Recurrent Laryngeal Nerve (RLN) and Parathyroid Identification & Preservation During Thyroid Surgery

PART 1: RECURRENT LARYNGEAL NERVE (RLN)

A. Anatomy - Why it "Recurs"

• Loops under the ligamentum arteriosum at the aortic arch (deeper in the chest)
• Travels in a straight, deep course in the tracheoesophageal groove
• More consistent and predictable position

• Loops under the right carotid-subclavian artery junction (higher up)
• Enters the neck at a more oblique, anterolateral angle - more variable
• Lies slightly more anterior and superficial than the left

B. Functional Importance

C. Surgical Landmarks for RLN Identification

• Low in the neck, the RLN is first found in the angle between the trachea and the inferior thyroid artery
• Identify it here first - it is safer, away from the thyroid
• Then follow it cephalad toward the gland

• A posterior projection of thyroid tissue on the posterior surface of the thyroid lobe
• The RLN lies just posteromedial to it
• One of the most reliable landmarks - find the tubercle, find the nerve

• The RLN crosses the ITA near the cricoid cartilage
• In ~70% of cases, the ITA lies anterior to the RLN
• The nerve can be found posterior to, anterior to, or between branches of the ITA - always identify before ligating

• Inferior parathyroid: RLN lies posteromedial to it
• Superior parathyroid: RLN lies inferolateral to it
• So if you find a parathyroid gland, the RLN is nearby

• At the level of the cricoid cartilage, the nerve has an intimate relationship with the Berry ligament (posterior suspensory ligament of thyroid to trachea)
• The RLN typically curves anteriorly toward the ligament before diving posteriorly to enter the larynx
• This is where the nerve is closest to the thyroid - most dangerous zone

D. Step-by-Step RLN Exposure Technique

1. Medialize the thyroid lobe - retract it anteromedially with finger, retractor, or clamp
2. Excise the investing fascia in a craniocaudal direction to expose the posterior aspect
3. Identify ITA and trachea - RLN is at their bisected angle, low in the neck
4. Dissect the nerve cephalad - follow it toward the gland, always keeping it in view
5. Identify Tubercle of Zuckerkandl - nerve lies just behind it
6. Carefully dissect around Berry ligament - most dangerous step; the nerve often curves anteriorly here
7. Do NOT apply traction or thermal energy near the nerve at any step

• Dissect bluntly near the nerve
• Do not stretch or pull the nerve
• Don't use monopolar cautery near it (bipolar only, if at all)

E. Non-Recurrent Laryngeal Nerve (NRLN) - An Exam Favourite

• Occurs on the right side in ~1% of people
• Associated with aberrant right subclavian artery (arteria lusoria) arising directly from the aortic arch, crossing behind the esophagus
• Because the normal loop-down never happens embryologically, the nerve takes a direct, lateral, horizontal course from the vagus to the larynx
• If you are looking for the RLN in the TE groove and cannot find it - suspect NRLN
• A CT scan showing aberrant vascular anatomy should raise preoperative suspicion

F. RLN Branching

• The RLN may branch before entering the larynx in 20-30% of cases
• The anterior branches are predominantly motor - most important to preserve
• Always preserve ALL branches

G. Intraoperative Neuromonitoring (IONM)

• EMG-based monitoring: endotracheal tube with surface electrodes on the vocal cords monitors muscle activity when the nerve is stimulated
• A positive signal = nerve is intact and responsive
• Loss of signal = potential nerve injury
• Does not replace visual identification - it supplements it
• Particularly useful in: reoperation, large goiters, thyroid cancer with posterior extension
• Evidence shows routine identification alone is as effective as identification + IONM in experienced hands, but IONM reduces anxiety and aids teaching

PART 2: PARATHYROID GLAND IDENTIFICATION AND PRESERVATION

A. Anatomy Recap - Why It's Hard

• 80% of people have exactly 4 glands; 10%+ have more than 4
• Superior glands are more predictable - at the level of the cricoid, medial to the RLN-ITA intersection
• Inferior glands are highly variable - can be found anywhere from the lower thyroid pole to the mediastinum along the thyrothymic ligament
• A useful rule: the position on one side mirrors the other side - use contralateral gland as a guide

B. How to Identify Parathyroid Glands

• Slightly browner / caramel color compared to the surrounding yellow fat
• With trauma, they become mahogany colored (sign of devascularization)
• They are small oval nodules on the posterior thyroid surface
• Distinguish from fat (yellow, less structured), lymph nodes (paler, firmer), and thyroid tissue (darker red)

• Ideally identify parathyroids before the thyroid is fully medialized
• Certainly identify before ligating the ITA (main blood supply)

1. Methylene blue - injected IV; parathyroids take up less dye than thyroid, making them lighter against the stained background (now rarely used due to neurotoxicity)
2. Nanocarbon suspension - lymphatic tracer that stains thyroid and lymph nodes black; parathyroids remain unstained (light) = "negative opacification" method
3. Near-infrared autofluorescence (NIRAF) - newest technique; parathyroids emit a strong autofluorescent signal under near-infrared light, allowing real-time identification without any dye

C. Technique of Parathyroid Preservation

1. Identify early - before clamping or dividing anything on the posterior thyroid
2. Do NOT grasp the gland directly - grasp the surrounding fat/connective tissue for retraction; parathyroids are fragile and bruise easily
3. Preserve the lateral vascular pedicle - the blood supply enters from the lateral side via a branch of the ITA. Do NOT ligate the ITA trunk proximally - ligate branches close to the thyroid capsule instead ("capsular ligation")
4. Sweep the gland laterally - carefully divide the thin attachment between parathyroid and thyroid using bipolar cautery, then mobilize the gland laterally with its pedicle intact
5. Leave a broad lateral attachment - this ensures vascularity is maintained
6. After mobilization, check viability - a healthy parathyroid is caramel-tan; a dark purple/mahogany gland is devascularized

D. When Parathyroid is Inadvertently Removed or Devascularized - Autotransplantation

• Any parathyroid found on the surgical specimen
• Any gland that appears dark and non-viable after mobilization (confirm it's not just a lymph node - send a sliver for frozen section)

1. Confirm it is parathyroid tissue (frozen section if in doubt)
2. Mince the gland into small 1-mm³ fragments
3. Place into multiple pockets in the ipsilateral sternocleidomastoid (SCM) muscle or brachioradialis (if future ipsilateral surgery is anticipated)
4. Mark with a non-absorbable suture and clip so it can be found if the patient later develops hyperparathyroidism

E. Consequences of Parathyroid Loss - Hypoparathyroidism

• Post-thyroidectomy hypocalcemia is the most common complication of total thyroidectomy
• Check serum Ca²⁺ at 6 hours and 24 hours post-op routinely

PART 3: INTEGRATED SURGICAL STEPS SUMMARY

Expose thyroid → Divide middle thyroid vein
        ↓
Ligate SUPERIOR thyroid vessels CLOSE to capsule (protect EBSLN)
        ↓
Medialize thyroid lobe medially
        ↓
Identify PARATHYROIDS early (before ITA ligation)
Sweep them laterally with intact pedicle
        ↓
Identify RLN low in neck (TE groove / angle of ITA & trachea)
Trace cephalad past Tubercle of Zuckerkandl to Berry ligament
        ↓
Ligate inferior thyroid vessels AFTER RLN is in full view
(Ligate branches close to thyroid, NOT trunk)
        ↓
Divide Berry ligament carefully (RLN closest to thyroid here)
        ↓
Remove thyroid - inspect specimen for any parathyroid tissue
Autotransplant if found

PART 4: HIGH-YIELD EXAM POINTS (MNEMONICS)

• T - Tracheoesophageal groove (initial search zone)
• B - Bisected angle of trachea + ITA (first identification point)
• Z - Zuckerkandl's tubercle (RLN just posterior to it)
• B - Berry ligament (danger zone - nerve closest to thyroid)

• S - Spot them early (before ITA ligation)
• L - Leave lateral pedicle intact
• I - Invest surrounding tissue (not the gland itself) for traction
• M - Mobilize laterally with bipolar only

1. Right RLN is more oblique than left (due to different anatomical origin)
2. Non-RLN = aberrant right subclavian artery = approaches from lateral NOT from below
3. RLN branches in 20-30% of cases - anterior branch is predominantly motor
4. ITA is anterior to RLN in 70% of cases
5. Superior parathyroids = 4th pouch (more constant), Inferior = 3rd pouch (more variable)
6. "Treat every parathyroid as the patient's last" = cardinal rule
7. IONM supplements but does NOT replace visual identification
8. Autotransplant into SCM; mark with clip/suture

• Fischer's Mastery of Surgery, 8th Ed. - p. 1073 (dedicated section on RLN exposure and parathyroid identification)
• Sabiston Textbook of Surgery, 21st Ed. - p. 1488 (RLN anatomy, NRLN, neuromonitoring)
• Cummings Otolaryngology Head and Neck Surgery, 7th Ed. (surgical anatomy, parathyroid management)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, 8th Ed. (IONM evidence)
• Mulholland and Greenfield's Surgery, 7th Ed. (parathyroid anatomy during thyroidectomy)