A collapse may involve extreme weakness of the front or rear limbs, falling to the ground, or unconsciousness, in which case the cat is unresponsive to sound or touch. The severity and symptoms will often be related to the cause of collapse.
Often consciousness is maintained but the cat has an expression of confusion or of anxiety. A "glassy-eyed" appearance may be evident. The collapse may last for only a few seconds, or it may take many minutes to hours before your cat can stand again.
Numerous diseases can cause acute collapse. Often a disease is fairly advanced when such an extreme manifestation as collapse occurs. However, there may not have been prior symptoms.
Examples of illnesses that may cause collapse include: Heart disease, including congenital heart disease (birth defects in the heart), acquired valvular heart disease (leaky heart valves), heartworm disease, tumors of the heart, pericardial disease (disease of the lining of the heart) and primary cardiac arrhythmias (erratic heartbeat). When blood is not properly pumped through the body, the brain is most vulnerable. It can be momentarily "starved" of oxygen, causing collapse or fainting.
Fainting (syncope) can occur due to abnormal blood pressure control mechanisms (neurocardiogenic syncope). This can be difficult to diagnose without a very full evaluation.
Diseases of the blood. These include internal hemorrhage from a ruptured tumor or organ; severe anemia, leukemia, and polycythemia (abnormally thick blood caused by an excess of red blood cells). The brain and the muscles need an appropriate amount of blood to function (and the oxygen carried by red blood cells). Failing this, collapse can occur.
Respiratory diseases including blockage of the throat by a foreign object or by laryngeal paralysis (the inability to open the voice box so that air can enter the lungs). Other causes include respiratory disease such as bronchitis, collapsing trachea, pneumonia, or pulmonary edema (fluid in the lungs). CAREFUL! Many conditions cause gasping during collapse, but without a foreign object in the mouth or throat. Do not risk being bitten by trying to remove from the throat an object that is not there. "Choking on something" is a common owner's description of animals that have respiratory problems without any foreign body in the mouth or throat.
Diseases of the nervous system are common reasons for collapsing. These include fibrocartilagenous emboli (when clots in the bloodstream damage the spinal cord), intervertebral disk disease ("slipped disk" in the neck or back), degenerative myelopathy (degeneration of the spinal cord), and myasthenia gravis (defective conduction from the nerves to the muscles). In diseases of the spinal cord and muscles, the animal's consciousness and mental abilities are generally unchanged during collapse, whereas in brain diseases disturbances of consciousness such as seizures can occur during collapse.
Musculoskeletal diseases, including hip dysplasia (abnormality of the hips), lumbosacral disease (arthritis of the lower back), and others. Generally, with the musculoskeletal causes of collapse, symptoms such as limping, difficulty getting up, or inability to sit up or jump have been present and getting worse for some time (days, weeks, months) before collapse occurs.
Toxicity. Poisonings of many kinds can cause sudden weakness and collapse. Any known exposure to substances that are deliberately poisonous (e.g. rat poison, snail/slug poison) should be reported to your veterinarian, even if the poisoning may have taken place several days earlier.
Drugs and medications. A simple example would be an overdose of insulin causing an excessively low blood sugar. Many human drugs that might be mistakenly eaten by your cat (or maliciously administered by someone) could lead to low blood pressure. Similarly, some veterinary prescription medicines can lead to low blood pressure and collapse.
The most serious cases of collapse are life-threatening.