MUSCLE DISORDERS 355.0 Ϯ 193 U/l, respectively.
Serum vitamin E and whole-blood selenium concentrations are normal in draft horses with PSSM.7 Pathogenesis mon draft breeds in the United States.
To determine the true prevalence of PSSM in the Belgian horse population, a prospective study was performed by Firshman et al.7 Muscle biopsies obtained from 113 Belgian horses on four farms revealed a prevalence of PSSM of 36% using amylase-resistant glycogen as the diagnostic criteria. Genetics The prevalence of PSSM did not vary between different Belgian breeding farms in the study by Firshman et al.7 The notable sharing of bloodlines among all the horses in that study made pedigree analysis problematic in determining a pattern of inheritance.
Several full- and half-sibling Belgian horses were identiﬁed with PSSM and many Belgian/light breed crosses have been identiﬁed with PSSM, indicating that further genetic studies of this disorder is warranted. Clinical Signs The average age of draft horses diagnosed with PSSM is 8 yr of age.8,21 No particular gender predilection has been identiﬁed for PSSM in draft horses.
It is notable that clinical signs are not consistently present with PSSM, because many of the Belgian draft horses that were positive for PSSM in the study by Firshman et al.7 had no clinical signs.
PSSM in draft horses likely is the same disorder described as “Monday Morning Disease” in work horses in the early 20th century.32 Exertional rhabdomyolysis is a manifestation of PSSM in draft horses as well as draft crosses and can be so severe that it leads to recumbency and death.11,32,33 In addition, post-anesthetic myopathy may also be a complication of PSSM in draft breeds.10 A number of draft horses with PSSM present with signs of progressive weakness and muscle loss resulting in difﬁculty rising.11,16 In these cases, serum CK activity is frequently normal.
Gait abnormalities such as excessive limb ﬂexion, fasciculations, and trembling are commonly seen with PSSM in draft horses.11,16,21 Although the condition shivers was previously attributed to PSSM,34 a recent study found no causal association between these two conditions.7 The very high prevalence of PSSM in draft horses in essence means that there is a 36% chance that any clinical sign could be falsely associated with the disease PSSM.
Thus, clinical judgment is required to determine whether the muscle biopsy results could reasonably be associated with a myopathic process or if other possible causes of muscle weakness or gait changes should also be investigated.
Serum muscle-enzyme activities are often normal in draft horses with PSSM.7 The median serum CK and AST activity in draft horses from which biopsies were sent to the NDL at the University of Minnesota was 459 and 537 U/l, respectively.
Mean CK and AST activities in the Belgian horse study by Firshman et al.7 were 326 Ϯ 380 and 376 2006 ր Vol. 52 ր AAEP PROCEEDINGS Similar to Quarter Horses, muscle-glycogen concentrations in draft horses with PSSM are ϳ1.8-fold higher than in healthy horses.7 In addition, abnormalities in enzymes involved in glycogenolysis or glycolysis have not been identiﬁed in draft breeds (unpublished observation).
Unlike Quarter Horses with PSSM, Belgian horses do not seem to have heightened insulin sensitivity as evidenced by normal results during euglycemic hyperinsulinemic clamping.35 Detailed studies of energy metabolism have not been performed in draft horses with PSSM and are necessary to further elucidate the biochemical basis for this disorder.
Based on difference in clinical signs and physiological responses between draft and Quarter Horses, it cannot be assumed at this point that this glycogen-storage disorder is identical to that found in Quarter Horses. 6.
PSSM in Warmbloods Prevalence The true prevalence of PSSM in other horse breeds remains to be established.
Based on the number of horses diagnosed with PSSM from muscle biopsies submitted to the NDL in Minnesota, PSSM seems to be a common neuromuscular disorder in Warmblood horses with ϳ50% of Warmblood biopsies being diagnosed with PSSM.21 This included a wide variety of Warmblood breeds such as Dutch Warmbloods, Hanoverian, Westfalian, Canadian Warmblood, Irish Sport Horse, Gerdlander, Hussien, and Rheinlander. Genetics No reports on the potential inheritance of PSSM in Warmbloods have been published. Clinical Signs The mean age of onset of clinical signs in Warmbloods is reported to be between 8 and 11 yr of age.12,21 A gender predilection for PSSM has not been identiﬁed.21 The most common clinical signs reported in Warmbloods with PSSM are painful ﬁrm back and hindquarter muscles, reluctance to collect and engage the hindquarters, poor rounding over fences, gait abnormalities, and atrophy.12,21,36 Overt signs of exertional rhabdomyolysis, such as stiffness, shortness of stride, and reluctance to move after exercise, were reported in Ͻ15% of Warmbloods with PSSM.21 The median CK and AST activity for Warmbloods diagnosed with PSSM at the NDL in Minnesota is 323 and 331 U/l, respectively. Pathogenesis — Rest PSSM horses that are conﬁned for days to weeks after an episode of rhabdomyolysis often have persistently elevated serum CK activity.24 In contrast, PSSM horses kept on pasture with little grain supplementation often show few clinical signs of rhabdomyolysis and have normal serum CK activity.20,22 As a result, a common recommendation for horses with PSSM is to limit stall conﬁnement to Ͻ48 h after an episode of rhabdomyolysis and then provide turnout in paddocks of gradually increasing size.
Providing horses with as much free exercise as possible on pasture seems to be beneﬁcial in the long term.
After an acute episode, excitable horses may require tranquilization before turnout to avoid excessive galloping.
Hand-walking horses recovering from an episode of PSSM for Ͼ5–10 min at time may trigger another episode of rhabdomyolysis. Exercise Regimens adequate time for adaptation to a new diet before commencing exercise, (2) recognize that the duration of exercise, not the intensity, is of primary importance, (3) ensure that the program is gradually introduced and consistently performed, and (4) minimize any days without some form of exercise.8,24 If horses have experienced an episode of rhabdomyolysis recently, 2 wk of turnout and diet change are often beneﬁcial before recommencing exercise.
Exercise should be very relaxed, and the horse should achieve a long, low frame without collection.
For many horses, this is most readily done in a round pen or on a lunge line.
Successive daily addition of 2-min intervals of walk and trot, beginning with only 4 min of exercise and working up to 30 min, after 3 wk is often recommended.8,12,24 Owners often do not recognize that walking the horse for 10 min or more initially can trigger muscle soreness in PSSM horses.
Advancing the horse too quickly often results in an episode of rhabdomyolysis and repeated frustration for the owner.
Work can usually begin under saddle after 3 wk of ground work and can gradually be increased by adding 2-min intervals of collection or canter to the initial relaxed warm-up period at a walk and trot.
Unless a horse shows an episode of overt rhabdomyolysis during the initial ﬁrst 4 wk of exercise, reevaluating serum CK activity is not usually helpful for the ﬁrst month.
This is because it is very common to have subclinical elevations in CK activity when exercise is reintroduced, and a return to normal levels often requires 4 – 6 wk of gradual exercise.23,24 Keeping horses with PSSM ﬁt seems the best prevention against further episodes of rhabdomyolysis.
This gradual approach to reintroducing exercise aims to enhance the oxidative capacity of skeletal muscle without causing further cellular damage.
The oxidative capacity of locomotor muscles in most Quarter Horses is very low but can be increased with daily exercise.23,26 The objective of enhancing oxidative metabolism is to facilitate the metabolism of fat as an energy substrate. Dietary Management of PSSM Important principles to follow when starting exercise programs in PSSM horses include (1) provide The dietary modiﬁcation for PSSM horses is designed to reduce the glucose load and provide fat as an alternate energy source.
Anecdotally, owners report that this type of diet improves clinical signs of muscle pain, stiffness, and exercise tolerance in draft horses, Warmbloods, Quarter Horses, and other breeds.8,12,16 Dietary change seems to have less impact on alleviating gait changes such as shivers.12 The value of low-starch, high-fat diets in reducing exercise-induced muscle damage has only been shown under controlled experimental conditions in Quarter Horses.23 In PSSM Quarter Horses with increased sensitivity to insulin, dropping dietary starch to Ͻ10% of daily digestible energy and increasing dietary fat to 13% of daily digestible energy resulted in normal serum CK activity 4 h post-exercise during a 6-wk trial.
ProviAAEP PROCEEDINGS ր Vol. 52 ր 2006 377 IN-DEPTH: Table 1. MUSCLE DISORDERS Feeding Recommendations for an Average-Sized Horse (500 kg) with PSSM at Various Levels of Exertion Maintenance Digestible energy (Mcal/day) % DE as NSC % DE as fat Forage % bwt Protein (grams/day) Calcium (g) Phosphorus (g) Sodium (g) Chloride (g) Potassium (g) Selenium (mg) Vitamin E (IU) 16.4 Ͻ10% 20% 1.5–2.0% 697 30 20 22.5 33.8 52.5 1.88 375 Light Exercise 20.5 Ͻ10% 20% 1.5–2.0% 767 33 22 33.5 50.3 78.3 2.2 700 Moderate Exercise 24.6 Ͻ10% 15%–20% 1.5–2.0% 836 36 24 33.8 50.6 78.8 2.81 900 Intense Exercise 32.8 Ͻ10% 15%–20% 1.5–2.0% 906 39 26 41.3 62 96.4 3.13 1000 — dition, selection of a low-starch, fat-supplemented feed that is particularly high in dietary ﬁber may be the best means of providing dietary fat without causing excessive weight gain.
PSSM horses that have normal body weight can be fed forage at the rate of 1.5–2% of body weight.
A wide variety of low-starch, high-fat diets are available for horses.
The most important dietary principle seems to be that of the total daily calories required or digestible energy (DE); Ͻ10% should be supplied by starch, and Ն13% should be supplied by fat.
Some authors recommend that 20% of daily caloric intake be supplied by fat (0.5 kg of fat) based on clinical experiences,16 whereas others report improvement in clinical signs when 10 –15% of DE is supplied as fat.8,12,23,37 There is a great deal of variation in individual tolerance to dietary starch; however, horses with more severe clinical signs of PSSM seem to require the greatest restriction in starch intake.23 A number of well-balanced, low-starch, high-fat commercial diets are suitable for horses with PSSM.
There is, at present, no research to suggest that one form of fat is more beneﬁcial than another.
Some commercial feeds meet the recommended nutritional needs of PSSM horses in one pelleted ration.
These feeds typically contain Ͼ10% of fat by weight and Ͻ20% of starch or non-structural carbohydrate (NSC) by weight.
Some feed companies offer similar nutritional content by blending their manufactured feeds or by supplementing with oils or rice bran.
Palatability of pelleted feeds is usually higher than feeds containing oils or loose rice bran.
At present, the NSC content of equine-feed products is not listed on the feed tag, and consultation with the feed manufacturer is necessary to obtain this information.
Nutritional support is available through most feed manufacturers in designing an appropriate diet using the recommendations provided in Table 1.
The NDL provides a list of sug- IN-DEPTH: gested diets together with the results of musclebiopsy evaluation. Expectations of Fat Supplementation MUSCLE DISORDERS The time required for improvement in signs of exertional rhabdomyolysis is controversial.
Serum CK activity declined to normal in PSSM Quarter Horses within 3 wk of beginning a high-fat, low-starch diet combined with regular daily exercise.23 Others have suggested that a minimum of 4 mo of supplementation is required.16 Firshman et al.8 found that 75% of Quarter Horses showed clinical signs of improvement when both diet and exercise were changed; however, signiﬁcantly fewer horses showed improvement if dietary changes were instituted without introducing a gradually increasing exercise regime.
Hunt et al.12 found the 54% of Warmbloods showed improved clinical signs if both diet and exercise regimens were followed.
A portion of the proﬁts from the sale of Re-Leve are donated to the University of Minnesota. References and Footnote 1.
DiMauro S, Lamperti C.
Muscle Nerve 2001;24:984 –999. 2.
Tsujino S, Nonaka I, DiMauro S.
Glycogen storage myopathies.
Neurol Clin 2000;18:125–150. 3.
Arad M, Benson DW, Perez-Atayde AR, et al.
Constitutively active AMP kinase mutations cause glycogen storage disease mimicking hypertrophic cardiomyopathy.
J Clin Invest 2002;109:357–362. 4.
Valberg SJ, Cardinet GH III, Carlson GP, et al.
Polysaccharide storage myopathy associated with recurrent exertional rhabdomyolysis in horses.
Neuromuscul Disord 1992;2:351– 359. 5.
Valberg SJ, Geyer C, Sorum SA, et al.
Familial basis of exertional rhabdomyolysis in Quarter horse-related breeds.
Am J Vet Res 1996;57:286 –290. 6.
Valentine BA, Cooper BJ.
Incidence of polysaccharide storage myopathy: necropsy study of 225 horses.
Vet Pathol 2005;42:823– 827. 7.
Firshman AM, Baird JD, Valberg SJ.
Prevalence and clinical signs of polysaccharide storage myopathy and shivers in Belgian draft horses.
J Am Vet Med Assoc 2005;227:1958 – 1964. 8.
Firshman AM, Valberg SJ, Bender JB, et al.
Epidemiologic characteristics and management of polysaccharide storage myopathy in Quarter horses.
Am J Vet Res 2003;64:1319 – 1327. 9.
Valberg SJ, Mickelson JR, Gallant EM, et al.
Exertional rhabdomyolysis in Quarter horses and Thoroughbreds: one syndrome, multiple aetiologies.
Equine Vet J 1999; 30(Suppl):533–538. 10.
Bloom BA, Valentine BA, Gleed RD, et al.
Postanaesthetic recumbency in a Belgian ﬁlly with polysaccharide storage myopathy.
Vet Rec 1999;144:73–75. 11.
Valentine BA, Credille KM, Lavoie JP, et al.
Severe polysaccharide storage myopathy in Belgian and Percheron draught horses.
Equine Vet J 1997;29:220 –225. 12.
Hunt LM, Valberg SJ, Steffenhagen K, et al.
A retrospective study of myopathies and associated gait abnormalities in 65 warmblood horses.
J Vet Int Med 2005;19:428 – 429. 13.
Valberg SJ, Macleay JM, Billstrom JA, et al.
Skeletal muscle metabolic response to exercise in horses with ‘tying-up’ due to polysaccharide storage myopathy.
Equine Vet J 1999; 31:43– 47. 14.
Valberg SJ, Townsend D, Mickelson JR.
Skeletal muscle glycolytic capacity and phosphofructokinase regulation in horses with polysaccharide storage myopathy.
Am J Vet Res 1998;59:782–785. 15.
Valentine BA, Habecker PL, Patterson JS, et al.
Incidence of polysaccharide storage myopathy in draft horse-related breeds: a necropsy study of 37 horses and a mule.
J Vet Diag Invest 2001;13:63– 68. 16.
Valentine BA, Van Saun RJ, Thompson KN, et al.
Role of dietary carbohydrate and fat in horses with equine polysaccharide storage myopathy.
J Am Vet Med Assoc 2001;219: 1537–1544. 17.
Valentine BA, McDonough SP, Chang YF, et al.
Polysaccharide storage myopathy in Morgan, Arabian, and Standardbred related horses and Welsh-cross ponies.
Vet Pathol 2000;37:193–196. 18.
Firshman AM, Valberg SJ, Bender JB, et al.comparison of histopathologic criteria and skeletal muscle ﬁxation techniques for the diagnosis of polysaccharide storage myopathy in horses.
Vet Pathol 2006;43:257–269. 19.
Aleman M, Lecouteur RA, Nieto JE, et al.
Sarcoplasmic masses in equine skeletal muscle.
Neuromuscul Disord 2005;15:147–153. 20.
De La Corte FD, Valberg SJ, MacLeay JM, et al.
Developmental onset of polysaccharide storage myopathy in 4 Quarter horse foals.
J Vet Int Med 2002;16:581–587. 21.
McCue M, Ribiero W, Lewis S, et al.
Prevalence of polysaccharide storage myopathy in horses with neuromuscular disorders.
Equine Vet J 2006;20:743. 22.
McCue ME, Ribeiro WE, Valberg SJ.
The prevelance of polysaccharide storage myopathy in the Quarter horse population.
J Vet Int Med 2006;20:743. 23.
Ribeiro WP, Valberg SJ, Pagan JD, et al.
The effect of varying dietary starch and fat content on serum creatine kinase activity and substrate availability in equine polysaccharide storage myopathy.
J Vet Int Med 2004;18:887– 894. 24.
Valberg SJ, MacLeay JM, Mickelson JR.
Polysaccharide storage myopathy associated with exertional rhabdomyolysis in horses.compend Cont Educ Pract Vet 1997;19:1077– 1086. 25.
Park HB, Marklund S, Jeon JT, et al.
Molecular characterization and mutational screening of the PRKAG3 gene in the horse.
Cytogenet Genome Res 2003;102:211–216. 26.
Annandale EJ, Valberg SJ, Mickelson JR, et al.
Insulin sensitivity and skeletal muscle glucose transport in horses with equine polysaccharide storage myopathy.
Neuromuscul Disord 2004;14:666 – 674. 27.
De La Corte FD, Valberg SJ, MacLeay JM, et al.
Glucose uptake in horses with polysaccharide storage myopathy.
Am J Vet Res 1999;60:458 – 462. 28.
De La Corte FD, Valberg SJ, Mickelson JR, et al.
Blood glucose clearance after feeding and exercise in polysaccharide storage myopathy.
Equine Vet J 1999;30(Suppl):324 –328. 29.
Firshman AM, Valberg SJ, Karges TL, et al.
Serum creatine kinase response to exercise during dexamethasone-induced insulin resistance in Quarter horses with polysaccharide storage myopathy.
Am J Vet Res 2005;66:1718 –1723. 30.
Raben N, Danon M, Lu N, et al.
Surprises of genetic engineering: a possible model of polyglucosan body disease.
Neurology 2001;56:1739 –1745. 31.
Annandale EJ, Valberg SJ, Essen-Gustavsson B.
Effects of submaximal exercise on adenine nucleotide concentrations in skeletal muscle ﬁbers of horses with polysaccharide storage myopathy.
Am J Vet Res 2005;66:839 – 845. 32.
Carlstro ¨ m B.
Uber die atiologie and pathogenese der kreuzlahme des pferdes (haemaglobinaemia paralytica).
Scand Arch 1932;62:1– 69. 33.
Sprayberry KA, Madigan J, LeCouteur RA, et al.
Renal failure, laminitis, and colitis following severe rhabdomyolysis in a draft horse-cross with polysaccharide storage myopathy.
Can Vet J 1998;39:500 –503. 34.
Valentine BA, de Lahunta A, Divers TJ, et al.
Clinical and pathologic ﬁndings in two draft horses with progressive muscle atrophy, neuromuscular weakness, and abnormal gait AAEP PROCEEDINGS ր Vol. 52 ր 2006 379 IN-DEPTH: MUSCLE DISORDERS rum of showjumpers and dressage horses with back pain.
Equine Vet J 2002;34:171–176. 37.
McGowan CM, Menzies-Gow NJ, McDiarmid AM, et al.
Four cases of equine polysaccharide storage myopathy in the United Kingdom.
Vet Rec 2003;152:109 –112. a characteristic of shivers syndrome.
J Am Vet Med Assoc 1999;215:1661–1665. 35.
Firshman AM, Baird JD, Hunt L, et al.
Hyperinsulinemic euglycemic clamping and insulin sensitivity in Belgian draft horses with polysaccharide storage myopathy.
J Vet Int Med 2006;20:743. 36.
Quiroz-Rothe E, Novales M, Aguilera-Tejero E, et al.
Polysaccharide storage myopathy in the M.
Longissimus lumbo- Re-Leve, Hallway Feeds, Lexington, KY 40508. 380 2006 ր Vol. 52 ր AAEP PROCEEDINGS
Read more about Shivers : 11,16,21 Although the condition shivers was previously attributed to PSSM….: