Pelviperineology

Basic Science

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Structural-functional correlation in bladder dysfunction: Is there a role for detrusor ultrastructural analysis?

Original article by ALISON BLATT - LEWIS CHAN - VINCENT TSE
Department of Urology, Concord Repatriation General Hospital, Sydney, Australia.

INTRODUCTION

For clinicians with an interest in bladder dysfunction, accurate diagnosis and prognosis remains elusive. The current diagnostic gold standard is fluoroscopic urodynamics despite its acknowledged morbidity and inaccuracies.^1-5 Some non-invasive techniques have been evaluated and shown to be useful in the management of bladder dysfunction.^6,7

However, if the edict “structure determines function” were true, dysfunction must lend its cause to abnormal structure. Using this edict in attempting to solve the puzzle of bladder dysfunction, the last three decades had witnessed an upsurge in the study of detrusor structure, especially in the field of ultrastructure.

This paper seeks to review our current knowledge on normal, as well as abnormal detrusor ultrastructure seen in non-neurogenic detrusor dysfunction. In particular, a summary of current clinical correlative evidence which may guide future research in this area will be presented.

THE DETRUSOR BIOPSY

Comparison of different biopsy techniques and sites has demonstrated very little variation in ultrastructure of the detrusor throughout its body (which is defined as being above the level of the ureteric orifices).^8-10 This includes transabdominal ultrasound-guided needle biopsy and autopsy specimens.^8,9 However, the standard biopsy is taken transurethrally from the body detrusor, approximately 2 cm supero-lateral to the ureteric orifice.^10-13
No study has been able to detect a difference between detrusors of different ages or sexes.^10,11,13-16

NORMAL DETRUSOR ULTRASTRUCTURE

On light microscopy, the detrusor musculature consists of spindle shaped smooth muscle cells with a single nucleus.¹⁷
Under electron microscopy, numerous mitochondria and occasional endoplasmic reticulum and Golgi apparatus may be seen.¹⁸ Clusters of ribosomes and dense bodies may be seen throughout the sarcoplasm. The cell membrane (sarcolemma) consists of alternating dense and thin bands. Rows of caveolae (flask-shaped surface vesicles) may appear at the thin zones (Fig 1, 2).

The gross architecture of the detrusor is an interwoven non-layered network of muscle bundles.^17,19 This arrangement can be appreciated by light microscopy or by electron microscopy at low magnifications (x 1000-3000).
The muscle “fascicle” is the smallest recognisable compact unit consisting of four to twelve cells closely aligned (less than 0.2um spaces), unidirectional and separated by a thin microsepta of collagen fibrils and occasional elastin.⁸

An aggregate of these fascicles is known as a muscle “bundle” and is surrounded by a thick interstitial macrosepta of collagen, elastin and occasional fibroblasts (Fig. 2, 3). Only a proportion of myocytes are directly stimulated by neuroeffector junctions, the majority receiving the stimulus to contract via mechanical coupling or electrical coupling via intercellular junctions.¹⁸ Therefore axon bundles are sparse in the interstitium and may be difficult to identify on electron microscopy (Fig. 4).
Intercellular junctions are an important ultrastructural feature of the detrusor. The most common junction is the intermediate cell junction (ICJ or “zonulae adherents”).¹²

This consists of two closely apposed (25-70nm wide gap) sarcolemma lying parallel to each other for a length of up to 10mm with paired symmetrical dense plaques. Other junctions are less common in the normal detrusor and are known as “gap junctions” owing to their tight apposition. These include “protrusion junctions” which are slender finger-like projections between cells with tip-to-tip contact, and “ultraclose abutments”, which are a tight apposition of parallel surfaces in a shallow bump-impression configuration (Fig. 5).¹⁵

ABNORMAL DETRUSOR ULTRASTRUCTURAL FEATURES

True pathological ultrastructural features may be considered as changes to the overall architecture, to the interstitium, the myocyte or to the nature of the cell junctions. In various combinations, these features have been correlated with urodynamically diagnosed voiding disorders.

Abnormal Detrusor Architecture

Detrusor fascicle arrangement becomes pathologically distorted in diseased bladders.^{13,14,16,20-22}

This is largely a qualitative description of the uniformity and close apposition of adjacent myocytes within fascicles. Various gradings have been proposed, with the clearest (albeit qualitative) from Hailemarium and Elbadawi as:

1. “Compact” – a fascicle-bundle unit with only mild uneven myocyte separation;
2. “Intermediate” – a mixture of uniform units and occasional intermediate or loose fascicles;
3. “Loose” – mostly moderate to marked myocyte separation or indistinct arrangement with rarely seen uniform units (Fig. 6-8).⁸

These changes are seen most markedly in the hypocontractile detrusor and in bladder outlet obstruction.^{13,14,16,20,21}

This abnormal architecture is closely associated with the abnormal interstitial content. The amount of collagen and elastin increase with increasingly distorted fascicle arrangements. In normal amounts collagen reinforces mechanical cell coupling, and has a key role in the eventual summated unitary detrusor contraction, leading to the complete expulsion of urine. However, excessive intercellular collagen is thought to dissipate forces achieved by contracting myocytes.¹⁵

This may lead to the clinical correlate of residual urine. Also, collagen being one of the stiffest biomaterials known, could increase rigidity and reduce compliance. Elastin, the most stretchable biomaterial known, would promote distensibility and may allow the bladder capacity to increase dramatically in chronic retention with overdistension (Fig 6-8).

Abnormal Cellular Profiles

Myocytes may hypertrophy, typically correlating with bladder outlet obstruction (Fig. 6). Cell shape, density and content must also be considered. Abnormally shaped myocytes may be branched (like fork-prongs), braided and intertwined (“hugging” each other) or bizarre shapes such as cannelloni. These abnormal shapes (versus the normally spindle-shaped myocyte) may also cause contractile forces to dissipate and wasted upon itself. Hypertrophic contorted myocytes are usually seen in association with loose fascicles, and therefore do not guarantee a stronger detrusor contraction as forces are attenuated, especially by the increased interfascicle distance by increased collagen deposition.

The myocyte shape and content can be indicative of detrusor degeneration, with the clinical correlate being hypocontractility. This may range from a cell containing vacuoles and debris, to a fragmented cell with sequestration or even a shrivelled dense cell without internal structure (Fig. 7). These disrupted cells may be the cause or effect of bladder dysfunction. They would presumably impair contractility or may be the result of a poorly contractile bladder with overdistension and patchy hypoxia in stretched muscle (Fig. 8).¹⁰

Abnormal Intercellular Junctions

The ultrastructural feature requiring the highest magnification to accurately assess is the intercellular junctions (ICJ). Initial researchers noted increased gap junctions in patients with urodynamics detrusor overactivity and they were considered an abnormal feature.¹⁵

However, gap junctions are seen in urodynamically normal detrusor. What has clearly been demonstrated is that their ratio compared to normal ICJs increases with detrusor overactivity.¹²

These detrusors demonstrate a syncytium pattern of indiscernible gaps between cell processes linking up to ten myocytes or more. This ultimately changes the nature of the summated detrusor contraction. Instead of predominant mechanical cell coupling via the ICJs during detrusor contraction, a low resistance pathway is created by the presence of these gap junctions, thus mediating rapid electrical coupling. This results in the unstable contractions seen on urodynamic studies of subjects with an overactive detrusor.

HOW DO THESE ULTRASTRUCTURAL FEATURES CORRELATE WITH CLINICAL FINDINGS?

Many of the original ultrastructural descriptions were based on very small numbers of patients, with variable urodynamic definitions.^11,14-16

Subsequent studies have confirmed and refined these features. Ultrastructural categories were created and those which persist today are defined as the following:

1. the myohypertrophy pattern of bladder outlet obstruction (with hypertrophied, contorted myocytes and loose fascicles);
2. the dysjunction pattern of detrusor overactivity (with presence of gap junctions and intermediate fascicles);
3. the degenerative pattern of detrusor failure (vacuolated lytic cells with intraand extra-cellular debris).

As a subcategory, hyperelastosis (excessive elastin deposition) seems to correlate with chronic urinary retention,^10,21 although not all studies agree.²³
Clinical studies show some discrepancies in their findings. The greatest variables between studies are the definitions used for voiding diagnoses (including definitions of controls), use of urodynamics, the technique of ultrastructural analysis, and the specimen processing technique.

Starting with the myohypertrophy pattern, Elbadawi originally studied seven patients with bladder outlet obstruction determined by fluoroscopically guided micturitional urethral pressure profilometry, in some patients corroborated by pressure-flow analysis.¹⁴ In a further validation study five patients with clinically moderate obstruction had more pronounced loose fascicles than 3 patients with mild obstruction (according to Schafer’s nomogram).²⁴

Brierly assessed 12 patients awaiting prostatectomy for urodynamically-proven bladder outlet obstruction.²⁰ Only eight patients demonstrated the myohypertrophy pattern and there was no apparent association between the urodynamic variables and the presence of myohypertrophy.

Three of the 17 controls showed some limited myohypertrophic features, although these were not extensive enough to fulfil the criteria used. In Holm’s study of 25 patients with bladder outlet obstruction, the myohypertrophy pattern was not observed in any of the biopsies when compared to six controls.²² This raises many questions, however the methodology in this paper was clearly different to that of the positive correlative studies. These differences included specimen processing technique and inclusion of patients with “equivocal” obstruction.

Elbadawi’s original description of the dysjunction pattern as the presence of abnormal gap junctions was based on 15 patients with detrusor overactivity.¹⁵ In a later validation study by the same researchers it was found that patients with normal urodynamics also displayed gap junctions. More recently, Tse’s quantitative analysis showed a significant stepwise increase in the ratio of gap junctions to ICJs in four groups respectively: controls, idiopathic sensory urgency, bladder outlet obstruction with overactivity, and idiopathic detrusor overactivity.¹²

However, Carey’s qualitative ultrastructural study of 13 women with idiopathic overactivity did not detect any gap junctions.²⁵ In this study there was considerable interobserver variability between two reporters, with only two patients for which there was tabulated agreement. This brings into question the qualitative nature of ultrastructural reporting on the whole and the need to objectify all aspects as much as possible. Degeneration was originally described in ten elderly patients with hypocontractile bladders, defined as post-void residuals (PVR) of greater than 250ml.¹⁶

The definition of normal PVR is subject to controversy²⁶, although a consistently “significantly” elevated PVR is considered indicative of relative detrusor failure.²⁷ Collins studied 19 patients with PVRs greater than 250ml.²¹ Eleven underwent prostatectomy. The degenerative pattern was seen in conjunction with myohypertrophy in all patients and the severity of degeneration correlated with post-prostatectomy voiding. Hindley found the degeneration pattern in all 20 patients with PVRs greater than 300ml.¹⁰

A study of 12 patients awaiting prostatectomy demonstrated the full degenerative pattern in the six patients with the highest PVRs (all greater than 150ml).²⁰ In a study of 14 patients with PVRs greater than 500ml compared to controls (with PVRs up to 160ml), Holm found degeneration in all patients and five of six controls.²³ Four of his six controls suffered from lower urinary tract symptoms and two had previous bladder cancer. Accepting that some degenerative features were often noted in controls, Brierly conducted a quantitative study by counting the “disrupted cell ratio” as the number of disrupted cells per 500 cells.¹³ A significant difference was demonstrated between controls and patients with detrusor hypocontractility.

Finally, in a study aimed to assess all of Elbadawi’s originally described ultrastructural patterns, Mastropietro was unable to significantly correlate any of the patterns with clinical diagnoses in 24 women.²⁸ The overall agreement between pattern and diagnosis was only 30%. There are several possible explanations for this negative result. In 35% of patients urodynamic diagnosis was equivocal or difficult, and in 35% of specimens the pathological diagnosis was equivocal or difficult. Fixation technique was not clearly explained and is acknowledged by the authors as another possible source of discrepancy.²⁸

CONCLUSION

Evidence has been accumulating of a definite relationship between structure and function of the detrusor for many years. Starting with the breakthrough work of Elbadawi, distinct ultrastructural features were for the first time correlated with functional urodynamic parameters.^11,14-16

This work has been subsequently expanded upon by many authors, and the methodology and definitions refined. Despite all this work, a definite clinical role for ultrastructural detrusor analysis has yet to be established.

Inconsistent findings of degeneration, myohypertrophy, hyperelastosis and dysjunction patterns in clinical validation studies suggest a need to reassess the significance of individual features rather than rely on the traditional patterns originally outlined by Elbadawi.¹¹ These individual features could then be analysed by multivariate analysis with urodynamic and outcome parameters. One potential future research area is whether the ultrastructure reverts towards normal after treatment of the bladder dysfunction. E.g. after successful treatment of overactivity or bladder outlet obstruction.

Current work is underway by the authors in looking at gap-junctional protein expression in urodynamically abnormal detrusors in the quest for further refining a structuralfunctional correlate in common bladder dysfunctions. We also hope this paper may assist and encourage future research in identifying a diagnostic and prognostic role for ultrastructural study of the detrusor.

REFERENCES

Correspondence: Dr. ALISON BLATT Department of Urology Concord Repatriation General Hospital Hospital Rd., Concord, NSW, 2139, AUSTRALIA Tel: 61-2-9767 8334 Fax: 61-2-9767 8331