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differential diagnosis in small animal medicine by alex gough ma vetmb certsam certvc mrcvs

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© 2007 by alex gough blackwell publishing editorial offices blackwell publishing ltd 9600 garsington road oxford ox4 2dq uk tel +44 01865 776868 blackwell publishing professional 2121 state avenue ames iowa 50014-8300 usa tel +1 515 292 0140 blackwell publishing asia pty ltd 550 swanston street carlton victoria 3053 australia tel +61 03 8359 1011 the right of the author to be identified as the author of this work has been asserted in accordance with the copyright designs and patents act 1988 all rights reserved no part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise except as permitted by the uk copyright designs and patents act 1988 without the prior permission of the publisher first published 2007 by blackwell publishing ltd isbn 978-1-4051-3252-7 library of congress cataloging-in-publication data gough alex differential diagnosis in small animal medicine by alex gough p cm includes bibliographical references and index isbn-13 978-1-4051-3252-7 pbk alk paper isbn-10 1-4051-3252-3 pbk alk paper 1 dogs ­ diseases ­ diagnosis ­ handbooks manuals etc 2 cats ­ diseases ­ diagnosis ­ handbooks manuals etc 3 diagnosis differential ­ handbooks manuals etc i title [dnlm 1 animal diseases ­ diagnosis ­ handbooks 2 diagnosis differential ­ handbooks sf 748 g692d 2006 sf991.g672 2006 636.0896075 ­ dc22 2006013926 a catalogue record for this title is available from the british library set in 9/11.5 pt sabon by snp best-set typesetter ltd hong kong printed and bound in india by replika press pvt ltd kundli the publisher s policy is to use permanent paper from mills that operate a sustainable forestry policy and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices furthermore the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards for further information visit our subject website www.blackwellvet.com

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contents introduction part 1 historical signs 1.1 general systemic and metabolic historical signs 1.1.1 polyuria/polydipsia 1.1.2 weight loss 1.1.3 weight gain 1.1.4 polyphagia 1.1.5 anorexia/inappetence 1.1.6 failure to grow 1.1.7 syncope/collapse 1.1.8 weakness gastrointestinal/abdominal historical signs 1.2.1 ptyalism/salivation/hypersalivation 1.2.2 gagging/retching 1.2.3 dysphagia 1.2.4 regurgitation 1.2.5 vomiting 1.2.6 diarrhoea 1.2.7 melaena 1.2.8 haematemesis 1.2.9 haematochezia 1.2.10 constipation/obstipation 1.2.11 faecal tenesmus/dyschezia 1.2.12 faecal incontinence 1.2.13 flatulence/borborygmus cardiorespiratory historical signs 1.3.1 coughing 1.3.2 dyspnoea/tachypnoea 1.3.3 sneezing and nasal discharge 1.3.4 epistaxis 1.3.5 haemoptysis 1.3.6 exercise intolerance dermatological historical signs 1.4.1 pruritus neurological historical signs 1.5.1 seizures 1.5.2 trembling/shivering xiii 1 1 1 3 4 5 6 8 9 13 16 16 18 19 20 21 26 31 33 34 36 38 39 40 40 40 42 43 44 46 47 48 48 51 51 55 iii 1.2 1.3 1.4 1.5

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iv contents 1.5.3 1.5.4 1.5.5 1.5.6 1.5.7 1.5.8 1.5.9 1.6 ataxia/conscious proprioceptive deficits paresis/paralysis coma/stupor altered behaviour ­ general changes altered behaviour ­ specific behavioural problems deafness multifocal neurological disease 57 65 70 72 74 75 77 80 80 82 83 83 87 90 91 91 92 94 95 97 98 98 100 101 101 101 102 102 104 ocular historical signs 1.6.1 blindness/visual impairment 1.6.2 epiphora/tear overflow musculoskeletal historical signs 1.7.1 forelimb lameness 1.7.2 hind limb lameness 1.7.3 multiple joint/limb lameness reproductive historical signs 1.8.1 failure to observe oestrus 1.8.2 irregular seasons 1.8.3 infertility in the female with normal oestrus 1.8.4 male infertility 1.8.5 vaginal/vulval discharge 1.8.6 abortion 1.8.7 dystocia 1.8.8 neonatal mortality urological historical signs 1.9.1 pollakiuria/dysuria/stranguria 1.9.2 polyuria/polydipsia 1.9.3 anuria/oliguria 1.9.4 haematuria 1.9.5 urinary incontinence/inappropriate urination 1.7 1.8 1.9 part 2 physical signs 2.1 general/miscellaneous physical signs 2.1.1 abnormalities of body temperature ­ hyperthermia 2.1.2 abnormalities of body temperature ­ hypothermia 2.1.3 enlarged lymph nodes 2.1.4 diffuse pain 2.1.5 peripheral oedema 2.1.6 hypertension 2.1.7 hypotension gastrointestinal/abdominal physical signs 2.2.1 oral lesions 2.2.2 abdominal distension 2.2.3 abdominal pain 2.2.4 perianal swelling 2.2.5 jaundice 2.2.6 abnormal liver palpation 106 106 106 110 111 113 114 115 116 118 118 120 120 123 124 126 2.2

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contents v 2.3 cardiorespiratory physical signs 2.3.1 dyspnoea/tachypnoea 2.3.2 pallor 2.3.3 shock 2.3.4 cyanosis 2.3.5 ascites 2.3.6 peripheral oedema 2.3.7 abnormal respiratory sounds 2.3.8 abnormal heart sounds 2.3.9 abnormalities in heart rate 2.3.10 jugular distension/positive hepatojugular reflux 2.3.11 jugular pulse components 2.3.12 alterations in arterial pulse dermatological signs 2.4.1 scaling 2.4.2 pustules and papules including miliary dermatitis 2.4.3 nodules 2.4.4 pigmentation disorders coat or skin 2.4.5 alopecia 2.4.6 erosive/ulcerative skin disease 2.4.7 otitis externa 2.4.8 pododermatitis 2.4.9 disorders of the claws 2.4.10 anal sac/perianal disease neurological signs 2.5.1 abnormal cranial nerve cn responses 2.5.2 vestibular disease head tilt nystagmus circling leaning falling rolling 2.5.3 horner s syndrome 2.5.4 hemineglect syndrome 2.5.5 spinal disorders ocular 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6 2.6.7 signs red eye corneal opacification corneal ulceration/erosion lens lesions retinal lesions intraocular haemorrhage/hyphaema abnormal appearance of anterior chamber 128 128 133 133 135 136 136 137 138 143 145 145 146 147 147 149 151 153 155 157 158 160 162 163 164 164 167 171 172 172 174 174 178 179 180 182 183 184 185 185 186 187 187 187 189 190 2.4 2.5 2.6 2.7 musculoskeletal signs 2.7.1 muscular atrophy or hypertrophy 2.7.2 trismus lockjaw 2.7.3 weakness urogenital physical signs 2.8.1 kidneys abnormal on palpation 2.8.2 bladder abnormalities 2.8.3 prostate abnormal on palpation 2.8

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vi contents 2.8.4 2.8.5 2.8.6 uterus abnormal on palpation testicular abnormalities penis abnormalities 191 191 192 193 193 193 193 195 199 201 202 203 203 205 205 206 209 211 212 213 214 217 217 219 221 224 230 230 233 234 236 238 239 239 240 241 241 242 243 243 243 244 245 246 247 part 3 radiographic and ultrasonographic signs 3.1 thoracic radiography 3.1.1 artefactual causes of increased lung opacity 3.1.2 increased bronchial pattern 3.1.3 increased alveolar pattern 3.1.4 increased interstitial pattern 3.1.5 increased vascular pattern 3.1.6 decreased vascular pattern 3.1.7 cardiac diseases that may be associated with a normal cardiac silhouette 3.1.8 increased size of cardiac silhouette 3.1.9 decreased size of cardiac silhouette 3.1.10 abnormalities of the ribs 3.1.11 abnormalities of the oesophagus 3.1.12 abnormalities of the trachea 3.1.13 pleural effusion 3.1.14 pneumothorax 3.1.15 abnormalities of the diaphragm 3.1.16 mediastinal abnormalities abdominal radiography 3.2.1 liver 3.2.2 spleen 3.2.3 stomach 3.2.4 intestines 3.2.5 ureters 3.2.6 bladder 3.2.7 urethra 3.2.8 kidneys 3.2.9 loss of intra-abdominal contrast 3.2.10 prostate 3.2.11 uterus 3.2.12 abdominal masses 3.2.13 abdominal calcification/mineral density skeletal 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7 3.3.8 3.3.9 radiography fractures altered shape of long bones dwarfism delayed ossification/growth plate closure increased radiopacity periosteal reactions bony masses osteopenia osteolysis 3.2 3.3

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contents vii 3.3.10 3.3.11 3.4 mixed osteolytic/osteogenic lesions joint changes 248 248 251 251 251 252 252 253 253 254 255 255 256 257 257 258 258 259 261 262 263 265 265 266 266 267 270 272 272 274 277 279 279 280 281 283 284 285 288 288 288 290 292 292 292 radiography of the head and neck 3.4.1 increased radiopacity/bony proliferation of the maxilla 3.4.2 decreased radiopacity of the maxilla 3.4.3 increased radiopacity/bony proliferation of the mandible 3.4.4 decreased radiopacity of the mandible 3.4.5 increased radiopacity of the tympanic bulla 3.4.6 decreased radiopacity of the nasal cavity 3.4.7 increased radiopacity of the nasal cavity 3.4.8 increased radiopacity of the frontal sinuses 3.4.9 increased radiopacity of the pharynx 3.4.10 thickening of the soft tissues of the head and neck 3.4.11 decreased radiopacity of the soft tissues of the head and neck 3.4.12 increased radiopacity of the soft tissues of the head and neck radiography of the spine 3.5.1 normal and congenital variation in vertebral shape and size 3.5.2 acquired variation in vertebral shape and size 3.5.3 changes in vertebral radiopacity 3.5.4 abnormalities in the intervertebral space 3.5.5 contrast radiography of the spine myelography thoracic ultrasonography 3.6.1 pleural effusion 3.6.2 mediastinal masses 3.6.3 pericardial effusion 3.6.4 altered chamber dimensions 3.6.5 changes in ejection phase indices of left ventricular performance abdominal ultrasonography 3.7.1 renal disease 3.7.2 hepatobiliary disease 3.7.3 splenic disease 3.7.4 pancreatic disease 3.7.5 adrenal disease 3.7.6 urinary bladder disease 3.7.7 gastrointestinal disease 3.7.8 ovarian and uterine disease 3.7.9 prostatic disease 3.7.10 ascites ultrasonography of other regions 3.8.1 testes 3.8.2 eyes 3.8.3 neck 3.5 3.6 3.7 3.8 part 4 laboratory findings 4.1 biochemical findings 4.1.1 albumin

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viii contents 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.1.9 4.1.10 4.1.11 4.1.12 4.1.13 4.1.14 4.1.15 4.1.16 4.1.17 4.1.18 4.1.19 4.1.20 4.1.21 4.1.22 4.1.23 4.1.24 4.1.25 4.1.26 4.1.27 4.1.28 4.2 alanine transferase alkaline phosphatase ammonia amylase aspartate aminotransferase bilirubin bile acids/dynamic bile acid test c-reactive protein cholesterol creatinine creatine kinase ferritin fibrinogen folate fructosamine gamma-glutamyl transferase gastrin globulins glucose iron lactate dehydrogenase lipase triglycerides trypsin-like immunoreactivity urea vitamin b12 cobalamin zinc 293 295 296 297 298 298 299 300 301 301 302 303 303 304 304 305 306 306 307 309 310 311 312 313 313 316 317 317 317 320 323 324 327 328 330 331 332 333 334 335 335 335 336 337 338 338 haematological findings 4.2.1 regenerative anaemia 4.2.2 poorly non-regenerative anaemia 4.2.3 polycythaemia 4.2.4 thrombocytopenia 4.2.5 thrombocytosis 4.2.6 neutrophilia 4.2.7 neutropenia 4.2.8 lymphocytosis 4.2.9 lymphopenia 4.2.10 monocytosis 4.2.11 eosinophilia 4.2.12 eosinopenia 4.2.13 mastocythaemia 4.2.14 basophilia 4.2.15 increased buccal mucosal bleeding time disorders of primary haemostasis 4.2.16 increased prothrombin time disorders of extrinsic and common pathways 4.2.17 increased partial thromboplastin time or activated clotting time disorders of intrinsic and common pathways 4.2.18 increased fibrin degradation products

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contents ix 4.2.19 4.2.20 4.3 decreased fibrinogen levels decreased antithrombin iii levels 338 339 339 339 342 343 345 347 348 350 353 354 354 354 354 354 356 360 362 364 364 366 367 369 369 370 372 373 373 374 375 376 376 376 377 377 378 378 379 379 379 380 380 381 381 electrolyte and blood gas findings 4.3.1 total calcium 4.3.2 chloride 4.3.3 magnesium 4.3.4 potassium 4.3.5 phosphate 4.3.6 sodium 4.3.7 ph 4.3.8 pao2 4.3.9 total co2 4.3.10 bicarbonate 4.3.11 base excess urinalysis findings 4.4.1 alterations in specific gravity 4.4.2 abnormalities in urine chemistry 4.4.3 abnormalities in urine sediment 4.4.4 infectious agents cytological findings 4.5.1 tracheal/bronchoalveolar lavage 4.5.2 nasal flush cytology 4.5.3 liver cytology 4.5.4 kidney cytology 4.5.5 skin scrapes/hair plucks/tape impressions 4.5.6 cerebrospinal fluid csf analysis 4.5.7 fine needle aspiration of cutaneous/subcutaneous masses hormones/endocrine testing 4.6.1 thyroxine 4.6.2 parathyroid hormone 4.6.3 cortisol baseline or post-acth stimulation test 4.6.4 insulin 4.6.5 acth 4.6.6 vitamin d 1,25 dihydroxycholecalciferol 4.6.7 testosterone 4.6.8 progesterone 4.6.9 oestradiol 4.6.10 atrial natriuretic peptide 4.6.11 modified water deprivation test in the investigation of polyuria/polydipsia faecal analysis findings 4.7.1 faecal blood 4.7.2 faecal parasites 4.7.3 faecal culture 4.7.4 faecal fungal infections 4.7.5 undigested food residues 4.4 4.5 4.6 4.7

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x contents part 5 electrodiagnostic testing 5.1 ecg findings 5.1.1 alterations in p wave 5.1.2 alterations in qrs complex 5.1.3 alterations in p-r relationship 5.1.4 alterations in s-t segment 5.1.5 alterations in q-t interval 5.1.6 alterations in t wave 5.1.7 alterations in baseline 5.1.8 rhythm alterations 5.1.9 alterations in rate electromyographic findings nerve conduction velocity findings electroencephalography findings 382 382 382 383 384 387 387 388 388 389 392 394 395 395 397 397 398 399 400 401 401 402 402 402 403 404 406 406 407 408 409 410 410 411 412 412 413 413 414 414 414 5.2 5.3 5.4 part 6 diagnostic procedures 6.1 6.2 6.3 6.4 6.5 fine-needle aspiration fna bronchoalveolar lavage gastrointestinal gi endoscopic biopsy electrocardiography ecg magnetic resonance imaging mri 6.5.1 brain 6.5.2 spine 6.5.3 nasal passages ultrasound-guided biopsy cerebrospinal fluid csf collection bone marrow aspiration thoraco pericardio cysto and abdominocentesis 6.9.1 thoracocentesis 6.9.2 pericardiocentesis 6.9.3 cystocentesis 6.9.4 abdominocentesis/diagnostic peritoneal lavage 6.6 6.7 6.8 6.9 6.10 blood pressure measurement 6.10.1 central venous pressure 6.10.2 indirect blood pressure measurement by doppler technique 6.11 dynamic testing 6.11.1 acth stimulation test 6.11.2 low-dose dexamethasone suppression test lddst 6.11.3 bile acid stimulation test 6.12 haematological techniques 6.12.1 in saline autoagglutination test 6.12.2 preparation of a blood smear

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contents xi 6.12.3 6.12.4 buccal mucosal bleeding time arterial blood sampling 415 416 416 418 419 419 420 421 421 422 423 425 426 426 429 430 431 432 433 434 435 436 437 438 439 440 441 443 445 448 451 453 6.13 water deprivation test 6.14 serial blood glucose curve 6.15 skin scraping 6.16 schirmer tear test 6.17 nasal flush cytology/nasal biopsy 6.18 contrast radiography 6.18.1 barium meal/swallow 6.18.2 intravenous urography 6.18.3 contrast cystography 6.18.4 myelography 6.19 contrast echocardiography 6.20 cranial nerve cn examination part 7 diagnostic algorithms 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 bradycardia tachycardia hypoalbuminaemia non-regenerative anaemia regenerative anaemia jaundice hypokalaemia hyperkalaemia hypocalcaemia 7.10 hypercalcaemia 7.11 systemic hypertension appendix appendix appendix appendix a history record b physical examination record c neurological examination chart d cardiology consultation form bibliography and further reading index colour plate section follows p 240

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to naomi and abigail for their patience and support and mac for a lifetime of companionship.

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introduction this book was written to fill what i felt was a gap in the market while working up difficult medical cases i have often wanted a single ready reference to help me formulate a differential list from the clinical information i have available unfortunately i found myself frequently having to consult multiple textbooks to bring all the information i needed together i decided therefore to write a book that would serve as a ready reference for differential diagnoses of the majority of presentations that are encountered in practice including both common and uncommon conditions this text should be of use to veterinary students general practitioners university interns residents and anyone who like me cannot fully carry these lists around in their heads i hope other clinicians find it as useful as i do the differential diagnosis list is one of the most important aspects of the problemoriented approach to clinical diagnosis for those who are not familiar with the problemoriented approach a brief outline follows as the name implies problem-oriented medical management pomm concentrates on the individual problems of a patient a differential diagnosis list should be made for each and every problem that is found in a patient whether in the history the physical examination imaging or clinicopathological tests although superficially this may not sound very `holistic in fact if all the patient s problems are considered individually the whole patient will have been evaluated without falling into the trap of presuming that all of the findings are caused by a single condition the problem-oriented approach starts with a detailed history and it is important to discover what the owners perceive to be the main problems ­ after all they usually know their animal better than the clinician does however there may be relevant historical signs that the owners had not considered significant so failing to systematically ask all the questions which could be of importance in a case can lead to overlooking important information using a checklist or form such as the one in appendix a can be useful as an aide-memoire in every case a detailed physical examination should be carried out including body systems that are not apparently of immediate concern again a checklist or form such as the one in appendix b can help ensure a systematic approach once the history has been taken and the physical examination has been completed the clinician should list every problem that has been discovered problems may include such findings as exercise intolerance pruritus pyrexia or a heart murmur a differential diagnosis list should then be created for every problem the list should be appropriate to that animal there is no point listing feline leukaemia virus as a likely diagnosis in a dog an attempt should also be made to categorise the conditions in order of likelihood or at least into common and uncommon although the more common conditions have been indicated in this book with an asterisk there are few objective data regarding the true incidence of conditions and the estimate of incidence is largely subjective and influenced by the author s geographical location and caseload familiarity with how xiii

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xiv introduction common conditions are and their local incidence will help prioritise differential lists the clinician can then select diagnostic tests in a rough order of probability although rarer but life-threatening conditions such as hypoadrenocorticism should also be ruled out early in the course of investigations some authorities rightly point out that emphasis should be placed on historical and physical signs and that `over-investigating can be expensive and potentially detrimental to the patient chesney 2003 it is this author s opinion however that it is possible to place too much importance on probabilities and how commonly a condition occurs the newly-qualified veterinary surgeon will often look for the rare but exciting and memorable condition they learned about at college while the experienced practitioner will often remind them that `common things are common and suggest they restrict their investigations only to commonly-encountered conditions the ideal approach is probably somewhere in between although it is self-evidently true that common things are common it is also true that uncommon things are encountered relatively often to take a hypothetical example if a common problem is caused by common conditions a and b with a frequency of 80 and by rare conditions c to z the rest of the time with conditions c to z occurring with equal frequency then each individual condition c to z will be responsible for the problem approximately 0.9 of the time making each individual condition quite uncommon however 1 in 5 presentations of this problem will be caused by an uncommon condition and so uncommon conditions will be diagnosed commonly provided they are looked for the problem-oriented approach ensures that these uncommon conditions are not overlooked some authorities prefer to categorise the initial approach to a case differently and describe the subjective and objective assessment of a patient as part of the soap approach subjective objective assessment plan the principle is the same however in that a detailed history or physical examination is the basis of the initial differential list once the differential diagnosis list has been formulated the clinician is in a position to select appropriate tests to aid in making a definitive diagnosis prioritising the selection of diagnostic tests helps avoid placing undue financial strain on the client and inappropriate or unnecessary testing on the patient tests may be prioritised on such factors as the number of conditions which will be ruled in and out the sensitivity and specificity of the tests the risk/benefit to the patient ratio the financial cost/benefit to the client ratio the incidence or prevalence of the condition being tested for the importance of the condition being tested for e.g hypoadrenocorticism is uncommon but the consequences of failing to diagnose it may be serious after the results of initial testing have been obtained the clinician may be in a position to make a definitive diagnosis often however it is necessary to refine the differential list and select further appropriate testing the differential list may be reformulated as often as is necessary until a single diagnosis for that problem is made often a single diagnosis will tie in all the problems satisfactorily however in many cases particularly in geriatric patients concurrent disorders will require multiple diagnoses for problem cases in which a clear diagnosis is not made or the patient fails to respond to treatment as expected returning to the beginning with the history and physical examination with the condition often having progressed can be helpful however very few tests are 100 sensitive and specific and many `definitive diagnoses in fact leave room for some doubt the clinician should never be afraid to revise the initial diagnosis if further evidence comes to light those who are concerned that failing to make the correct diagnosis in every case is somehow a sign of inferior clinical abilities

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introduction xv should take heart from a recent study from the school of veterinary medicine at the university of california kent et al 2004 in this paper clinical and post mortem diagnoses of 623 dogs treated between 1989 and 1999 at the veterinary teaching hospital were compared it was found that the post mortem diagnosis presumed to be the correct diagnosis differed from the clinical diagnosis in approximately 1/3 of cases this book is organised into seven parts part 1 deals with signs likely to be uncovered during history taking part 2 deals with signs encountered at the physical examination part 3 deals with imaging findings part 4 with clinicopathological findings and part 5 electrophysiological findings part 6 outlines the techniques involved in some common diagnostic procedures and part 7 contains some algorithms to aid in the diagnosis of common clinical presentations four appendices containing checklists for diagnostic investigations and a bibliography follow the individual lists are categorised as i felt was logical for example by the damnitv organisation damnit-v is a mnemonic for remembering the various pathological processes that may cause a disease d ­ degenerative a ­ anomalous usually listed as congenital in this book m ­ metabolic n ­ nutritional neoplastic i ­ inflammatory infectious immune-mediated iatrogenic idiopathic t ­ traumatic toxic v ­ vascular this categorisation is not appropriate in all cases however the individual lists are largely organised alphabetically the more common conditions are labelled with an asterisk but as stated above whether or not a condition is considered to be common is largely a matter of subjective opinion those conditions that are predominantly or exclusively found only in dogs are marked with a d and those in cats are marked with a c sources for the information in this book are wide ranging a large number of textbooks listed in the bibliography were consulted but in most cases it was necessary to expand the lists found in these sources using information from veterinary journals and conference proceedings although there are undoubtedly omissions from some of the lists encompassing as this book does virtually the whole of small animal veterinary medicine i have tried to make it as comprehensive as possible i would be happy to hear of any omissions corrections or comments on the text which can be e-mailed with any supporting references to alex.gough@btconnect.com i am grateful to simon platt bvm&s dipacvim dipecvn mrcvs chris belford bvsc dvsc facvsc rcvs specialist pathologist dip wldl mgt theresa mccann bvsc certsam mrcvs rosie mcgregor bvsc certvd certvc mrcvs and mark bush ma vetmb certsas mrcvs for comments on the text i am equally grateful to alison thomas bvsc certsam mrcvs mark maltman bvsc certsam certvc mrcvs panagiotis mantis dvm dipecvdi mrcvs axiom laboratories stuart caton ba vetmb certsam mrcvs tim knott bsc bvsc certvetophth mrcvs lisa phillips certvr bvetmed mrcvs roderick macgregor bvm&s certvetophth certsas mrcvs and mark owen bvsc certsao mrcvs for their comments on the text any errors are of course mine and not theirs i am also grateful to samantha jackson at blackwell publishing for her support in this project.

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