Larynx & Vocal-Cord Palsy — RLN & SLN

🗣️ Larynx & vocal-cord palsy

"Why the anaesthetist cares: the recurrent laryngeal nerve runs in the tracheo-oesophageal groove and is at risk in thyroid, parathyroid, oesophageal, carotid and anterior cervical-spine surgery. Bilateral recurrent laryngeal palsy is the airway emergency — the cords sit near the midline and cannot abduct, so extubation can be followed by stridor and complete obstruction."

Synthesised from Dhingra — Diseases of Ear, Nose & Throat; Scott-Brown's Otorhinolaryngology; Miller's Anesthesia.

The five vocal-cord positions

Cord position is described by the distance of the free (medial) edge from the midline. Both branches of the vagus control it: the recurrent laryngeal nerve (RLN) works every intrinsic muscle except one, and the external branch of the superior laryngeal nerve (SLN) works the cricothyroid (the cord tensor).

Superior view of the glottis with the vocal cord shown at five positions: median, paramedian, intermediate or cadaveric, slight abduction and full abduction — The vocal cord swings out (abducts) from the midline through five described positions.

Position	Distance from midline	Occurs normally during
Median	Midline (0)	Speech / phonation
Paramedian	1.5 mm	Whisper
Intermediate (cadaveric)	3.5 mm	— (no muscle tone)
Gentle / slight abduction	7 mm	Normal quiet respiration
Full abduction	9.5 mm	Deep inspiration

The cadaveric (intermediate) position is the neutral position the cord adopts when it has no nerve supply at all (complete vagal / combined RLN + SLN palsy) — neither adducted nor abducted.

Nerve supply — and why the left RLN is longer

Course of the superior and recurrent laryngeal nerves; the right recurrent nerve loops under the right subclavian artery and the left under the aortic arch — Both recurrent nerves ascend in the tracheo-oesophageal groove, but the **right** hooks under the right subclavian artery while the **left** takes the long path under the aortic arch — so the left RLN is longer and more often injured (e.g. by a mediastinal or aortic lesion — *Ortner's syndrome*).

SLN — internal branch: sensory to the larynx above the cords (afferent limb of the cough/laryngospasm reflex).
SLN — external branch: motor to cricothyroid (tenses/lengthens the cord → raises pitch).
RLN: motor to all other intrinsic muscles, including the only abductor — posterior cricoarytenoid ("the muscle of respiration"; PCA opens the cords). Sensory below the cords.

Recurrent laryngeal nerve (RLN) palsy

Unilateral RLN palsy at respiration and phonation — the paralysed cord sits paramedian while the normal cord moves — **Unilateral RLN palsy** — the affected cord sits paramedian; the intact cord crosses to meet it at phonation, so the voice is often near-normal.

Bilateral RLN palsy at respiration and phonation — both cords fixed near the midline — **Bilateral RLN palsy** — both cords are stuck near the midline: voice is preserved but the airway is dangerously narrow (stridor, the most life-threatening palsy).

⚠️ Bilateral RLN palsy after thyroid surgery = airway emergency

Suspect it if the patient develops stridor after extubation. Both cords cannot abduct, so the glottic chink is tiny. Be ready to re-intubate and involve ENT — definitive options include cordotomy, arytenoidectomy or vocal-cord lateralisation (type II thyroplasty).

Superior laryngeal nerve (SLN) palsy

Unilateral SLN palsy: the affected cord is bowed and lax and the glottis is askewed at phonation — Losing the cricothyroid (cord tensor) leaves the cord **bowed and lax**; it sits at a slightly different level and the glottis looks **askewed**, rotating toward the normal side on phonation. The classic complaint is a **weak voice that tires and cannot reach high notes**.

Palsy comparison — position, voice, airway & management

Type of palsy	Position of VC	Speech	Aspiration	Respiration	Management
U/L RLN	Mostly median, sometimes paramedian	Normal; sometimes mild hoarseness initially	None	Normal	Conservative (the palsy with the least consequences)
B/L RLN	Mostly median	Largely normal; sometimes mild hoarseness	None	Stridor & dyspnoea on exertion — the most life-threatening palsy	Usually one-sided VC lateralisation (cordectomy or type II thyroplasty)
U/L SLN	Normally moving, curved (bowed) VC	Weak voice, voice fatigue, loss of timbre, cannot raise pitch	Occasional	Normal	Conservative
B/L SLN	Normally moving VC, but both curved	Husky voice	Present	Normal	Tracheostomy and epiglottopexy (to prevent aspiration)
U/L complete	Cadaveric	Initial dysphonia/aphonia, subsequently hoarseness	Sometimes	Normal	Teflon/fat injection or medialisation of VC (type I thyroplasty)
B/L complete	Both VC cadaveric	Aphonia	Severe aspiration episodes	Normal	Tracheo-oesophageal diversion (gold standard) / epiglottopexy or VC plication with tracheostomy

🎓 Exam pearls

Posterior cricoarytenoid is the only abductor — "the muscle of respiration". Lose it bilaterally and you lose the airway.
Semon's law — in a progressive nerve lesion the abductor fibres fail before the adductors, so the cord drifts from paramedian toward the midline.
Cadaveric = intermediate position = no nerve supply (complete/combined palsy), not isolated RLN palsy.
Unilateral RLN = best voice/least danger. Bilateral RLN = good voice but worst airway. Bilateral complete = aphonia + severe aspiration.
The left RLN is longer (loops under the aortic arch) → more often involved by mediastinal disease; hoarseness from a left-sided intrathoracic cause is Ortner's syndrome.