VetInfo Digest                                                           June 2006

 

Table of Contents:

Differential Diagnoses

Seizures in Dogs and Cats

 

This Month's Note:

Sometimes I wonder if answering questions online was a good idea. Answering questions online presents some very difficult problems. The first is that I inevitably get only one side of the story when people have questions about the care their pets have received. This means that there are situations in which some or even all of the necessary information I need to accurately answer a question is missing or distorted. I chose early on to be sure to mention this when I answered questions but to assume that the writer was presenting the facts to the best of their ability. I know that there have been times when this has not been an accurate assumption. The second problem is that when information is fairly and accurately presented I still have had no ability to perform an examination or to ensure that the lab work was done correctly. I think that this is a severe handicap but it mostly interferes with giving very specific advice about a particular problem. For me, the answer to this problem was to point out the list of possible problems rather than to attempt to make a specific diagnosis. I really believe that this is a legitimate approach to helping people but sometimes it has been very hard not to say that I knew exactly what was wrong with a pet, if the history and clinical signs presented were correctly reported. I have always wished to keep my license to practice veterinary medicine so I have tried hard to avoid providing a specific diagnosis online since it is obvious that there can not be a complete doctor/patient relationship without an examination of a pet. In the long run, though, I really believe this has been a beneficial restraint. Over the course of my career I have come to the conclusion that it is far better to have a really good list of what might be wrong than to reach a quick diagnosis, even when that diagnosis seems very probably correct. Part of this VetInfo is devoted to trying to explain why I feel that way.

 

The Importance of Differential Diagnoses

A diagnosis is a specific guess as to what is causing a particular set of symptoms or lab values. Many of my clients come to their conclusions about what their pet's diagnosis is based on what I think of as a "hunt and peck" system. They read about a disease and recognize that their pet has one or more symptoms of that disease. So they decide their pet has that particular disease. Then they read something else and decide their pet has that disease, instead. They jump from one conclusion (diagnosis) to another without trying to apply a logical system to their diagnostic process. This sometimes works but a great deal of the time it doesn't. Some clients take a short cut in this process and just learn three or four diseases that each have a different major symptom, such as diarrhea, and all pets with that symptom then have that disease. This is why at least one of your friends thinks your dog has parvovirus every time it has diarrhea. Sometimes these people are right because parvovirus does exist. Most of the time it is better to have an organized process for getting from a set of symptoms to a diagnosis, though.

A differential diagnosis is a list of all the things that might produce that same set of symptoms or lab values. Many clients seem to prefer a veterinarian who skips making a differential diagnosis and moves straight to giving a specific diagnosis. I worked for a veterinarian once who told me that I had to give every client a specific diagnosis at the end of each office visit. I asked, "What if I don't know what's wrong?" He said, " That doesn't matter, if you decide tomorrow that you were wrong you can just give them a new diagnosis, people just want you to be sure of yourself." Or something similar to that. I really couldn't believe that people felt that way but a couple of years later, after I had left that practice, a client came to me and told me how much she liked that particular vet because "He always knew exactly what was wrong with my pet." Before you think that way, consider that even at the very best veterinary facilities a recent study suggests that about half of the initial diagnoses are wrong. If your veterinarian does not have the ability to produce a good list of possible diagnoses other than the one they initially favor and the mindset to abandon a diagnosis when it becomes questionable, the potential to get an accurate diagnosis is severely affected.

Constructing a list of differentials can be done in a number of ways. Getting to an accurate list is the most important thing but knowing several different ways to think of list building can also be helpful.

When I first started in practice I constructed most of my differential diagnosis lists by writing down every symptom and lab result and then writing a list of possible causes for every symptom and every lab result that was abnormal. After doing this, I would cross reference the lists to try to come up with a disease that appeared on every list. If so, I tended to think that was the diagnosis, as long as there wasn't a compelling reason not to. A compelling reason might something like a disease that only occurs in Africa since I'm in Virginia. It is entirely possible to come up with a good list of differentials this way but it is a lot of work. In addition, it becomes pretty clear that there are a number of times when a disease doesn't have all the symptoms it is supposed to or when a pet has all the symptoms of one disease, plus some.

At this time in my career, I tend to approach the idea of a differential diagnosis a little differently. I still think about all the possibilities that I can and try to keep them in mind but I usually try to work through algorithms instead of producing lists. An algorithm is a logical pathway to follow when presented with certain initial signs or certain abnormal laboratory values. It is a series of steps to take in based on a yes or no answer to a specific question. A good algorithm provides a way to work through a list without have to compare large numbers of symptoms and lab values every time. Well written algorithms lead you through the diagnostic process through a series of questions with yes and no answers. A yes answer leads you down one path and a no answer leads you down another path. There are lots of points where the paths backtrack or branch off based on various combinations of answers but ultimately they lead to a logical conclusion. In theory, at least. Just like my old method there are still many days when I follow carefully constructed algorithms and reach a state of confusion instead of a conclusion.

It is probably important at this point to point out the first major pitfall in using a differential diagnosis list or an algorithm to reach a single diagnosis. Some pets have more than one problem at the same time. There are two times when this is particularly important, when a pet is very young and very susceptible to contagious illnesses and when a pet ages. In older pets there is a greater and greater likelihood that there is more than one problem affecting them. Just as an example, it is estimated that up to 25% of dogs will develop Cushing's disease during their lifetime if they live long enough and the same percentage of cats may be affected by hyperthyroidism. Many times there are few or subtle signs of either of these diseases early on. Almost any disease can look different in a pet with an underlying hormonal disease than it looks in a pet who doesn't have an underlying disease.

Veterinarians are trained to look for the simplest explanation for a series of symptoms and lab values. When one disease explains the entire set of problems satisfactorily veterinarians tend to really latch on to that disease as the ultimate answer to the pet's problem. Clients tend to think that there should be a single explanation for their pet's problems, too. Despite this training and this natural inclination, it is critical to keep in mind that a significant portion of sick pets have more than one problem and some have three or more problems at the same time. It is entirely possible to correctly identify one problem, to treat it properly and to still have an ill pet. When this happens it is important to keep in mind that the original treatment may have been successful for the problem identified and that more than one problem may be present. It is equally important to keep in mind that that the original diagnosis may have been incorrect and that it is now necessary to search for another answer. Try not to get judgmental about the process of treating the pet to this point just because the response to therapy isn't what you wanted it to be. Recognize that you need additional help but don't get down on your vet, just yet. Keep in mind the possibility that your vet was partially right.

The most difficult problem associated with multiple illnesses in a single pet is that the interaction of illnesses may produce an entirely new set of symptoms that is unexpected or may change the nature of some symptoms enough that they are not easily recognized. This is one of the strongest reasons to continue to work through differential diagnoses lists or algorithms when facing a diagnostic challenge. Forcing yourself to look for other possible problems that might produce the symptoms seen is sometimes the only way to figure out that there is more than one disease present.

A significant advantage of using differential lists is that it can also help to point out to the client the need for information they may not have considered. For me, the most common example of this come from my retired clients who are traveling the country in RVs. When I run through a list of differential diagnoses with a client I sometimes say something like, "If your dog had been to Connecticut recently, I would have to include Lyme disease in this list but a dog who stays right here in the county isn't likely to have that problem." Which fairly often prompts the client to say something like "Well, Doc, we did take the RV up to Maine last month, does that count?". This is an important piece of information that may not have surfaced if I was not discussing all the possible causes of a set of symptoms with the client.

There are times when I put together a list of differentials for a problem and realize that I'm going to have to take steps to rule out one of the diseases first, even if I don't think that it is the most likely problem. A good example of this problem is the itchy dog or cat. Often my first instinct is to think that allergic skin disease is the most likely problem, if for no other reason than the majority of itchy pets in my practice have allergic skin disease. I might jump right to treating for this except for the fact that my list of differential diagnoses contains some other problems, such as a bacterial skin infection, that might get dramatically worse it is actually the cause of the symptoms and I choose to try to treat for allergic disease first. In this situation I have to look at my list and recognize that if I treat for bacterial skin disease first, even though it is less likely, the pet won't suffer serious consequences if I have made the wrong choice. On the other hand, if I choose to treat for allergies first, especially if I use corticosteroids, I might make a bacterial skin infection seriously worse and potentially even life threatening. So I treat for the problems that have to be eliminated in order to safely treat for the problem I actually think is present. This can be a little frustrating for clients but it is safer for my patients.

There are times when I am working through a list of possible diseases that might cause a pet's symptoms when I recognize that the remaining diseases all have the same treatment. In this situation I usually ask the client if they really want to know specifically what is wrong with their pet or if they want to elect to treat for the likely problems since they all have the same treatment. Most clients prefer to save the cost of additional testing. Some clients really want to know the specific problem present. It is possible to make a good case for either choice in this situation but I think that it is important to let the client know the situation and to let them make the choice for themselves.

With all of the information available to veterinary clients and all the medical shows about unusual illnesses on TV now it is surprising to me how many of my clients still expect me to give them a single specific diagnosis during an office visit. A dog or cat's physiology is not markedly different than a human's. There are lots of differences but one of them isn't complexity. You are much better off if your vet takes the time to give you a list of the possible problems and an explanation of the process necessary to work though that list than a "best guess" about what is wrong with your pet. Just hearing the list can be very useful if it makes you remember facts or symptoms that are helpful in sorting through the list. When you want to pursue a diagnosis a plan exists for getting to that diagnosis. If money is tight you can still ask your vet to make a good guess and to treat accordingly. The point is to remember that there is almost always more than one disease that might cause a particular set of symptoms. Just remembering that makes it necessary for you to reevaluate your plan when it isn't working well.

 

Seizure Disorders

A seizure is a temporary loss of proper neurologic function, often for unknown reasons. It is estimated that 20% of the canine population will have at least one medical event in their lifetime that is a seizure or closely resembles a seizure. Cats are less likely to have seizures but it is likely that at least 5% of cats have an seizure or seizure like episode during their lifetime. Approximately 2% of dogs and 1% or less of cats will have repeated seizure episodes during their lifetime.

An epileptic seizure is excessive activity in the cerebral cortex. Epilepsy is usually considered to be susceptibility to repeated episodes of epileptic seizures. It is important to note that more than one seizure event is usually necessary before a dog or cat is thought to be epileptic.

It is critical that some effort be made to determine if the episodes that occur are actually seizures. There are lots of things that can look like seizures. Heart disease causing syncope, or fainting, is frequently confused with seizure activity. It can be very hard to document some heart problems, such as intermittent arrhythmias, that cause syncope so it is important to remember this possibility. Severe pain can closely resemble seizure activity, especially severe gastrointestinal pain since animals can't really communicate what is happening to them. We have pets in our practice with separation anxiety whose owners genuinely thought were having seizure episodes. A very careful examination of the history of the event or events can be the key to understanding their cause. Seizures are random events in most animals but will sometimes follow patterns that make it possible to obtain a video of the pet during an episode. If you can get a video of the behavior this can also be extremely helpful to your vet.

Seizures are split up a great many ways. Sometimes it almost reminds me of the statements that start with "there are two kinds of people in the world... ". Remembering that all people fit in more than two categories and almost all seizures do to, is very helpful.

Primary epileptic seizures are repeated episodes of seizure events that have no discernible cause. These are also known as idiopathic epilepsy or cryptogenic seizures.

Secondary epileptic seizures are seizures in which a defect in the brain structure or function that leads to seizures can be identified. This group of seizures includes seizure activity from encephalitis, which can be an infectious disease process in some cases.

Reactive epileptic seizures are a normal brain reaction to outside influences, such as liver disease, low blood sugar, toxins or other identifiable causes of brain irritation.

Seizures are also classified by whether they affect large areas of the brain or small focused areas of the brain only. A partial seizure affects a small area of the cerebral cortex and is more likely to be associated with a specific problem in the brain. A generalized seizure affects a large area of the brain and it is less likely that an identifiable brain problem will be found when generalized seizures occur. Generalized seizures are the more common type of reactive seizure.

Just to make things even more confusing, partial seizures are divided into simple partial seizures and complex partial seizures. Simple partial seizures usually affect only one side of the body, usually do not involve a loss of consciousness and may cause facial tics, biting at a particular body part or similar activities. Complex partial seizures (CPS) are more complicated as the name implies. There usually is some alteration in behavior when complex partial seizures occur, although there may not be complete loss of consciousness. CPS causes symptoms such as "fly chasing" or "fly-biting" behaviors where a pet snaps at the air as if trying to catch a fly. They have been argued to be the underlying cause for sudden aggression in some dogs, most notably "rage syndrome" in English springer spaniels. This is a hotly debated topic and it is likely that there are multiple causes for this particular syndrome but CPS may be one of them. Sudden howling in dogs and episodes of vocalization in cats have also been associated with complex partial seizure activity.

Generalized seizures may be convulsive seizures or they may involve loss of consciousness without convulsive activity. When convulsions occur the seizure is sometimes referred to as a "grand mal" seizure and when convulsions do not occur the seizure may be referred to as a "petit mal" seizure.

Generalized seizure are more common than partial seizures and are the ones that most people are familiar with. When generalized seizures occur there is a loss of consciousness and there tends to be fairly equal involvement of both sides of the body, although this can be masked if a pet is lying on one side during the event. In pets most generalized seizures do involve rhythmic movement at some point but this can be a very short period and so occasionally it is missed. Seizures can occur without movement but this is thought to be uncommon in pets.

Generalized seizures seem to occur most frequently when pets are at rest and often occur when they are asleep. It is thought that this occurs because it is easier for a seizure event to take control of the brain when brain activity is low. This can be a very important differentiating point in the history so it is really important to make sure that you document when seizures are occurring. Some of my clients think that their dog seizures when it is excited but when they are closely questioned they realize that their dog has seizures on a day it is very excited but that the seizure actually occurs after it has settled down. An example of this is a dog who has been boarded, is picked up and is very excited but is also tired from staying up all day at the kennel. So it greets the family enthusiastically, finds its bed, lays down and then has a seizure. The seizure is closely associated with the excitement but it occurs when the pet is at rest. Heart related episodes tend to occur during the actual excitement or when the pet is up and moving.

Most of the people who call me about seizure activity initially insist that the seizure went on for five to ten minutes. After really careful questioning, though, it usually becomes apparent that the actual period in which the pet was unconscious, actively convulsing or exhibiting aberrant behavior was really about 30 to 90 seconds and then there was a period of confusion that went on for several minutes after the episode. It is important to try to pay attention and to differentiate between the seizure itself and the time after the seizure that the pet was not normal. This latter period is referred to as the post-ictal period. A small number of pets seem to be aware that a seizure is going to occur before it happens. These pets are experiencing a seizure aura or a pre-ictal period.

Laboratory testing doesn't provide a diagnosis for the cause of seizures very often. For the pets who do have a systemic cause for their seizures, such as liver disease or diabetes, though, the lab work is critical. So this is one of those instances I talked about in the differential diagnosis section where I make the choice to do testing to rule out the things that I think aren't present because missing one of them could threaten the life of a pet. We may not do testing the first time a pet has a seizure event because somewhere around 10% of pets have a single seizure episode during their lifetime and never have another one. We really want to do lab work after the second or third seizure event, though. Especially if they occur in a short period of time.

X-rays of the brain almost never provide much useful information on seizure activity. MRI scans are very useful in ruling out abnormal brain structure, past episodes of stroke and similar causes of seizure activity but are not available to everyone. Ideally all dogs over 5 years of age who have new seizure events would have MRI examination but it will take some time for this type of testing to that widely available.

Examination of cerebrospinal fluid can be helpful but this is something that a lot of general practitioners can't offer because the fluid has to be examined pretty quickly after it is drawn for really useful information to be obtained from it. There probably are general practitioners who do this frequently enough to be confident in the results but in many cases it will be better if your veterinarian refers you to an internal medicine specialist or neurologist if CSF examination seems important.

Despite the availability of laboratory testing procedures the single most important piece of information in the diagnostic process is frequently the observations of the pet owner as to when seizures occur, how long they last, whether there is a distinct post-ictal period, how long the post-ictal period lasts when it occurs and what was happening prior to the seizure activity both immediately and in the few days prior to the seizure. Keeping a good journal of seizure activity helps your vet a great deal and sometimes results in identification of the cause of seizure activity. Even if your vet doesn't emphasize this aspect of caring for a pet with seizures you should do your best to keep a record of the events and of things that might be influencing them.

In the journal that you keep it is important to note a number of things:

There are some significant differences between dogs and cats when it comes to the underlying cause of seizures. While firm figures are hard to obtain it is likely that the great majority of cats have reactive seizures or secondary seizures while the majority of dogs have primary seizures. For this reason, it is important to look carefully for an underlying cause when dealing with seizures in cat even on the first incidence of seizure activity where it is reasonable in a dog to wait and see if a second seizure episode occurs prior to doing a complete work-up, in most cases.

There are some general guidelines for how to approach seizure diagnosis that are formulated primarily for dogs but work for cats as long as you keep in mind that in all age categories in cats an underlying cause is more likely to be found.

When a pet has seizures that start when they are less than one year of age it is more likely that the cause will be a disease or congenital defect than that the cause will be primary epilepsy.

When a pet has seizures that start when they are between one and five years of age it is more likely that the seizure will be primary epilepsy. There is still a possibility of underlying disease, reaction to previous trauma, toxins or cancer but in this age range these problems are a little less common than in younger or older pets. Remember that for cats there is still a strong possibility of underlying diseases such as feline leukemia in this age range.

When pets are over five years of age cancer becomes the most common cause of new seizure activity but all of the other causes are still possibilities. The older a pet is when new occurrences of repeated seizure activity occur the more likely it is that the underlying cause is cancer. Careful examination to try to rule out undetected cancer is justified in this age range, including chest X-rays, MRI examination of the brain and testing for hormonal diseases such as Cushing's disease that are caused by underlying cancers are reasonable testing procedures in this age range.

Many of our clients do not wish to pursue a diagnosis if the underlying cause of seizure activity once it is clear that their pet does not have a treatable underlying condition such as liver disease. I think that this is a reasonable choice to make in most cases but there is no reason not to pursue a specific diagnosis if you believe that is the best decision for you and your pet. Even though many of the defects identified by MRI scans can not be treated it is possible to have many types of brain tumors removed and pets often do very well after brain surgery to remove tumors.

While the general guideline for when to treat seizures in dogs is to wait until there is more than one seizure event, there are some exceptions to this rule. These same rules apply to cats. Even though we may try harder to pursue a diagnosis on the first seizure event in a cat we still usually wait until more than one seizure event has occurred prior to treating them, except in these circumstances:

1) Any seizure in which the actual seizure episode lasts for more than five minutes should prompt a complete work-up and most pets will require anti-seizure medications after such an event.

2) Anytime more than one seizure episode occurs within a 24 hour period it is necessary to consider seizure control. Seizures that occur close together are referred to as cluster seizures.

3) Seizures occur within 1 week of a traumatic event.

4) An identifiable cause of seizure activity is present that can be controlled but for which the seizures themselves may be damaging before that happens.

While the above mentioned problems require immediate therapy for long seizures and cluster seizures and urgent treatment for the other two problems, most of the time there is some decision making to do before choosing to treat seizures.

Clients often want to treat seizures as soon as they occur. Many people find seizures to be very scary. The sense of helplessness when watching one can be disturbing. Fortunately most seizure activity is not life threatening. There is usually time to decide when the seizures become more of a problem than the potential side effects of treatments. Your vet will help you decide when it is necessary to treat for seizures. At the present time there is a range of advice but the majority of vets probably choose to treat seizures when they are observed or detected more than 6 to 12 times a year.. To some extent this decision making depends on the exact nature of the seizure. It may be reasonable to wait on seizure therapy even when seizures seem to be occurring more often than this in some cases.

Next month I will cover seizure control medications.

 

Thanks for your Support!

VetInfo Digest
P.O. Box 476
Cobbs Creek VA 23035

All opinions in this newsletter are those of the author, Michael Richards, DVM

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