What is neurogenic orthostatic hypotension (NOH)?

Neurogenic Orthostatic Hypotension: Definition and Characteristics

Neurogenic orthostatic hypotension (NOH) is a subtype of orthostatic hypotension caused by dysfunction of the autonomic nervous system affecting central or peripheral autonomic nerves, resulting in impaired vasoconstriction upon standing due to defective norepinephrine release from sympathetic terminals. 1

Diagnostic Criteria

NOH is characterized by:

• A sustained reduction in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of assuming upright posture 1
• In patients with supine hypertension, a systolic BP drop ≥30 mmHg should be considered significant 2
• The BP drop is due to autonomic nervous system dysfunction rather than environmental factors like dehydration or medications 1

Pathophysiology

NOH occurs due to:

• Failure of noradrenergic neurotransmission in the autonomic nervous system 3
• Defective activation of sympathetic nerves leading to reduced norepinephrine release upon standing 4
• Impaired vasoconstriction and inadequate cardiac output causing excessive BP drop and inadequate perfusion 5

Underlying Causes

NOH is associated with:

Primary Autonomic Failure

• Parkinson's disease
• Multiple system atrophy
• Pure autonomic failure
• Dementia with Lewy bodies 1, 6

Secondary Autonomic Failure

• Diabetes
• Amyloidosis
• Spinal cord injuries
• Autoimmune autonomic neuropathy
• Paraneoplastic autonomic neuropathy
• Kidney failure 1
• Dopamine beta-hydroxylase deficiency
• Non-diabetic autonomic neuropathy 6

Clinical Manifestations

Common symptoms include:

• Dizziness and lightheadedness
• Visual disturbances (tunnel vision, graying out)
• Weakness and fatigue
• Palpitations
• Tremulousness
• Blurred vision
• Exercise intolerance
• Feeling of impending faint or "about to black out" 2, 6

Distinguishing Features from Other Forms of OH

Unlike other forms of orthostatic hypotension:

1. NOH vs. Classic OH: NOH specifically involves autonomic nervous system dysfunction rather than environmental or medication-related causes 1

2. NOH vs. Initial OH: Initial OH occurs within 15 seconds of standing and resolves quickly (within 40 seconds), while NOH is sustained 1

3. NOH vs. Delayed OH: Both involve autonomic dysfunction, but delayed OH takes >3 minutes to develop, while classic NOH occurs within 3 minutes 1

Clinical Significance

NOH is associated with:

• Increased morbidity and mortality 4
• Cognitive impairment 5
• Increased risk of syncope and falls 5
• Reduced quality of life 7
• Approximately 50% of patients have associated supine hypertension, complicating treatment 8

Common Pitfalls in Diagnosis

• Failing to measure BP beyond 3 minutes can miss delayed OH, which is common in Parkinson's disease 2
• Overlooking the contribution of medications (especially in Parkinson's disease) to orthostatic hypotension 2
• Not accounting for supine hypertension when assessing the significance of BP drops 2
• Failing to distinguish between neurogenic and non-neurogenic causes of OH 2

NOH represents a significant clinical challenge requiring careful diagnosis and management to improve symptoms, prevent complications, and enhance quality of life in affected individuals.