenerative disease 1