Trigone

1. Overview

The trigone is a smooth, triangular region located on the posterior wall of the urinary bladder, defined by three openings: the two ureteric orifices and the internal urethral orifice. Unlike the rest of the bladder, which expands and contracts with urine volume, the trigone remains relatively fixed in shape and position. It serves as a functional funnel directing urine from the ureters into the urethra and is highly sensitive to stretch and pressure changes.

2. Location

The trigone is located within the base (posterior inferior surface) of the bladder, specifically:

• Between the openings of the left and right ureters (superolateral angles).

• Above the internal urethral orifice (inferior apex of the triangle).

It is situated anterior to the rectum in males and vagina in females, and lies immediately superior to the prostate gland in males.

3. Structure

The trigone is characterized by:

• Triangular shape: Bounded by two ureteric orifices and the internal urethral orifice.

• Smooth mucosa: Unlike the rest of the bladder, the trigone’s mucosa is firmly attached to the underlying detrusor muscle and does not form folds when the bladder is empty.

• Ureteral ridges (interureteric bar): A slight ridge of tissue connecting the two ureteric orifices, forming the base of the triangle.

Histologically, the trigone is lined by transitional epithelium (urothelium), with underlying dense connective tissue and muscularis layers. It is richly innervated and vascularized, making it sensitive to stretch and irritation.

4. Function

The primary functions of the trigone include:

• Directing urine flow: Channels urine from the ureters into the bladder and then toward the urethra during micturition.

• Stretch detection: Its mucosa contains stretch receptors that help initiate the micturition reflex as the bladder fills.

• Preventing reflux: Its positioning and muscular backing assist in preventing vesicoureteral reflux (backflow of urine into the ureters).

5. Physiological Role(s)

The trigone plays a key role in:

• Bladder sensation and emptying: Sends afferent signals to the spinal cord when stretched, aiding in coordinated bladder contraction.

• Maintaining flow direction: Acts as a controlled entry and exit point for urine flow.

• Ureteric valve function: Helps maintain proper closure of the ureteric orifices during bladder contraction, minimizing risk of upper urinary tract infections.

Because the trigone does not expand or contract significantly, it serves as a stable landmark during urological procedures.

6. Clinical Significance

Cystitis (Bladder Inflammation)

The trigone is particularly susceptible to inflammation due to its fixed position and rich innervation. In trigonitis, patients may experience:

• Urinary frequency and urgency

• Suprapubic pain

• Dysuria (painful urination)

Trigonitis may be seen in both infectious and interstitial cystitis.

Vesicoureteral Reflux (VUR)

In some individuals, especially children, the valve mechanism at the ureteric orifices within the trigone fails, allowing urine to reflux back into the ureters and kidneys. This can lead to:

• Recurrent UTIs

• Renal scarring

• Chronic kidney disease (in severe cases)

Diagnosed via voiding cystourethrogram (VCUG).

Neurogenic Bladder Dysfunction

Damage to the afferent innervation of the trigone can impair the micturition reflex, leading to urinary retention or incontinence. Common in patients with:

• Spinal cord injuries

• Multiple sclerosis

• Diabetes mellitus

Urethral Obstruction and BPH

In men, benign prostatic hyperplasia (BPH) may distort the trigone region by compressing the bladder neck, leading to:

• Difficulty initiating urination

• Incomplete emptying

• Post-void dribbling

Cancer Involvement

Bladder cancers often arise in the trigone region, and because of its proximity to the ureteric orifices and urethra, early tumors in this area may cause:

• Hematuria

• Obstructive uropathy

• Ureteric obstruction

Surgical Importance

The trigone is preserved during supratrigonal cystectomy due to its functional importance. In reconstructive surgeries (e.g., neobladder formation), its anatomy must be replicated to maintain physiologic urine flow and sensation.