Collapsing or episodic weakness of unknown origin

Brief and/or infrequent episodes of collapse present a diagnostic challenge, particularly when they are often normal at the time of examination. It is of paramount importance in such cases to obtain a detailed and thorough history - a 5 to 10 minutes consultation may not be sufficient. Either reallocating more time, or referral of the case, is therefore necessary.

It is most important to establish whether the collapse is a seizure or not. Try to avoid using the term ‘fits’ to mean any form of collapse, it’s a synomym for seizures. Apart from the usual features of seizure activity such as paddling movements, it is often useful to ask the owner if, at the very beginning of the collapse, the animal was relaxed (sleep-like) or went straight into seizure activity (the latter being suggestive of true seizures).

Differential diagnosis :
The following are very broad categories of collapse that I see in referred dogs.

• Cardiac output failure such as an intermittent arrhythmia (idiopathic arrhythmias or secondary to organic heart disease) or forward heart failure (e.g. pericardial effusion)
• Metabolic/systemic causes (eg. Addison’s, hypoglycaemia, anaemia, hepatic shunts)
• Muscular or neurological disorders (requires neurological examination, muscle enzymes)
• Occult haemorrhage may produce collapse with shock, then a gradual recovery. (Beware of a haemoabdomen, it can sometimes be quite subtle).
• Tussive-syncope (in the history, coughing precedes and leads to the collapse)

 Further investigations required to reach a diagnosis : 

• A comprehensive blood profile is indicated: haematology and biochemistry (eg. TP, Alb & globulin, urea & creatinine, liver enzymes & bile acids, cholesterol, glucose, Na, K & Cl, P04 & Ca).  Note: the blood should be taken from a fasted animal (12 to 18 hours), and the serum separated for accurate assessment of electrolytes prior to posting. Post-prandial bile acids (2 hours) is useful to check liver function.
• If occult haemorrhage is suspected then serial protein levels and haematology (including a reticulocyte count & MCV) need to be run. PCV drops 12 to 24 hours after the haemorrhage, with regenerative signs seen after 4 days and maximal at 7 days. Protein levels drop after a few hours, up to 24 hours. Check urine for evidence of blood break-down products. Consider paracentesis or peritoneal lavage to screen for abdominal haemorrhage. Ultrasound is the diagnostic test of choice to screen for abdominal fluid and also guided paracentesis.
• Screening for hypothyroidism and/or Addison’s disease may be important in some cases.
• Survey thoracic radiographs and/or echocardiography may be of help to rule in/out an organic heart disease (e.g. occult dilated cardiomyopathy, pericardial disease) or a thoracic abnormality.
• Ideally an ECG at the time of collapse is required to check if a dysrhythmia is the cause of the collapse. Since this is almost impossible with infrequent collapse it might be more practical to teach the owner how to examine the colour of the gums (they usually go pale with cardiac syncope) and feel the heart rate during future collapses (for an extreme bradycardia or tachycardia). It is, of course, important to have the owners’ practice daily to be familiar with what is normal.
• If the dog is collapsing very frequently, ie. at least once a day, then a patient activated ECG monitor or 24 hour Holter monitor can be used to record the heart rhythm during a collapse - we have these available for use on referred cases.  Alternatively an implantable Reveal device for those dogs that don’t collapse within the 24 hour period.
• In some cases, if the owner can video-record the event with a camcorder, this can provide valuable information.
• If a muscular disorder is possible, then muscle enzymes (AST & CK) pre- and 24 hours post-exercise might be of value.
• Measurement of serum titres for myasthenia gravis, Neospora and Toxoplasma may be of value in some cases.
• If a muscular disorder is suspected then referral to a Diplomate/Specialist in Veterinary Neurology for EMG and muscle biopsy would be advisable.

Recommended reading Martin & Corcoran (2006) Notes on: Cardiorespiratory disease of the dog and cat, 2nd ed. Blackwell Science. ISBN 0-632-03298-7