Introduction
The upper oesophageal sphincter (UOS) is a 3–4 cm intraluminal high-pressure zoneReference Lawson, Bachy, Desgain, Matar, Jamart and Remacle 1 , Reference Jones, Hammer, Hoffman and McCulloch 2 separating sub-atmospheric oesophageal pressure from atmospheric pharyngeal pressure.Reference Goyal, Martin, Shapiro and Spechler 3 The prevalence of dysphagia, a characteristic of UOS dysfunction, is high in neurological disease cohorts. If conservative measures fail to improve UOS functioning, surgery may be proposed.
Cricopharyngeal myotomy may be performed via open, transcervical or endoscopic approaches.Reference Campbell, Bruce, Tuominen and Toohil 4 Endoscopic cricopharyngeal myotomy was initially performed using potassium-titanyl-phosphate lasers; however, surgical trends have recently favoured use of the carbon dioxide (CO2) laser given the possible benefits regarding reduced thermal damage and blood vessel coagulation.Reference Lawson and Remacle 5 , Reference Bergeron and Chhetri 6 Endoscopic cricopharyngeal myotomy is performed transorally; it involves cricopharyngeal diverticuloscope visualisation and exposure, with pulsed or non-pulsed laser incision to the buccopharyngeal fascia.Reference Lawson and Remacle 5 , Reference Damrose and Ho 7 , Reference Silver and Gal 8 Buccopharyngeal fascia and retropharyngeal space violation increase the risk of mediastinitis because of air extravasation and the potential for contamination with abscess formation.Reference Lawson and Remacle 5 , Reference Damrose and Ho 7 If the buccopharyngeal fascia is damaged, the surgical bed is closed using fibrin glue or endoscopic stapling.Reference Lawson and Remacle 5 , Reference Pitman and Weissbrod 9
Lawson et al.Reference Lawson, Bachy, Desgain, Matar, Jamart and Remacle 1 retrospectively evaluated endoscopic cricopharyngeal myotomy effects in a neurological disease cohort, a post-chemoradiation therapy cohort and a cricopharyngeal spasm cohort. Fibre-optic endoscopic swallowing and videofluoroscopic swallowing were evaluated pre- and post-operatively, along with self-assessments of symptom improvement. The authors reported no complications, yet recurrence of clinical symptoms was noted in 62.5 per cent of neurological disease patients.
Linke et al.Reference Linke, Bockmuhl and Haake 10 retrospectively reviewed outcomes of patients with neuromuscular diseases, reporting 100 per cent enteral feeding tube removal post-operatively. However, cricopharyngeal myotomy was combined with glottopexy and glottal closure in 58 per cent of patients, thus confounding the results.
Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 reviewed endoscopic cricopharyngeal myotomy outcomes in an unspecified population, although five patients presented with neurological disease diagnoses such as amyotrophic lateral sclerosis, stroke and myopathy. Results were confounded by the fact that 13.8 per cent of participants underwent endoscopic cricopharyngeal myotomy following failed open or transcervical cricopharyngeal myotomy. The authors reported significant objective improvement and no complications. However, only short-term follow up was assessed and there was no information on long-term symptom evolution.
Bergeron and ChhetriReference Bergeron and Chhetri 6 retrospectively reviewed outcomes in a mixed population that included patients with UOS dysfunction following stroke and neuronal damage. All participants demonstrated significant objective improvements post-operatively. Several patients required concurrent dilation and endoscopic cricopharyngeal myotomy; however, details regarding who received this procedure were not available. In addition, assessment was non-blinded, which increased the risk of detection bias.
Hazarika and colleaguesReference Hazarika, Parul, Kailesh, Balakrishnan, Hazarika and Singh 12 retrospectively investigated combined endoscopic cricopharyngeal myotomy and dilation in a mixed population that included neurological disease patients. Objective improvement was reported in 87.5 per cent of patients. However, one participant died as a result of mediastinitis caused by intra-operative perforation, indicating the morbidity and mortality potential of endoscopic cricopharyngeal myotomy.
There is controversy in the literature regarding endoscopic cricopharyngeal myotomy.Reference Singh and Hamdy 13 This is related to under-reporting of complications,Reference Pitman and Weissbrod 9 , Reference Takes, van den Hoogen and Marres 14 and limited rigorous efficacy, coupled with varying clinical indications and follow-up durations.Reference Lawson, Remacle, Jamart and Keghian 11 Therefore, large prospective studies with rigorous clinical indicators and validated assessment tools are requiredReference Lawson, Remacle, Jamart and Keghian 11 , Reference Takes, van den Hoogen and Marres 14 to direct further research and clinical practice in the area. This systematic review aimed to examine the evidence for endoscopic cricopharyngeal myotomy as an intervention for UOS dysfunction in patients with dysphagia associated with neurological disease.
Research questions
These were: (1) does endoscopic cricopharyngeal myotomy performed for UOS dysfunction reduce or eliminate aspiration of food and/or fluids and laryngeal penetration in adults with UOS dysfunction and neurological disease?; (2) does endoscopic cricopharyngeal myotomy performed for UOS dysfunction reduce mean UOS resting pressure in adults with UOS dysfunction and neurological disease?; (3) does endoscopic cricopharyngeal myotomy performed for UOS dysfunction change oral intake status in adults with UOS dysfunction and neurological disease?; (4) what are the adverse events associated with endoscopic cricopharyngeal myotomy performed for UOS dysfunction in adults with UOS dysfunction and neurological disease?; and (5) does endoscopic cricopharyngeal myotomy performed for UOS dysfunction result in a change in quality of life (QoL) for adults with UOS dysfunction and neurological disease?
Materials and methods
The process of conducting and reporting this systematic review was guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (‘PRISMA’) statement.Reference Moher, Liberati, Tetzlaff and Altman 15
Eligibility criteria
A systematic search of eight electronic databases and grey literature, from inception to June 2015, was conducted (Table I).
Resources searched
*Both published in Dysphagia. AMED = Allied and Complementary Medicine Database; CINAHL = Cumulative Index to Nursing and Allied Health Literature
A highly sensitive search strategy was employed using filters, Medical Subject Headings and key-text terms, developed with assistance from Trinity College Dublin librarians. No language or date limitations were applied. This search examined all published and unpublished studies with a quasi-experimental design that investigated endoscopic cricopharyngeal myotomy effects on upper oesophageal sphincter (UOS) dysfunction in humans and which satisfied the inclusion and exclusion criteria (Table II).Reference De Morton 16 – Reference Woisard and Lepage 18
Inclusion and exclusion criteria
UOS = upper oesophageal sphincter
Studies that investigated endoscopic cricopharyngeal myotomy combined with another intervention were omitted as this hindered clear data analysis. Studies comprising participants of mixed demographics were considered if extraction of data for participants aged 19 years and over was possible.
Study selection process
Searches were independently conducted by two authors (ÓG and PK). Following the deletion of duplicate articles, the double screening of titles, key words and abstracts was conducted (by ÓG and PK) to minimise reporting bias and human error. References were managed using the bibliographic management system Zotero (free online software). Hand searches of conference abstracts from the European Society for Swallowing Disorders and the Dysphagia Research Society were also conducted (by ÓG and PK), as these societies primarily aim to investigate normal and abnormal swallowing physiology, and dysphagia and its related functions. The reference lists of eligible studies were searched (by ÓG) to identify further relevant articles. A third independent reviewer (SB) mediated disputes if they occurred.
Data extraction process
Data were extracted electronically (by ÓG and PK independently). The extraction process was piloted on a representative sample of studies to ensure appropriateness and reviewer competency.Reference Brøndbo 19 A random sample of 20 per cent of data was reviewed by a third author (SB) to mitigate discrepancies. Disputes were documented and resolved via discussion of rationale, with input from an independent reviewer (SB) if required.
Data regarding both objective and patient-reported outcomes were extracted.Reference Richardson, Wilson, Nishikawa and Hayward 20 Primary outcome data extracted included objective post-operative improvements in laryngeal penetration and aspiration of food, oral intake status, and objective mean manometric UOS resting pressure reductions. Secondary outcome data extracted included adverse events directly related to surgery, such as increases in dysphagia, compromised medical health, negative psychological or social consequences, hospitalisation, or death. Post-operative QoL changes were also considered, to assess psychosocial intervention effects.
Time frames for analysis of effects (i.e. immediate, less than 1 month; medium, 1–6 months; and long-term, greater than 24 months) were chosen to capture long-term intervention effects.Reference Brennan, Leape, Laird, Hebert, Localio and Lawthers 21
Data were stratified into comparative subgroups using Microsoft™ Excel® spreadsheets, with planned statistical analysis performed using IBM™ SPSS® software.
Risk of bias assessment
Risk of bias was independently assessed by two authors (ÓG and PK) using the PEDro (Physiotherapy Evidence Database) scaleReference Maher, Sherrington, Herbert, Moseley and Elkins 22 and MINORS (Methodological index for non-randomised studies) tool,Reference Slim, Nini, Forestier, Kwiatkowski, Panis and Chipponi 23 with conflicts resolved via input from a third author (SB).
The PEDro scale assesses methodological quality across the study design spectrum, with sufficient reliability for use in systematic reviews.Reference De Morton 16 Maher et al.Reference Maher, Sherrington, Herbert, Moseley and Elkins 22 reported an inter-rater reliability generalised kappa statistic of between 0.40 and 0.75 for the PEDro scale, indicating moderate to good agreement between reviewers.
The MINORS tool is a methodological index that assesses risk of bias and quality of non-randomised surgical trials, providing requisite sensitivity to facilitate meta-analysis.Reference Slim, Nini, Forestier, Kwiatkowski, Panis and Chipponi 23 It has been widely used in systematic reviews of surgical trials because of high levels of intra-reliability and test–retest reliability, internal consistency, and validity.
The issue of missing data was addressed by contacting authors, for studies published within the last five years. In the case of no response following two contact attempts, the study was excluded.
Endpoints and data synthesis
A meta-analysis of primary and secondary endpoints (using calculations of risk ratios and 95 per cent confidence intervals for dichotomous outcomes, and standardised mean differences and 95 per cent confidence intervals for continuous outcomes) was planned if sufficient numbers of studies were identified via searches, and if adequate homogeneity (as indicated by a chi-square test of homogeneity) was present. If heterogeneity was present, causative factors were to be explored via a subgroup analysis of heterogeneity sources. The authors planned to construct a forest plot to display effect estimates and confidence intervals for dichotomous outcomes. A subgroup analysis considering long- and short-term intervention effects, with stratification as per acute versus progressive neurological disease and specialty training of the operating surgeon, was also planned.
Results
Study selection
A search of 8 databases and grey literature yielded 2938 articles (Figure 1). No additional articles were identified from searching the reference lists of included studies. After deleting duplications, 1711 records remained. Of these, 1609 records were excluded at the screening stage as they did not satisfy the inclusion criteria. Authors subsequently examined 102 full-text records. One author (ÓG) sought access to and/or clarification of missing data. Eight authors responded, leading to 12 exclusions on the basis of ineligible participants, inappropriate interventions or insufficient available data. Three studies were excluded as the authors were unable to access the full contents of the articles. After full-text reviews and author contact, two studies ultimately satisfied the inclusion criteria and were included in the systematic review.
Preferred Reporting Items for Systematic Reviews and Meta-Analyses (‘PRISMA’) flow diagram. CINAHL = Cumulative Index to Nursing and Allied Health Literature; AMED = Allied and Complementary Medicine Database
Study characteristics
Study participants
The two studies that satisfied the inclusion criteria presented data regarding five patients with confirmed neurological disease (Tables III–V).Reference Lawson, Remacle, Jamart and Keghian 11 , Reference Brøndbo 19
Stratified clinical demographics
ALS = amyotrophic lateral sclerosis
Only one study provided the overall average age of participants.Reference Lawson, Remacle, Jamart and Keghian 11 The average age of neurological disease patients was not provided in either study. The pooled age range for both studies was 38–83 years. Northern European, single-centre university hospital clinics were the setting for both studies. No details regarding participants' nationalities were provided.
Study design
Both included studies were published in the English language and both were non-randomised. One was a retrospective patient survey,Reference Brøndbo 19 and one was a retrospective case series with a chart review spanning the years 1995 to 2000 (Table IV).Reference Lawson, Remacle, Jamart and Keghian 11
Studies that fulfilled inclusion criteria
Wk = weeks; post-op = post-operatively; mth = months
Neither study recruited a comparison or control group; the inclusion of such a group would have enhanced internal validity.Reference Richardson, Wilson, Nishikawa and Hayward 20 , Reference Dawe, Patterson, Hamilton and Hartley 24 Study duration ranged from three years and eight monthsReference Lawson, Remacle, Jamart and Keghian 11 to approximately five years.Reference Brøndbo 19 Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 reported their ‘experience with [endoscopic cricopharyngeal myotomy] in 29 cases', and BrøndboReference Brøndbo 19 evaluated ‘functional [operative] results'. BrøndboReference Brøndbo 19 did not specify follow-up periods, while Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 considered the immediate post-operative stage, with a median long-term follow up of 21 months (range, 1–44 months).
Interventions
Both studies utilised continuous mode CO2 laser-assisted endoscopic cricopharyngeal myotomy. Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 used fibrin glue to conduct primary mucosal closure. It is unclear what procedure was used by Brøndbo.Reference Brøndbo 19 Only Lawson reported statistically significant effects of endoscopic cricopharyngeal myotomy (Table V).Reference Lawson, Remacle, Jamart and Keghian 11
Endoscopic cricopharyngeal myotomy intervention, outcome, results and adverse effects
F = females; M = males; CO2 = carbon dioxide; post-op = post-operative
Outcomes
As discussed, the outcomes considered were related to the research questions and were as follows: objective post-operative reductions in aspiration of food and laryngeal penetration and in mean upper oesophageal sphincter (UOS) resting pressure, reductions in adverse events, and improvements in oral intake status and QoL.
Primary outcomes
Only Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 addressed objective post-operative reductions in aspiration of food and laryngeal penetration, utilising both videofluoroscopic swallowing studies and fibre-optic endoscopic swallowing evaluations to enhance objectivity.Reference Ramsey, Smithard and Kalra 25 The median frequency of aspiration events, calculated using a non-validated scale, was significantly reduced from 3 (uncommon) (inter-quartile range, 2–4) to 4 (none) (inter-quartile range, 4–4).
Manometry, which is used to examine changes in mean UOS resting pressure pre- and post-surgery, was not used in either study. No post-operative changes in oral intake status were reported in either study.Reference Lawson, Remacle, Jamart and Keghian 11 , Reference Brøndbo 19
Secondary outcomes
Both included studies referenced post-operative adverse events; however, no post-operative adverse events were documented and no post-operative changes to QoL were discussed (Table V).
Intervention efficacy
Given the low numbers and heterogeneity of eligible studies, meta-analysis could not be conducted. Similarly, subgroup analysis of long- and short-term operative effects, along with stratification according to acute versus progressive neurological disease and specialty training of operating surgeons, could not be conducted. Only Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 specified follow-up timeframes, reporting both short- and long-term outcomes (with follow-up assessments at week 2–3 and at a median of 21 months, respectively).
The two included studies reported on five patients eligible for inclusion (two with acute disorders, two with progressive diseases and one with an unspecified neurological disease); however, details regarding the location, severity and type of the presenting conditions were not provided. Thus, there was insufficient evidence to determine the effectiveness of endoscopic cricopharyngeal myotomy in this population of neurological disease patients.
Risk of bias
Independent reviewers reached consensus regarding risk of bias ratings, without dispute. The study by BrøndboReference Brøndbo 19 was given an overall score of 3 out of a possible 16 using the MINORS tool,Reference Slim, Nini, Forestier, Kwiatkowski, Panis and Chipponi 23 and the study by Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 was given a score of 8 (Table VI).
MINORS scores for included studies
Note regarding scoring: 0 = not reported; 1 = reported but inadequate; 2 = reported and adequate
Neither of the two included studies recorded whether blinding occurred; hence, there was a possibility of detection and performance bias. Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 provided information regarding the statistical significance of the results. According to the PEDro scale,Reference Maher, Sherrington, Herbert, Moseley and Elkins 22 neither study was rated ‘high’ in terms of quality (Table VII).
PEDro scores for included studies
Discussion
No conclusions can be drawn regarding the effectiveness of endoscopic cricopharyngeal myotomy in adult neurological disease populations. There is insufficient evidence to determine whether this procedure is associated with the post-operative reduction or elimination of aspiration of food and laryngeal penetration. Lawson et al.Reference Lawson, Remacle, Jamart and Keghian 11 noted that the median aspiration frequency was significantly reduced, which is in agreement with research by Takes et al.Reference Takes, van den Hoogen and Marres 14 in non-neurological disease populations. Takes et al.Reference Takes, van den Hoogen and Marres 14 found that endoscopic cricopharyngeal myotomy significantly reduced aspiration in adults with unspecified cricopharyngeal bar disorders. However, Dawe et al.Reference Dawe, Patterson, Hamilton and Hartley 24 reported that the aspiration of food and laryngeal penetration in adults with an upper oesophageal sphincter (UOS) dysfunction following chemoradiation therapy was not significantly improved with endoscopic cricopharyngeal myotomy, as measured using the Modified Barium Swallow Impairment Profile.Reference Martin-Harris, Brodsky, Michel, Castell, Schleicher and Sandidge 26 Aspiration pneumonia costs the US healthcare system approximately $4.4 billion per annum.Reference Niederman, McCombs, Unger, Kumar and Popovian 27 , Reference Kaplan, Angus, Griffin, Clermont, Watson and Linde-Zwirble 28 Therefore, it is essential to investigate endoscopic cricopharyngeal myotomy outcomes to determine if the relative risk of laryngeal penetration and aspiration of food, and the associated morbidity and mortality, is reduced post-operatively.
Endoscopic cricopharyngeal myotomy can result in changes in diet. For example, Bergeron and ChhetriReference Bergeron and Chhetri 6 investigated post-operative dietary changes in a mixed population who had undergone endoscopic cricopharyngeal myotomy and dilation, and found that Functional Outcome Swallowing ScaleReference Crary, Mann and Groher 29 scores significantly improved, with 50 per cent of neurological disease patients who received pre-operative enteral feeding progressing to oral intake post-operatively. Linke et al.Reference Linke, Bockmuhl and Haake 10 reported that all neuromuscular disease patients who received enteral feeding pre-operatively progressed to varying levels of oral intake following combined endoscopic cricopharyngeal myotomy, glottopexy and glottal closure.
Endoscopic cricopharyngeal myotomy is reported to greatly improve oral intake in those with non-neurological UOS dysfunction. Takes et al.Reference Takes, van den Hoogen and Marres 14 reported a reduction, from 100 per cent to 40 per cent, in patients with idiopathic dysfunction requiring dietary alterations following surgery. In a study by Silver and Gal,Reference Silver and Gal 8 approximately 67 per cent of patients with UOS dysfunction following chemoradiation therapy who received total enteral feeding progressed to soft diets, and 17 per cent progressed to a regular diet. Similarly, 100 per cent of those on a liquid diet pre-operatively progressed to a regular diet post-operatively. Bachy et al.Reference Bachy, Matar, Remacle, Jamart and Lawson 17 reported dietary changes in an unspecified population, stating that 75 per cent of patients reported post-operative improvements in swallowing liquids, while 81 per cent experienced improvements in swallowing solids, as measured using the Deglutition Handicap Index.Reference Woisard and Lepage 18 Ho et al.Reference Ho, Morzaria and Damrose 30 investigated an unspecified population, noting that both endoscopic cricopharyngeal myotomy and open or transcervical cricopharyngeal myotomy resulted in comparably significant Functional Outcome Swallowing ScaleReference Crary, Mann and Groher 29 score changes.
The authors of the studies included in this systematic review did not detail changes in pre- and post-operative oral intake levels, and therefore no determination regarding intervention effects may be made. As UOS dysfunction often necessitates dietary changes,Reference Sudhakara, Satishchandra and Murthy 31 it is essential to address this in future studies to aid informed decision making and treatment planning.
Both of the included studies considered the adverse events associated with surgery, noting that no complications occurred. There is a dearth of evidence pertaining to endoscopic cricopharyngeal myotomy complications, which may relate to publication bias. With increasing clinical usage, the reporting of complications has proportionally increased, although this issue remains contentious.Reference Pitman and Weissbrod 9 In the USA and Australia, up to 16.6 per cent of hospital admissions across clinical specialties (e.g. orthopaedics, neurosurgery, thoracic and cardiac surgery, and general medicine) result in complications, with 7 per cent of complications producing permanent disabilities and 14 per cent causing death.Reference Brennan, Leape, Laird, Hebert, Localio and Lawthers 21 , Reference Vincent, Neale and Woloshynowych 32 – Reference Zeeshan, Dembe, Seiber and Lu 35 Therefore, reliable reporting of complications must be promoted.Reference Asadi-Lari, Tamburini and Gray 36 , Reference Gervais and Dorion 37
Regarding QoL, there is a small body of evidence to indicate positive changes following cricopharyngeal myotomy in non-neurological disease populations.Reference Hazarika, Parul, Kailesh, Balakrishnan, Hazarika and Singh 12 , Reference Vincent, Neale and Woloshynowych 32 Bachy et al.Reference Bachy, Matar, Remacle, Jamart and Lawson 17 retrospectively investigated post-operative QoL outcomes in an unspecified population. These authors found that endoscopic cricopharyngeal myotomy significantly improved psychosocial well-being, as measured using the Deglutition Handicap Index.Reference Woisard and Lepage 18 Gervais and DorionReference Gervais and Dorion 37 investigated QoL outcomes in patients presenting with oculopharyngeal syndrome who underwent open or transcervical cricopharyngeal myotomy or blepharoplasty. They reported significant QoL improvements following open or transcervical cricopharyngeal myotomy.
Neither of the studies included in this review addressed post-operative QoL changes. Nevertheless, consideration of the relationships between QoL, health needs and patient satisfactionReference Asadi-Lari, Tamburini and Gray 36 are important for future research studies.
Review limitations
This study has some limitations. For example, combined treatments were not considered, resulting in the exclusion of studies that may have enhanced findings. Bergeron and ChhetriReference Bergeron and Chhetri 6 found that the post-operative outcomes of those receiving intra-operative dilation were not significantly different to those receiving endoscopic cricopharyngeal myotomy alone. Therefore, future reviews may be justified in expanding criteria to accommodate treatment combinations.
Another limitation relates to the inability to conduct a meta-analysis to produce a unified statistical estimate of clinical effectiveness for endoscopic cricopharyngeal myotomy in adults with UOS dysfunction and neurological disease, because of the limited numbers of eligible studies identified. This limitation reflects a larger issue at the level of the wider literature relating to this field, and indicates an urgent need for methodologically rigorous and evidence-based research pertaining to endoscopic cricopharyngeal myotomy for UOS dysfunction.
Recommendations
High-quality evidence is required to support clinical decision-making. Directions for future research lie in reporting and conducting research studies. For example, trials should be reported according to either the Consolidated Standards of Reporting Trials (‘CONSORT’) statementReference Schulz, Altman and Moher 38 or the Strengthening the Reporting of Observational studies in Epidemiology (‘STROBE’) guidelines.Reference von Elm, Altman, Egger, Pocock, Gøtzsche and Vandenbroucke 39 Trials should provide precise patient demographic details. Outcome measures should be psychometrically robust, and assess the physiological, functional and psychological impact of impairment. Furthermore, trials should prioritise blinding where appropriate, to reduce detection and performance bias. In addition, patients should be followed up at immediate, short-term and long-term time points post-operatively.
Conclusion
There is insufficient evidence to determine the effectiveness of endoscopic cricopharyngeal myotomy in adults with upper oesophageal sphincter (UOS) dysfunction and neurological disease. Despite this, one cannot assume that endoscopic cricopharyngeal myotomy is an ineffective treatment, rather that there is a need for high-quality research to support this procedure. It is essential that the methodological quality of future trials is addressed comprehensively in order to: optimally select candidates for surgical intervention, safeguard patients against adverse effects, and ultimately improve QoL and overall dysphagia management in this vulnerable population.
Despite the discussed limitations, this review provides a clear description of the current status of literature pertaining to this topic. It should act as a point of departure for future methodologically rigorous research that can inform evidence-based practice guidelines for the care of adults with UOS dysfunction and neurological disease.
Acknowledgements
The authors wish to acknowledge the help of Trinity College Dublin librarians Isolde Harpur and Greg Sheaf.
