Elaborate. About. Rima Glottidis with respect to... KNR UHS VSAQ pattern.
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Rima Glottidis anatomy VSAQ KNR UHS exam important questions
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Rima Glottidis - KNR UHS VSAQ Pattern
Definition
The rima glottidis is the narrowest part of the laryngeal cavity - a triangular slit-like opening between the two vocal folds (true vocal cords) and the adjacent mucosa-covered parts of the arytenoid cartilages. It separates the middle chamber of the laryngeal cavity (above) from the infraglottic cavity (below).
- It is narrower than the rima vestibuli (the opening between the vestibular/false folds above it)
- The apex of the triangular opening is anterior; the base is formed by the interarytenoid fold at the bottom of the interarytenoid notch
Parts of the Rima Glottidis
The rima glottidis has two distinct parts:
| Part | Also Called | Location | Bounded By |
|---|---|---|---|
| Anterior (longer) | Intermembranous part | Lies over the vocal ligament | Between the two vocal folds |
| Posterior (shorter) | Intercartilaginous part | Between the arytenoid cartilages | Between the vocal processes/bodies of the arytenoids |
Both portions can be opened to varying degrees independently.
Dimensions (Clinically Important)
- The rima glottidis is the narrowest part of the larynx in adults
- It is approximately 23 mm long in males and shorter in females
- In children, the narrowest point of the larynx is actually the subglottic region (cricoid ring level) - this is a frequently tested distinction
Shape Changes with Function
The shape of the rima glottidis changes according to functional state - this is a high-yield VSAQ topic:
| Functional State | Shape of Rima Glottidis | Mechanism |
|---|---|---|
| Quiet respiration | Triangular (narrow) - only intercartilaginous part open | Arytenoids slightly abducted |
| Forced inspiration | Wide rhomboid/diamond shape | Posterior cricoarytenoid (PCA) rotates arytenoids laterally, widely abducting vocal folds |
| Whispering | Intermembranous part closed; intercartilaginous part open as a triangle | Lateral cricoarytenoid adducts vocal processes; transverse arytenoid leaves posterior gap |
| Phonation | Closed (adducted) - then opened by expiratory airstream causing vibration | Lateral cricoarytenoid + transverse arytenoid + vocalis |
| Effort closure | Completely closed | All adductors active; rima vestibuli also closed |
| Swallowing | Completely closed | Same as effort closure; larynx also moves up and forward |
| Coughing | First closed, then opens explosively | Rima closes against high subglottic pressure, then bursts open |
Muscles Acting on the Rima Glottidis
Abductors (Open the rima glottidis)
- Posterior cricoarytenoid (PCA) - the only abductor; draws the muscular process posteriorly, rotating the vocal process laterally, widening the rima glottidis. Innervated by the recurrent laryngeal nerve (RLN)
Adductors (Close the rima glottidis)
| Muscle | Action | Nerve |
|---|---|---|
| Lateral cricoarytenoid | Rotates vocal process medially | RLN |
| Transverse (interarytenoid) | Approximates arytenoid bodies | RLN |
| Oblique arytenoid | Assists transverse arytenoid | RLN |
| Thyroarytenoid/Vocalis | Fine tension adjustment of vocal folds | RLN |
Key mnemonic: "PCA is the only Abductor" - always asked in exams. All other intrinsic muscles are adductors or tension-adjusters.
Nerve Supply (High-Yield)
- Above vocal folds (rima glottidis level): mucosa innervated by internal branch of superior laryngeal nerve (SLN) - sensory
- Below vocal folds: recurrent laryngeal nerve (RLN) - sensory
- All intrinsic muscles (including those acting on rima): RLN (recurrent laryngeal nerve, branch of vagus [CN X])
- Exception - cricothyroid: external branch of SLN (tensor of vocal fold, indirectly affects pitch)
Clinical Correlates (VSAQ favourites)
1. Cricothyrotomy
If there is life-threatening closure of the rima glottidis (e.g., laryngospasm, severe edema, foreign body), an emergency airway is established by incising through the median cricothyroid ligament - which lies below the level of the rima glottidis. The median cricothyroid ligament is subcutaneous, avascular, and readily accessible.
2. RLN Paralysis
- Unilateral RLN injury: all intrinsic muscles on that side are paralyzed. The affected vocal fold lies in a paramedian (adducted) position - voice is hoarse/breathy. The PCA is paralyzed, so the fold cannot abduct.
- Bilateral acute RLN injury: both folds lie in paramedian position, causing respiratory obstruction (life-threatening); the rima glottidis is dangerously narrowed.
3. Narrowest Part of the Airway
- Adults: rima glottidis is the narrowest part of the larynx
- Children: subglottic region (cricoid ring) is the narrowest - relevant in croup, endotracheal tube sizing
4. Foreign Body Aspiration
Involuntary closure of the rima glottidis occurs when a foreign body enters the airway (protective reflex). The cough reflex then causes it to reopen explosively.
5. Laryngoscopy
During laryngoscopy, the image is inverted - anterior areas of the laryngeal inlet appear at the top of the image and posterior at the bottom.
Summary Box (Quick Revision)
| Feature | Detail |
|---|---|
| Definition | Opening between vocal folds + arytenoids |
| Parts | Intermembranous (anterior) + Intercartilaginous (posterior) |
| Shape at rest | Triangular |
| Shape forced inspiration | Rhomboid/wide diamond |
| Only abductor | Posterior cricoarytenoid (PCA) |
| Muscle nerve supply | Recurrent laryngeal nerve (all except cricothyroid) |
| Narrowest part in adults | Rima glottidis |
| Narrowest part in children | Subglottic (cricoid ring) |
| Emergency airway below it | Cricothyrotomy via median cricothyroid ligament |
Sources: Gray's Anatomy for Students, p. 1199-1204; Color Atlas of Human Anatomy Vol. 2 (Internal Organs), p. 194-202
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