What is the cause and prevention of dizziness and lightheadedness in a pregnant woman when lying on her back with no significant past medical history and normal fetal investigations?

Supine Hypotension Syndrome in Pregnancy

D - Supine hypotension syndrome, lay down on the left side is the correct answer. This pregnant woman is experiencing supine hypotension syndrome (also called aortocaval compression syndrome), caused by the gravid uterus compressing the inferior vena cava when lying supine, and the treatment is immediate position change to the left lateral decubitus position 1, 2.

Pathophysiology and Mechanism

The enlarged gravid uterus compresses the inferior vena cava when the patient lies supine, particularly after 20 weeks of gestation, decreasing venous return to the heart by up to 85% at its origin and reducing cardiac output by approximately 16-24% 2, 3. This compression leads to an abrupt decrease in cardiac preload, which causes hypotension with weakness and lightheadedness 1.

• The hemodynamic changes are dramatic: inferior vena cava blood flow decreases by 85.3% at its origin and 44.4% at the level of the renal veins when supine 3
• Cardiac output drops by 16.4-24% in the supine position compared to left lateral positioning 2, 3
• While the azygos venous system attempts compensation by increasing flow by 220%, this collateral circulation is insufficient to prevent symptoms in susceptible women 3

Clinical Presentation and Diagnosis

The diagnosis is confirmed by immediate symptom resolution with position change - when the patient sits up or turns to her side, the symptoms rapidly disappear as inferior vena cava compression is relieved and venous return is restored 1, 2.

• Symptoms include dizziness, lightheadedness, weakness, nausea, and in severe cases, syncope 1, 2
• This typically occurs after 20 weeks gestation when the uterus is large enough to cause significant compression 2
• The normal fetal investigations and lack of significant past medical history make other pathological causes unlikely 2

Management and Prevention

Pregnant patients after 20 weeks gestation should avoid prolonged supine positioning during procedures, examinations, or sleep 2. The left lateral position or left pelvic tilt should be maintained during any prolonged recumbent positioning 2.

• Immediate treatment: Turn the patient to the left lateral decubitus position 1, 2, 4
• The left lateral position relieves aortocaval compression and restores venous return within seconds to minutes 2, 4
• For procedures requiring supine positioning, use a left pelvic tilt (wedge under right hip) to displace the uterus off the vena cava 2
• Pregnant women should be counseled to sleep on their left side, especially in the third trimester 2

Why the Other Options Are Incorrect

Option A (Anemia with iron supplementation) is incorrect because:

• While physiologic anemia of pregnancy can cause fatigue and lightheadedness, it does not cause positional symptoms that resolve immediately upon sitting up 5
• Anemia symptoms are constant and not triggered specifically by lying supine 5
• The immediate relief with position change is pathognomonic for supine hypotension syndrome, not anemia 2

Option B (Dehydration with hydration) is incorrect because:

• Dehydration causes orthostatic hypotension when moving from lying to standing, not symptoms specifically when lying flat 1
• The patient's symptoms improve with sitting up, which is opposite to what would occur with dehydration-related orthostatic hypotension 2

Option C (Orthostatic hypertension with increased salt intake) is incorrect because:

• This option contains a fundamental error - "orthostatic hypertension" is not a recognized clinical entity causing dizziness 1
• Orthostatic hypotension causes blood pressure to drop when moving from lying to sitting/standing positions, whereas this patient's symptoms improve when sitting up, indicating the opposite mechanism 1, 2
• The supine position is what triggers symptoms, not the upright position 2

Common Pitfalls to Avoid

• Do not dismiss these symptoms as "normal pregnancy complaints" - while supine hypotension syndrome is physiologic, it requires specific management and patient education 2
• Do not confuse this with orthostatic hypotension - the key distinguishing feature is that symptoms occur when lying flat and improve when sitting up, which is the opposite pattern of orthostatic hypotension 1, 2
• Do not delay position change while investigating other causes - immediate relief with left lateral positioning both treats the condition and confirms the diagnosis 2, 4
• Ensure all healthcare providers caring for pregnant women after 20 weeks are aware - this includes radiology technicians, physical therapists, and dental professionals who may position patients supine for procedures 2