FRANK J. MILNE STATE-OF-THE-ART LECTURE


very high (up to 300 umol/L) in some PSS foals without evidence of encephalopathic signs. The chronicity of the hyperammonemia and, in some cases, its gradual development may somehow allow the brain to adapt to the elevated ammonia levels. PSS can be confirmed by liver ultrasonography, by ultrasound-guided, percutaneous trans-splenic in- jection of 10 mL agitated saline into the spleen with simultaneous echocardiography of the right heart (“bubblegram”) or by transrectal hepatic scintigra- phy.201,205 The bubblegram may have higher sen- sitivity because air bubbles will appear in the right heart almost immediately after splenic injection if PSS is present.201 Theoretically, if portal circula- tion is normal the liver will filter the bubbles and the bubbles will not reach the heart. Trans-rectal scin- tigraphy for the diagnosis of PSS is performed by placing a soft rubber catheter as far into the rectum as safely possible and administering 30 mC of tech- netium pertechnetate followed immediately by ra- dioisotope scanning of both the heart and liver. If the majority of technetium is detected first in the liver then PSS can be ruled out. Conversely, if most of the radioactivity appears in the heart first then the foal either has a PSS or the technetium was not administered “high” enough in the distal bowel (substances absorbed from the distal rectum enter the caudal vena cava, not the portal circulation).205 If a liver biopsy is performed (usually unnecessary), lobular atrophy characterized by decreased distance between the central veins and portal areas and ar- teriolar proliferation in the portal tract with mild proliferation of interlobular arteries are usually seen.201 The number and size of portal veins may be subjectively less than normal. These findings may vary depending upon location of the shunt (ex- tra- vs. intrahepatic), but all of the changes develop as a result of decreased oxygen supply to the liver caused by the PSS (recall that the majority of oxygen to the liver is delivered via the portal vein, not the hepatic arteries). Positive-contrast portography following anesthesia and catheterization of a mes- enteric (usually jejunal) vein or computed tomo- graphic angiography (Fig. 9) with contrast and ultra-sonography may all be required to determine the shunt location prior to considering surgical re- pair. Shunts may occur as either intrahepatic or extrahepatic and be single or multiple shunts.198 Shunts may occur due to aberrant connections be- tween the systemic circulation (often the caudal vena cava or azygous vein) and virtually any gastric, splenic or mesenteric vessel.198 Surgical repair has been successful using suture ligation, cellophane bands, or casein–stainless steel rings (ameroid rings) placed around the shunt.198,201 Immediate, total surgical ligation of the shunt vessel can cause life-threatening portal hypertension with increased mesenteric venous pressures and congestion of the bowel. As opposed to suture ligation, cellophane bands or casein rings can be fitted around the shunt vessel, modestly occluding blood flow initially; over


Fig. 9. Computed tomographic slice through the liver of a foal with a portosystemic shunt. Three vascular structures are seen aorta (A), caudal vena cava (C) and portal vein (P). At this point, well within the hepatic parenchyma, the portal vein should have branched indicating an intrahepatic shunt.


the following weeks gradual attenuation of the shunt occurs due to either vascular constriction as- sociated with the inflammation caused by cello- phane banding or because of gradual casein swelling in the ameroid rings.201 Placement of transvenous coils or umbrellas into shunts could also result in slow embolization and progressive closure of the shunt, but experience has shown that the umbrellas made for children are too small to cause a sufficient thrombus in the vessel in neonatal large animals.204 While preparing for diagnostics and surgery, treat- ments for foals with HE secondary to PSS are sim- ilar to HE treatment caused by intrinsic liver disease and failure (page 92). Enemas are often used on foals with HE if the manure becomes abnor- mally firm. One foal with a PSS and a blood am- monia as high as 300 umol/L was maintained on lactulose and minocycline per os, in addition to en- emas, until surgery could be attempted.


Hyperammonemia Syndrome in Morgan Foals


An acute onset of clinical signs such as coma, blind- ness, and seizures has been associated with the hy- perammonemia syndrome seen in Morgan foals, usually 4–7 months of age.199,206 The onset of clin- ical signs often occurs soon after weaning. Liver enzymes may be mildly elevated and variable de- grees of liver pathology can be seen on microscopic examination but the degree of microscopic disease is modest and other liver function tests are not abnor- mal in affected foals. The cause of the disease is unknown but it seems to be a disorder of metabolism of ammonia and possibly other amino acids. The blood ammonia is extremely high in affected foals (300–500 umol/L), indicating that they have likely developed some compensatory or protective mecha- nism against the persistent elevation in ammonia to


AAEP PROCEEDINGS  Vol. 61  2015 89


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241  |  Page 242  |  Page 243  |  Page 244  |  Page 245  |  Page 246  |  Page 247  |  Page 248  |  Page 249  |  Page 250  |  Page 251  |  Page 252  |  Page 253  |  Page 254  |  Page 255  |  Page 256  |  Page 257  |  Page 258  |  Page 259  |  Page 260  |  Page 261  |  Page 262  |  Page 263  |  Page 264  |  Page 265  |  Page 266  |  Page 267  |  Page 268  |  Page 269  |  Page 270  |  Page 271  |  Page 272  |  Page 273  |  Page 274  |  Page 275  |  Page 276  |  Page 277  |  Page 278  |  Page 279  |  Page 280  |  Page 281  |  Page 282  |  Page 283  |  Page 284  |  Page 285  |  Page 286  |  Page 287  |  Page 288  |  Page 289  |  Page 290  |  Page 291  |  Page 292  |  Page 293  |  Page 294  |  Page 295  |  Page 296  |  Page 297  |  Page 298  |  Page 299  |  Page 300  |  Page 301  |  Page 302  |  Page 303  |  Page 304  |  Page 305  |  Page 306  |  Page 307  |  Page 308  |  Page 309  |  Page 310  |  Page 311  |  Page 312  |  Page 313  |  Page 314  |  Page 315  |  Page 316  |  Page 317  |  Page 318  |  Page 319  |  Page 320  |  Page 321  |  Page 322  |  Page 323  |  Page 324  |  Page 325  |  Page 326  |  Page 327  |  Page 328  |  Page 329  |  Page 330  |  Page 331  |  Page 332  |  Page 333  |  Page 334  |  Page 335  |  Page 336  |  Page 337  |  Page 338  |  Page 339  |  Page 340  |  Page 341  |  Page 342  |  Page 343  |  Page 344  |  Page 345  |  Page 346  |  Page 347  |  Page 348  |  Page 349  |  Page 350  |  Page 351  |  Page 352  |  Page 353  |  Page 354  |  Page 355  |  Page 356  |  Page 357  |  Page 358  |  Page 359  |  Page 360  |  Page 361  |  Page 362  |  Page 363  |  Page 364  |  Page 365  |  Page 366  |  Page 367  |  Page 368  |  Page 369  |  Page 370  |  Page 371  |  Page 372  |  Page 373  |  Page 374  |  Page 375  |  Page 376  |  Page 377  |  Page 378  |  Page 379  |  Page 380  |  Page 381  |  Page 382  |  Page 383  |  Page 384  |  Page 385  |  Page 386  |  Page 387  |  Page 388  |  Page 389  |  Page 390  |  Page 391  |  Page 392  |  Page 393  |  Page 394  |  Page 395  |  Page 396  |  Page 397  |  Page 398  |  Page 399  |  Page 400  |  Page 401  |  Page 402  |  Page 403  |  Page 404  |  Page 405  |  Page 406  |  Page 407  |  Page 408  |  Page 409  |  Page 410  |  Page 411  |  Page 412  |  Page 413  |  Page 414  |  Page 415  |  Page 416  |  Page 417  |  Page 418  |  Page 419  |  Page 420  |  Page 421  |  Page 422  |  Page 423  |  Page 424  |  Page 425  |  Page 426  |  Page 427  |  Page 428  |  Page 429  |  Page 430  |  Page 431  |  Page 432  |  Page 433  |  Page 434  |  Page 435  |  Page 436  |  Page 437  |  Page 438  |  Page 439  |  Page 440  |  Page 441  |  Page 442  |  Page 443  |  Page 444  |  Page 445  |  Page 446  |  Page 447  |  Page 448  |  Page 449  |  Page 450  |  Page 451  |  Page 452  |  Page 453  |  Page 454  |  Page 455  |  Page 456  |  Page 457  |  Page 458  |  Page 459  |  Page 460  |  Page 461  |  Page 462  |  Page 463  |  Page 464  |  Page 465  |  Page 466  |  Page 467  |  Page 468  |  Page 469  |  Page 470  |  Page 471  |  Page 472  |  Page 473  |  Page 474  |  Page 475  |  Page 476  |  Page 477  |  Page 478  |  Page 479  |  Page 480  |  Page 481  |  Page 482  |  Page 483  |  Page 484  |  Page 485  |  Page 486  |  Page 487  |  Page 488  |  Page 489  |  Page 490  |  Page 491  |  Page 492  |  Page 493  |  Page 494  |  Page 495  |  Page 496  |  Page 497  |  Page 498  |  Page 499  |  Page 500  |  Page 501  |  Page 502  |  Page 503  |  Page 504  |  Page 505  |  Page 506  |  Page 507  |  Page 508  |  Page 509  |  Page 510  |  Page 511  |  Page 512  |  Page 513  |  Page 514  |  Page 515  |  Page 516  |  Page 517  |  Page 518  |  Page 519  |  Page 520  |  Page 521  |  Page 522  |  Page 523  |  Page 524  |  Page 525  |  Page 526  |  Page 527  |  Page 528  |  Page 529  |  Page 530  |  Page 531  |  Page 532  |  Page 533  |  Page 534  |  Page 535  |  Page 536  |  Page 537  |  Page 538  |  Page 539  |  Page 540  |  Page 541  |  Page 542  |  Page 543  |  Page 544  |  Page 545  |  Page 546  |  Page 547  |  Page 548  |  Page 549  |  Page 550  |  Page 551  |  Page 552  |  Page 553  |  Page 554  |  Page 555  |  Page 556  |  Page 557  |  Page 558  |  Page 559  |  Page 560  |  Page 561  |  Page 562  |  Page 563  |  Page 564  |  Page 565  |  Page 566  |  Page 567  |  Page 568  |  Page 569  |  Page 570  |  Page 571  |  Page 572  |  Page 573  |  Page 574  |  Page 575  |  Page 576  |  Page 577  |  Page 578  |  Page 579  |  Page 580  |  Page 581  |  Page 582  |  Page 583  |  Page 584  |  Page 585  |  Page 586  |  Page 587  |  Page 588  |  Page 589  |  Page 590  |  Page 591  |  Page 592  |  Page 593  |  Page 594  |  Page 595  |  Page 596