Flat Chest Kitten (FCK) Defect
and
Pectus Excavatum
A number of chest wall deformities have been described in cats and dogs in the veterinary literature. In cats, the two most common are pectus excavatum ("funnel chest") and flat chest (FCK). There are over a dozen reports about pectus excavatum in the literature, including papers describing surgical correction, but very little about FCK. It is important to understand the difference between the two conditions so they can be identified correctly.
Cross sectional diagram of a normal chest:

The chest cavity is oval in shape, with the spinal column at the top and the sternum at the bottom. The black squares represent the costochondral junctions, the point where the cartilage portion of the rib attached to the sternum joins the boney portion of the rib attached to the spine.
Cross sectional diagram of a pectus excavatum chest:

In pectus excavatum, the chest cavity is narrower top to bottom as the sterum is displaced upward. In severe cases, the lack of space compresses the heart and lungs. Common signs in moderate to severely affected cats include exercise intolerance, trouble breathing, cough, weight loss or failure to gain weight.
Cross sectional diagram of FCK:

In FCK, the ribcage is angled sharply at the costochondral junction, causing the ventral part of the chest to be flattened. FCK varies from very mild to very severe and life-threatening. The condition is not apparent at birth, but becomes obvious within the first few weeks of life. Mildly affected kittens may appear perfectly normal as adults. Moderate to severely affected kittens will have difficulty breathing and poor weight gain. The worst affected kittens will die.
FCK has been reported in many breeds of cats. Some FCK kittens also have a spinal curvature or may have pectus excavatum. A 1997 study in the U.K. reported that 3 to 4% of all Burmese born there are affected with FCK. The same study tried to evaluate potential causes and concluded that the defect is inherited, but must also be influenced by some other factors, possibly environmental or nutritional. For example, Burmese kittens in the study had much higher blood and skeletal muscle taurine levels than normally expected for cats. It is unknown what association these high taurine levels may have with FCK. No association between FCK and any particular diet or any particular dietary supplements has been found.
 
In North America, FCK has been seen in many breeds, but the overall incidence in any breed is not known. Until more information is known about this defect, breeders should not use any FCK-affected cat in a breeding program, even if it appears normal as an adult. It would also be wise to avoid breeding a queen that has produced FCK kittens with a sire that has also produced FCK kittens.

References:

Boudrieau R et al. Pectus excavatum in dogs and cats. Comp Contin Edu Pract Vet 12(3): 341-355, 1990

Fossum TW et al. Pectus excavatum in eight dogs and six cats. J Am Anim Hosp Assoc 25:595-605, 1989

McAnulty JF, Harvey CE. Repair of pectus excavatum by percutaneous suturing and temporary external coaptation in a kitten. J Am Vet Med Assoc 194(8): 1065-1067, 1989

Sturgess C. Flat chested kittens - does taurine have a role to play? Burmese Cat Club News (U.K.), vol 12, no 8, 1995

Sturgess CP, Waters L, Gruffydd-Jones TJ et al. Investigation of the association between whole blood and tissue taurine levels and the development of thoracic deformities in neonatal Burmese kittens. Vet Rec 141:566-570, 1997
Ocicat kitten (on the bottom) with flat chest kitten defect. FCK has been reported in many pedigreed cat breeds, and in random bred cats too.
 
X-ray of a kitten with pectus excavatum. Note the pronounced upward deviation of the sternum indicated by the 2 black arrows.
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