Neuromuscular Diseases

 In this article I will explore the interrelationships between three disease processes: Equine Polysaccharide Storage Myopathy (EPSM, also known as PSSM), Stringhalt and Shivers. These three syndromes have been seen more frequently over the past 10 years. Shivers may be a new disease for some of you, but it has been present in Draft horses for quite some time. The Drafts still have issues with this, but its presence in Warmbloods is our greater concern. The gait abnormalities associated with this disease are particularly problematic for dressage horses, but even such routine procedures as farriery and the daily picking out of hooves can be affected by it.

EPSM

EPSM is a metabolic condition related to skeletal muscle dysfunction. The primary issue is an inability to properly metabolize carbohydrates from feed. Attempting to make dietary changes and maintaining a schedule of regular exercise seems to be the most effective methods to improve these horses. This is a lifelong condition and altering the carbohydrate to fat ratio will be a permanent change.

Potentially any type of horse could develop this condition, with the highest incidence involving Warmbloods, Draft-related breeds, Quarter Horses and Arabians. It is thought to be an inherited disorder, but due to variability in its onset this may not seem clear.

There are a variety of signs reported thus making it difficult to conclusively diagnose this based on history and signalment. Clients have reported: an onset of weakness, poor performance, an inability to move forward, back soreness, gait abnormalities with or without a lameness, attitude problems, poor muscling, decreased impulsion and general stiffness. From this list we could probably come up with 30 or more reasons for these signs. What also makes this difficult is that a horse can be progressing normally in a training program then some of these signs can develop subtly. Over time the gait abnormality when recognized may not be clearly linked to any particular change in the horse's diet, work or routine. An actual lameness may develop from a slight stiffness to an actual lameness that involves one or both hind limbs. From behind, the gait has been described as a "goose waddle". One of the more obvious horses I saw was a 4 year old Draft gelding that walked out of the stall stiffly and would trot, but appeared stiff and lame in both hind limbs. There was not muscle loss, but they were not developing well and they seemed remarkably tight and sore for the limited work the horse was doing. The lameness and sometimes vague signs make it difficult to distinguish EPSM from other causes of lameness, including Shivers and Stringhalt plus different neurologic conditions.

Attempting to diagnose EPSM with laboratory work is a worthwhile pursuit, but it still may elude an accurate diagnosis. The primary blood test involves looking at levels of muscle enzymes before and after exercise. Muscle enzymes will increase with exercise and the higher the rise the more likely that muscle inflammation is occurring. Unfortunately not every horse will exhibit the type of elevation necessary to diagnose this from bloodwork. A genetic test became available some years ago that evaluates DNA from hair or blood to determine if the horse carries the gene for what is being called Type 1 EPSM. The Type 2 - the more common form is only diagnosed presently through a muscle biopsy. The preferred method for making a more conclusive diagnosis is by doing a muscle biopsy, which allows a pathologist to make the diagnosis based on a microscopic examination of the muscle tissue. The procedure is straightforward and well tolerated by the horse. Since this is somewhat of an invasive procedure most clients opt to treat rather than to biopsy. In order to confirm the diagnosis and to get a sense of the degree that EPSM is involved in the unsoundness, biopsying is essential. As with many diseases more information develops over time and that has been true with EPSM as a subset exists that exhibit similar signs and it’s termed Myofibrillar Myopathy (MFM). It can only be tested for when submitting a muscle biopsy, no genetic test exists.

Clinical signs with MFM have been described as follows:

“Warmblood horses with MFM may have a history of satisfactory performance as young animals but a decline in performance as they reach 8 -10 yrs of age. Tying up is usually very intermittent or absent in Warmbloods. The most common clinical signs of MFM in Warmbloods are related to poor performance without elevations in serum CK and AST activity. Unwillingness to go forward, a vague, poorly localized hindlimb lameness, sore muscles and drop in energy level are common complaints. Reluctance to collect and engage the hindquarters, poor rounding over fences, poor quality of the canter and slow onset of atrophy especially when out of work are reported. Note that these are not specific signs to MFM and they can occur with poor saddle fit, lameness from hocks, stifles, suspensory injuries, sacroiliac disorders, inflammation in the thoracic or lumbar vertebrae and more. A thorough evaluation for lameness to rule out these more common issues should precede any testing for MFM in horses because MFM is likely much less common than these other equine maladies.

In Arabians, particularly those competing in endurance, the most common clinical sign is intermittent tying up (episodes of muscle pain, stiffness, reluctance to move). The severity of muscle stiffness can be much milder than that seen with classic tying-up (exertional rhabdomyolysis). After an endurance ride horses may just appear slightly stiff and have dark urine without being completely unwilling to move. If horses are rested for a few weeks, marked muscle stiffness often occurs approximately 5 miles into a light ride. Increased serum CK and AST activities can occur but may not consistently be as high as in classic forms of tying up.” Michigan State University, Equine Neuromuscular Diagnostic Laboratory

Clinical signs with EPSM:

“In Warmblood horses tying up may be reported occasionally with high serum CK. However, the most common clinical signs of type 2 PSSM in Warmbloods are often most closely related to poor performance without elevations in serum CK activity. An undiagnosed gait abnormality, sore muscles and drop in energy level and unwillingness to perform after 5 -10 min of exercise are common complaints with type 2 PSSM. Warmbloods with type 2 PSSM have painful firm back and hindquarter muscles, reluctance to collect and engage the hindquarters, poor rounding over fences, gait abnormalities, and slow onset of atrophy especially when out of work. The mean age of onset of clinical signs in Warmbloods is between 8 and 11 years of age with the median CK and AST activity being 323 and 331U/L, respectively.

In Arabians and Quarter Horses with PSSM2, the most common clinical sign is tying up (episodes of muscle pain, stiffness, reluctance to move) with increased serum CK and AST resembling signs described for Recurrent Exertional Rhabdomyolosis (RER). Muscle atrophy and high serum CK activity are also common complaints in Quarter Horses with PSSM2. In Quarter Horses less than one year of age, PSSM2 can present with an inability to rise or a stiff hind limb gait.” Michigan State University, Equine Neuromuscular Diagnostic Laboratory

Performing a muscle biopsy, a minimally invasive procedure occurs adjacent to the tail in the rump area and in the picture below the site and closing the skin with sutures after biopsying can be seen:

Stringhalt

This gait abnormality shares some of the signs seen with Shivers, yet there are a few things that make it unique. Potentially any breed can be affected by this condition and there is no age or sex predilection. The classic sign is described as an exaggerated flexion of the hindlimb that occurs during the walk stride. The flexion can be quite dramatic as the front of the fetlock can actually contact the horses abdomen. Once the horse continues moving and gets into trot and canter gaits the signs may dissipate altogether. It can be seen in one or both limbs. Horses with both legs involved generally have one leg more obviously affected than the other. In addition, most horses with this syndrome seem to be affected more during the cold weather and stress. It may show up in the walk or when you first pick up a hindlimb and most horses improve even after a few minutes of walking. The condition is frequently noticed when picking out the feet and when the farrier works with the horse. While starting to back a horse an abnormal flexion may be shown which could be consistent with either Stringhalt or Shivers.

There are several potential causes and some type of neuropathy - a nerve dysfunction is the most common. There are times when this neuropathy develops for no apparent reason and other times a traumatic event may be linked. I know of several horses where this developed after trauma to the area above and to the side of the hock as well as in the upper cannon bone region. Some type of contusion may have resulted in a permanent change in the ability of a nerve to properly conduct impulses or for a tendon attached to a flexor muscle to function properly. In the Northwestern U.S., parts of Europe and Australia the disease has been linked to a toxic agent in a plant. If Stringhalt can be linked to plants then the goal is to remove the horse as soon as possible from that location. Unfortunately toxicity is not a cause in the New England area and our attempts to treat and manage this condition are limited. I will cover dietary options at the end of this discussion. Besides attempt at dietary management, time and benign neglect are considered reasonable approaches, on the other end of the scale is surgery. A procedure that transects and removes a section of muscle and tendon of the Lateral Digital Extensor muscle has been available for years and, in my practice as well as in others, Veterinarians have reported mixed success with this surgery - some horses improved and for some no benefit.

Shivers

Initially this condition was diagnosed exclusively in Draft breeds, but it has been seen with an increasing frequency in Warmbloods. Perhaps because of the similarities with respect to abnormal gaits, some of the horses previously diagnosed as being affected with Stringhalt may in fact be Shivers horses. In the Veterinary literature there have been reported cases in other breeds, however in our practice the Warmbloods have been affected the most.

There may be several signs associated with this syndrome. The classic abnormality is an exaggerated flexion of the hindlimb, where the limb may be held in the flexed position and quiver for 10-30 seconds. The shaking is different from Stringhalt which is a quicker more spastic type of movement. In addition to the quivering, the leg may be held omewhat away from the body when it goes into flexion. The tail may be partially raised during one of these episodes. On occasion flickering of the eyelids and ears may occur along with a stretching or flexing motion of the neck. I frequently find they appear to be neurologically impaired yet when we do a neurologic exam with positional responses and reflexes these horses are normal. The signs may be evident in numerous situations: turning in a tight circle at a walk, starting to trot, walking backwards and when an attempt is made to pick up a hind foot. Some of these horses seem to have an inability to pick up the limb when asked and if they do "release the limb" and allow it to be held then the quivering will be seen and some of these horses may require using a wall to balance themselves on the opposing limb., then they may prefer to quickly put their foot back on the floor. Typically a farrier may be one of the first to note difficulty working with a hind limb, often owners too though are impacted as they try to lift the leg when picking out the feet.

Initially the gait abnormality may consist of an unevenness in a hind limb that may progress to a "hitch". Over time this could progress to involve both hindlimbs with the flexion abnormalities as previously described. Since the horse may have functionally normal gaits beyond the walk the syndrome may be misdiagnosed as Stringhalt, EPSM or an intermittently locked stifle. A hitch in the stride could run the gamut for any cause for a hindlimb lameness.

Determining a cause for this syndrome has taken researchers in several different directions with a neuropathy being originally considered likely:

“The ultimate causes of Shivers remain unknown. However, an extensive study published in February of 2015 revealed that the brains of horses with Shivers are damaged in a specific area of the cerebellum, a part of the brain largely devoted to regulating muscular activity. Horses with Shivers often move forward normally and can even reach high levels of athletic competition, but have extreme difficulty with less natural gaits such as backing up or lifting a hindlimb. Studies suggest that these slow, learned movements are regulated by a cerebellar pathway separate from spinal circuits involved in faster, natural forward gaits, providing a possible explanation for the unique deficits seen in Shivers. Michigan State University, Equine Neuromuscular Diagnostic Laboratory

Attempts to prove that a neurologic component is present in the peripheral limb has yet to be determined; instead affected horses share some of the same metabolic issues that affect those with EPSM.

Treatment involve dietary and exercise management and will be discussed below.

Treatment Strategies

As previously discussed, diagnosing these conditions may be difficult and treatments can be just as confusing. Generally attempts to improve affected horses involve dietary manipulation since metabolic issues play an important role in EPSM and Shivers. Only Stringhalt is amenable to a surgical procedure and success rate is generally in the 70% range. Since Stringhalt may not affect a horse's gait enough to warrant surgery most people elect to live with this condition. In the past, I have attempted using Vitamin E supplementation as a means of management, but at this point I am not convinced that this will offer many advantages.

Dietary recommendations for the two metabolic diseases (EPSM and Shivers) are designed to move the horse away from a high carbohydrate diet and to focus on the fat and protein content.

Dietary and exercise management is discussed below:

“Signs of muscle pain, atrophy and stiffness in horses with PSSM2 can be managed through specific diet and training regimes. Both diet and training must be changed to see a beneficial effect. The diet is altered to provide a moderate starch and sugar content, a slightly higher protein content with high quality amino acids and, if needed for energy, fat supplementation. This means reducing or replacing sweet feed, corn, wheat, oats, barley, and molasses with a ration balancer that contains vitamins, minerals and at least 20% protein. Consistent exercise enhances glycogen utilization, increases turnover of structural proteins in the muscle and builds enzymes needed to burn energy as fuel. At present, the best we can do is to use management techniques in order to reduce recurrence.

Avoid Rest: For chronic cases, prolonged rest after an episode appears to be counterproductive and predisposes PSSM horses to further episodes of muscle pain. Providing daily turn out with compatible companions can be very beneficial as it enhances energy metabolism in PSSM horses. Grazing muzzles may be of benefit to PSSM horses turned out on pastures for periods when grass is particularly lush. Most PSSM horses are calm and not easily stressed, however, if stress is a precipitating fact, stressful environmental elements should be minimized.

Reintroducing exercise: Re-introduction of exercise after several days of rest needs to be gradual. Exercise should begin with light slow uncollected work on a longe-line or under saddle beginning with once a day for 3-5 minutes at a walk and trot. This initial work should be very mild and very short in duration at a walk and trot and gradually increased each day. At least two weeks of walk and trot should precede work at a canter. Horses should be worked in a long and low frame for at least 4 weeks initially with a very gradual reintroduction of collected work.

Exercise: Regular daily exercise is extremely important for managing horses with PSSM. Once conditioned, some PSSM horses thrive with 4 days of exercise as long as they receive daily turn out. A prolonged warm-up with adequate stretching is recommended. The collected work should be performed in intervals lasting no more than 5 min with a period of stretching provided between intervals. The time of active collection can be gradually increased as the horse works more underneath himself and in balance.

Diet: We strongly encourage owners to utilize an equine nutritionist to provide a balanced diet. Most feed companies have a nutritionist that will provide support for veterinarians. The recommendations for diet for PSSM2 are similar to those for PSSM1. Ensuring adequate protein content in the diet is an additional recommendation for PSSM2 horses especially those with loss of muscle mass. Whey based formulations are recommended at the manufacturers recommended amount. Progressive’s Topline Xtreme or Purina’s Supersport are two such supplements. Feeding within 45 min of exercise may enhance development of muscle mass. If horses have evidence of a cresty neck, excessive fat pads or a history of metabolic syndrome or laminitis and sore feet consult your veterinarian before feeding higher protein feeds.

Expectations: With adherence to both the diet and exercise recommendations about 70% of Warmblood horses show notable improvement in clinical signs and many return to acceptable levels of performance. There is, however, a wide range in the severity of clinical signs shown by horses with PSSM; those horses with severe or recurrent clinical signs will require more stringent adherence to diet and exercise recommendations in order to regain muscle function.” Michigan State University, Equine Neuromuscular Diagnostic Laboratory

Fat sources are often sought out and just want to mention the following with respect to that: Some horses are supplemented with vegetable oils (sunflower, safflower, corn, cocosoya or soy ), high fat feeds and supplements such as Rice Bran. A change in a horses diet to one that contains these high fat levels is safe and may be part of a dietary "test" to evaluate if a horse is a candidate for these type of syndromes. Typically these diets may combine higher fat and protein levels - often alfalfa pellets are added - with lowering of the carbohydrate content. Alfalfa has been suggested as an adequate source of protein, calcium and Vitamin E with good palatability. Alfalfa hay can also be added to the mixture. If the oil is a factor, the palatability can be improved by using Cocosoya oil, which is a blend of coconut and soy oil. The two oils that should not be used are linseed and flax seed. Because of pro-inflammatory effects seen with Corn Oil this fat source is also not recommended. Rice Bran which is 20% fat can be used to supplement or to replace some of the oil. The conversion of the rice bran to an equivalent amount of oil is as follows: 5 pounds of rice bran provides the same amount of fat as 2 cups or 1 pound of oil. Feeding with rice bran as the primary fat source will substantially increase costs and not reduce the carbohydrate or starch content as much as desired. Dried fat sources are also available and provide an alternative to the oil. These products weigh half of their volume so twice the volume will be required.

Often the difficulty lies with the feeds that are reportedly high fat. The actual % of fat may be 10-12%, but what they do not report is the carbohydrate %. As more feeds are used in the feeding regime the potential for throwing off the carbohydrate to fat ratio exists. For example, a feed with 10% fat would require feeding up to 10 pounds in order to achieve 1 pound of fat from the feed. Obviously this will be too much grain and by increasing this in order to help manage fat content you will undoubtedly significantly increase carbohydrate levels. Therefore the use of oils as a 100% fat source is the safest and most efficient means to get the desired results.

Feeding extra sources of vitamins and minerals is generally not a bad idea if a general supplement is needed. However I feel particular attention should be given to Vitamin E which should be fed at higher levels. Generally we suggest giving the horse at least 1 unit of Vitamin E for each pond of the horse's weight. In my opinion these horse do better with at least 2-5 times that amount. I recommend several natural source Vitamin E products that provide 5000-7500 units per horse depending on size and current diet. If you are already feeding a Vitamin E-Selenium supplement we don't want to use this to increase Vitamin E as there is a risk of toxicity if Selenium is increased beyond the 1 mg per 1000 pounds of weight.

Following dietary and exercise changes improvement in the horse's musculature is to be expected. Potential set backs have occurred and may be associated with variables such as access to grass or hay. There can be a significant increase in grass carbohydrate content especially in the spring and after the initial onset of cool weather in the fall as grasses will concentrate sugars. Paying particular attention to the pastures and hay, such as first versus second cut will help.

While most of the dietary management discussed above related to EPSM there’s enough overlap with the other two syndromes that they too may benefit from some of these changes. Without a doubt regular exercise promotes better movement and will help with all the syndromes.