Could the mild pitting edema on the dorsum of both feet, medial ankles, and left shin be lymphedema secondary to prior prostate radiation rather than cardiac congestion, given my low absolute lymphocyte count and mild obstructive sleep apnea with possible nocturnal hypertension?

Distinguishing Lymphedema from Cardiac Edema After Prostate Radiation

Your bilateral lower extremity edema is most likely cardiac or multifactorial in origin rather than lymphedema, given the bilateral pitting pattern, distribution on dorsal feet and medial ankles, and presence of sleep apnea—however, prostate radiation does create lymphedema risk that warrants specific evaluation.

Key Diagnostic Distinctions

Clinical Features Favoring Cardiac/Systemic Edema in Your Case

• Bilateral pitting edema strongly suggests increased capillary hydrostatic pressure from cardiac, renal, or other systemic causes rather than lymphatic obstruction 1, 2
• Distribution pattern: Dorsal feet, medial ankles, and shin involvement with pitting is the classic presentation of dependent edema from cardiac congestion or venous insufficiency 1
• Sleep apnea connection: Obstructive sleep apnea can directly cause bilateral leg edema even without overt pulmonary hypertension, making this a highly relevant contributing factor in your case 3
• Response to elevation: If your edema improves with leg elevation or worsens throughout the day, this favors cardiac/venous causes over lymphedema 4

Clinical Features That Would Suggest Lymphedema Instead

• Stemmer sign: The inability to pinch and lift the skin fold at the base of the second toe indicates lymphedema rather than other causes of edema 1, 5
• Unilateral presentation: Lymphedema from pelvic radiation or lymph node dissection typically presents asymmetrically, though bilateral cases can occur 1, 5
• Non-pitting quality: Lymphedema characteristically becomes brawny and non-pitting as it progresses, unlike the pitting edema you describe 4, 3
• Lack of response to diuretics or elevation: Lymphedema does not improve with these interventions 4

Prostate Cancer Treatment and Lymphedema Risk

Your Specific Risk Profile

• Radiation therapy alone carries a relatively low risk of lower limb lymphedema (0-9%) and genital lymphedema (0-8%) 6
• Highest risk scenario (18-29% lower limb, 2-22% genital lymphedema) occurs when patients receive pelvic radiation after pelvic lymph node dissection 6
• Timing: Lymphedema may develop immediately after treatment or years later, so your 7-month post-radiation timeframe is plausible for either early or delayed onset 1, 4

Low Lymphocyte Count Consideration

Your persistently low absolute lymphocyte count is likely a radiation effect on bone marrow and lymphoid tissue rather than a direct indicator of lymphedema. Lymphedema results from structural lymphatic damage causing fluid accumulation, not from low circulating lymphocyte counts 5, 4.

Recommended Diagnostic Approach

Immediate Physical Examination Steps

1. Perform Stemmer sign test: Attempt to pinch and lift the skin at the base of your second toe on both feet. If you cannot lift the skin compared to normal, this suggests lymphedema 1, 5

2. Assess edema characteristics:

   • Does it pit when you press firmly for 5 seconds? (Pitting favors cardiac/venous) 1
   • Is the skin thickened or fibrotic? (Suggests lymphedema) 4, 3
   • Is it symmetric or asymmetric? (Asymmetric favors lymphedema) 5, 6

3. Check for cardiac signs: Jugular venous distention, S3 gallop, pulmonary rales, orthopnea, or paroxysmal nocturnal dyspnea would indicate heart failure 1

Laboratory and Imaging Evaluation

• BNP or NT-proBNP: Obtain to assess for heart failure. Levels >500 pg/mL (BNP) or >2000 pg/mL (NT-proBNP) suggest acute decompensated heart failure 1
• Echocardiogram: Essential to evaluate for systolic or diastolic dysfunction, valvular disease, or elevated filling pressures (E/e' >9) 1
• Ambulatory blood pressure monitoring (ABPM): Proceed with your plan to evaluate nocturnal hypertension from sleep apnea, as this is a highly relevant potential cause 3
• Venous duplex ultrasound: Consider if venous insufficiency is suspected, particularly given the medial ankle involvement 3

If Lymphedema Remains Suspected

• Referral to lymphedema specialist: Physical therapist, occupational therapist, or lymphedema specialist for formal evaluation 1, 4
• Lymphoscintigraphy: Gold standard imaging if clinical suspicion remains high despite negative Stemmer sign 1

Management Priorities Based on Most Likely Diagnosis

If Cardiac/Sleep Apnea Related (Most Likely)

• Optimize sleep apnea treatment: CPAP therapy can directly reduce lower extremity edema 3
• Control nocturnal hypertension: ABPM will guide antihypertensive therapy timing 1
• Diuretic therapy: Loop diuretics (furosemide) are appropriate for cardiac edema and will be effective if this is the cause 1, 4
• Compression stockings: May provide symptomatic relief for venous insufficiency component 3

If Lymphedema Confirmed

• Complete decongestive therapy (CDT): Manual lymphatic drainage, compression therapy, exercise, and skin care 4
• Avoid diuretics: These are ineffective and physiologically inappropriate for lymphedema 4
• Weight management: Critical for both prevention and treatment if overweight 1, 5
• Infection prevention: Meticulous skin care to prevent cellulitis, which can worsen lymphedema 1, 4

Critical Clinical Pitfall to Avoid

Do not assume all post-radiation edema is lymphedema. The bilateral pitting pattern, sleep apnea history, and distribution strongly suggest cardiac or multifactorial causes that require entirely different management than lymphedema 1, 4. Using diuretics for true lymphedema would be ineffective, but withholding them for cardiac edema would be harmful 1, 4.

Your next step should be checking the Stemmer sign yourself, obtaining BNP and echocardiogram, and proceeding with ABPM as planned to evaluate the sleep apnea-hypertension connection 1, 3.