Generalized weakness can present a formidable problem solving challenge for the veterinary clinician. Since there are a multitude
of causes, and body systems which when affected, may result in this clinical presentation, a very organized thought process
is crucial to obtaining a definitive answer. The history, physical examination and observation of the patient's gait may
each provide initial clues, but then an organized diagnostic plan must follow. We will first look at the clues that can be
derived from the history, physical exam and gait analysis and then discuss the diagnostic plan for this clinical problem.
History and signalment
Clarify the nature of the clinical signs and have the owner describe in their own their words what they believe is abnormal,
don't interject your thoughts too early, many times this will "talk the owner" into a comment. After the client describes
the problem then find out some other important points: 1) is the weakness a) constant, b) exercise induced, c) paroxysmal
(good days and then bad days). Constant weakness may point to 1) anemia 2) metabolic problems such as electrolyte imbalance,
hypoglycemia 3) muscle/nerve disorders 4) causes of hypoxemia or perfusion issues 5) obesity.
Exercise induced may point to some muscle disorders, neuromuscular junction disorders, metabolic (Addison's for example),
cardiovascular, respiratory, or orthopedic. Paroxysmal may point to cardiac arrhythmias, diseases that result in periodic
intra-abdominal hemorrhage such as splenic hemangiosarcoma or GI hemorrhage or disorders which result in sporadic hypotension
such as visceral mastocytosis with concomitant sporadic histamine release, or endocrine disease such as hypothyroidism. These
are all examples where the history and listening intently to the client may provide important initial clues with regard to
the etiology of the weakness. In addition, the history may also provide clues for which body system might be involved: such
as cough, respiratory distress, cyanosis for cardio-respiratory; lameness for orthopedic/joint disease; sudden collapse which
might point to syncope. Weakness secondary to glucose homeostasis issues may be more apparent after eating or fasting.
Physical examination (PE)
With regard to the gait an animal manifests when it has muscle disease or disease of any portion of the lower motor neuron
unit (LMN) unit, the animal has a tendency to walk with a very short strided, choppy, shuffling type gait. They tend to keep
their limbs under their trunk and do not take long protracted strides with their limbs. This type of gait may also be seen
in dogs which have joint pain or spinal column pain but there are usually other PE findings that help support pain that is
originating from these two locations.
One should always ask the following questions. 1. "What body system that if affected, could result in the clinical signs and/or
historical findings? This question should make one focus more intently on those body systems during the physical examination.
For the problem of weakness, most etiologies will fall into one of the following systems: 1) Neuromuscular 2) Orthopedic/Joint
3) Metabolic/Endocrine or 4) Cardiorespiratory. So how would the PE be helpful? How would your PE help direct you to one
of these four groups?
The other question to ask is: 2. "What is the mechanism or pathophysiology that causes the clinical problem?" In other words,
if one knows how a sign or symptom might be caused, then many times you already have a "ready list" of differentials. For
example, if one knows that electrolyte/glucose disturbances may result in weakness then Addison's disease, renal disease with
loss of potassium, or causes of hypoglycemia should be considered. If one knows that the mechanism of poor perfusion/oxygen
delivery to tissues could result in weakness, then heart disease, arrhythmias, causes of anemia should all be considered.
This type of thought process allows the clinician to become a problem solver versus a "let's pick a differential and prove
it type clinician."