16. Detrusor leak point pressure
a. This is the pressure obtained by filling the bladder until intravesical pressure overcomes urethral resistance and incontinence occurs. Detrusor pressure can force the sphincter open, whereas the abdominal pressure cannot.
b. It is an indirect measure of bladder storage.
c. It is an assessment of intravesical pressure, useful in the patient with a neuropathic bladder to determine if urine is being stored at pressures that will not cause upper urinary tract deterioration.
d. Normal detrusor leak point pressure is below 40 cm H2O. Pressures above this can over time cause upper tract damage.
17. Specialized testing
a. Rapid fill cystometry
(1) Used to rule out detrusor areflexia
(2) Rapid filling of the bladder at rates of flow up to 300 cc/mm
b. Bethanechol supersensitivity test
(1) Bethanechol chloride.
(b) Selective for bladder and gut (e) Cholinesterase resistant
(d) Causes bladder muscle to contract (spares ganglia and cardiovascular)
(e) Minimal effects on the normal bladder
(i) Decreases capacity slightly
(ii) Increases detrusor tone
(iii) Increases voluntary micturition pressure
(iv) Does not cause detrusor instability
(2) Basis for test: Cannon’s law of denervation; when an organ is deprived of its nerve supply, it will develop hypersensitivity to its own neurotransmitter substance.
(3) Method: Subcutaneous injection of 2.5 mg of bethanechol chloride followed in 15 to 30 minutes by repeat cystometro-gram study.
(a) A rise of at least 15 cm of water after 100-cc infusion at a medium rate of between 100 and 200 cc/min in excess of the pretreated cystometrogram pressure is indicative of parasympathetic dysfunction (denervation). It is seen with lower motor neuron lesions (also called decentralized bladder).
(b) This test is contraindicated in patients with cardiac disease, asthma, hyperthyroidism, peptic ulcer disease, enteritis bowel obstruction, and bladder outlet obstruction.
(c) Blaivas reported a false-negative rate of 24%, attributing such testing factors as repeated overdistension of the bladder, bethanechol absorption, emotional stress, time, and bladder volume to the high false-negative rate.
(d) False-positive test results occur in the presence of active urinary tract infections, azotemia, emotional stress, and detrusor hypertrophy.
(e) Studies by Mattiason14 and Malkowicz15 suggest that end organ supersensitivity is not an end organ response to denervation but may measure supersensitivity at the pelvic reflex arc or demonstrates another aspect of vesical pressure generation such as detrusor muscle hypertrophy.