Endocrine Disease Updates 

Two diseases that we deal with on a regular basis are Cushings Disease and Equine Metabolic Syndrome (EMS).  While Cushings has been around for a while, EMS is a relatively newly recognized disease.  Recently, the Equine Endocrinology Group put forth new guidelines on the best method to test for each of these diseases.  Here is a brief overview of how this will be changing.    

CUSHINGS DISEASE

In the past, there were two common methods of testing for Cushings.  The first (which we typically rely on) is to test the resting ACTH levels.  This is simple enough - if the measured level of ACTH is above or below a certain threshhold, the horse is positive or negative, respectively. 

The second test is known as a Dex Suppression Test, whereby a blood sample is taken, then a small amount of dexamethasone (a steroid) is administered.  A second blood sample is taken approximately 20 hours later.  This test should cause a significant drop in the amount of ACTH present.  If it does not, then the horse is positive.  However, this test can be concerning as we are giving steroids to a horse with possibly high circulating levels of steroids.  This situation can lead to laminits, which is quite rare but still possible. 

So what is recommended now?  Well, many times we can tell a Cushinoid horse by appearance, so just beginning treatment is sometimes an option.  However, for actual testing, a two step method is now recommended (the first step can be omitted if the owner would prefer).  The first step is just as we have done in the past - draw a single sample to determine circulating levels of ACTH.  Again, if it's high, we're positive and can begin treatment.  Some cases, however are a bit ambiguous, so the second step will need to be followed. 

The second step to this testing plan is a Thyrotroping Releasing Hormone (TRH) stimulation test - or TRH stim for short.  This test is similar to the dex suppression test in that a sample is drawn, the TRH is administered, and a second sample is drawn 10 minutes later.  Again, we will be looking at ACTH levels and how much they rise after administration of the hormone.  This gives us a very reliable test to determine the presence or absence of Cushings disease, without the added risk of giving Dexamethasone to a potentially laminitis-prone animal.   

EQUINE METABOLIC SYNDROME

EMS is another endocrine disease that we wrote at length about in last summer's newsletter.  (You can read it here in the archives)  Suffice it to say that EMS is defined by 3 criteria - generalized or regional adiposity (obesity), insulin resistance (IR), and a predisposition toward developing laminitis.  Similar to Cushings disease, we can often look at a horse and their appearance may be suggestive of EMS.  We often like to confirm it to be sure, as some horses are just overweight (especially after the holidays!)

Given that obesity and laminitis are quite apparent visually, we generally test mainly for the presence of insulin resistance.  This, too, is done in a two step method.  An initial blood sample is drawn to determine fasting insulin levels.  If the insulin level is above normal, then the patient is obviously positive.  If we still question the presence of IR, we can perform an oral sugar test.  With this test, the patient is fasted, then administered dark Karo syrup orally by the owner.  The vet can then come 60 minutes later to draw blood samples at 60 and 90 minutes to monitor for a rise in insulin.  It's quite simple, really!

Some owners may have a concern about administering sugar to a possibly pre-laminitic horse and that is valid.  However, the amount of suger that is being administered via the Karo syrup is quite a small amount and has not led to secondary issues in any of the studies that have been done.   

Endocrine diseases in horses are difficult - by the time we recognize them, the best time to begin treatment has already passed.  In the future, we hope to have the possibility of genetic testing for this disorder, but in the meantime we all have to be diligent in watching for the clinical signs. 

Equine Protozoal  Myeloencephatlitis (EPM) is a neurologic disease in horses that is caused by a migrating larva.  It is often carried by opossum and transmitted through their feces.  This "worm" finds it's way to the central nervous system of horses and migrates up the spinal cord, causing damage and scarring along the way.  Typically, we see hind end problems first, as the larva migrates from the hind end towards the head. 

Testing for this disease has always been difficult.  Multiple labs around the country have developed a variety of tests, and various proteins have been implicated as "the best marker to test for."  However, even with the purported best testing methods, we are left shaking our heads.  The reason for this is that new research has shown that the most accurate way to test for this disease is by obtaining a sample of cerebrospinal fluid (CSF) - the fluid that bathes the brain and spinal cord.  The antibodies contained in this fluid, as opposed to blood, are at a significantly higher concentration in the face of disease.  The tests relating to blood samples are "as good as flipping a coin", according to one researcher.     

While obtaining a sample of CSF is possible, it is more difficult and more invasive than just a simple blood draw. 

If you have ever seen a lameness exam performed, you may have seen us (or your vet) perform a "nerve block".  This is done by injecting a small amount of anesthetic locally over a nerve.  (Think of when you go to the dentist and they put novicaine in your gums).  We do this to try and accurately determine where the lameness is coming from.  Recently, there has been a lot of research into what we might actually be blocking when we do this procedure.

Many studies are performed where varying amounts of contrast material are injected at the typical site (depending on which nerve you're trying to block).  Radiographs are then taken to determine how far that injection has diffused from the original injection site.  This is important because if the anesthetic travels too far, it can give us inaccurate results.  For instance, if we are trying to block out everything below the fetlock, and the anesthetic migrates up the limb, we may actually have desensitized the fetlock to reduce the lameness.  This can alter the course of further diagnostics and treatment.  So we need to be aware of this possible diffusion when performing these procedures.

Some studies showed that, for certain blocking patterns, if too much anesthetic is used, it can diffuse up to 12 inches (a whole foot!) from the initial site.  There are also various pouches of the joints (such as the carpus, hock and fetlock) that can be entered accidentally.  This is one reason you will always see us thoroughly scrub the limb before placing a needle.  This indicates that it is not only important to be clean, but also that we need to be cautious in the amount of anesthetic that we use, as well as be accurate with needle placement to be sure we are injecting over the nerve. 

(The figure above was borrowed from a case report at Colorado State University.  It demonstrates the proximal diffusion of contrast material after a low 4-point nerve block. The red arrow demonstrates the level of needle placement for the injection. The white arrows demonstrate movement of the contrast material  up the limb from the injection site.)

To help reduce problems, researchers are continuously developing and comparing injection methods to determine the best way to approach various injections.  At this year's conference, new protocols were proposed for navicular bursa injections, digital tendon sheath injections, as well as palmar digital nerve blocks (for blocking the back half of the foot).  This information is always invaluable and can help us to alter the way we practice to better treat horses. 

Managing pain in laminits is one of the more difficult aspects of our profession.  We often end up euthanizing laminitic horses because we can't control their pain.  Our mainstay for therapy has typically been phenylbutazone (Bute) which is an NSAID.  The problem is that the pain pathways involved in laminitis are numerous, and one route of attack for us is not always successful.  While many drugs have been explored to treat this disease, few have shown promise.  However, some drugs do show some efficacy. 

Tramadol is an opiate-like drug which can be administered orally.  It has shown some promise in the first few days of a laminitic episode, however becomes ineffective after about 7 days.  In combination with other drugs, it may show some extended benefits, but at this time it is difficult to say how effective a drug it can be in our arsenal. 

A second drug which helps with a different pain pathway (neuropathic) is Gabapentin.  Again, this drug is given orally and can be included in a multimodal approach to laminitis.  It has not been shown to be effective in all cases, but some horses have benefited from it in regards to their pain management. 

It's important to note that there are a lot of rumors circulating out there about new cures for laminitis.  Laminitis is the number one researched disease in equine medicine, and the top researchers in the world are having trouble delineating the exact cause, let alone the best way to help horses recover.  Please keep this in mind as the next "wonder drug" for this disease is revealed on the internet. 

As always, our winter lecture series is coming soon.  We have not yet finalized the dates, but they will begin in February.  For now, please feel free to join us at an upcoming lecture on Chiropractic medicine at Steppping Stone Ranch.  All of the details of any of our lectures are available at our website under News/Events. 

This lecture will be held at Stepping Stone Ranch 201 Escoheag Hill Road, West Greenwich, RI 02817.  To sign up for the lectures, please contact Heidi at either (401) 397-3725 or steppingranch@cox.net.  There will be a small fee to attend these lectures.