OSTEOCHONDRODYSPLASIAS,
LEG DEFORMITIES,
AND
DWARFISM IN THE CANINE
Part
Two
(Warning: Pictures are slow to download. Please be patient.)
By Fred Lanting
There
has been renewed interest in the subject of “abnormal” bone lengths, joints, angles
between limbs, and related phenotypic variations from what I have called “the
ancestral type”. We need to establish some definitions of terms before entering
into a discussion of the subject. The “ancestral” phenotype in my arbitrary
definition (which, however, is in line with the views of many or most
professionals in animal science) is one that comes to mind when one thinks of
the Jackal, Northern Wolf, and descendants of the extinct Pale-footed Wolf
(such as sighthounds). The head is neither brachycephalic (pushed-in/shortened) nor exaggeratedly long
and narrow (the dolichocephalic Borzoi, etc.), the
leg length is such that the total height at withers is roughly twice the
distance from elbow or chest to the ground, and limbs give an impression of being
straight. Typical examples of ancestral types are the German Shepherd Dog,
Saluki, various Spitz breeds, and many pariah breeds such as found in every
corner of the world.
“Abnormal”
phenotypes (and this will rouse the ire of many people who love their dogs and
think of them as being “normal”) include breeds specifically bred to produce
the characteristics that would be agreed on as being “faults” in the ancestral
types. Think of the (“English”) Bulldog, Pekingese, Corgis, Dachshunds/Teckels, and others. I have long maintained that there is a
genetic defect affecting primarily the hypophysis or
pituitary gland, the “master gland” that so greatly influences the functions of
the others as well as developmental processes.
Some
variation within normal parameters results from the tremendous plasticity of
the canine genotype, but here we are more interested in the departure from
those limits of normality. Whatever the combinations of defective DNA
nucleotide pairs (adenine-thymine, cytosine-guanine, etc.), and which glands or
organs they initiate the changes in, many of the irregularities we are
discussing here manifest themselves in the cartilage that is on the ends of
bones and “bone centers”. Bone centers are those hard, mineralized portions of
a growing bone that become enlarged (almost entirely on their long-axis
surfaces) and fused together to form the eventual limb, and the cartilage
between most bone centers is called a “growth plate” or physis.
It gradually “calcifies” into bone tissue, thus uniting epiphysis (called a cap
or head, usually) and metaphysis (shaft); it
disappears during maturation. But if there is an abnormal coding of the
nucleotide pairs, there is an abnormal calcification process, a “growth-plate
disturbance”.
If
one bone in a two-bone limb segment (such as the tibia/fibula or ulna/radius
combinations) has more of a disturbance than the other, or if one end of the
growing bone’s cartilage is disrupted during remodeling into bone tissue, there
are unequal rates of growth and consequent bowing of that limb, with one part
wanting to be longer than the other part does. In some dogs, disruption of
normal cartilage-bone turnover at the ends can keep a single bone from growing
in length, or if the disruption is laterally asymmetrical, the femur or humerus may also become slightly bowed. The pull of muscles
and ligaments on different parts of such bones also has some effect on shape.
The
general definition of “dysplasia” is poor or abnormal
(dys-) shape or form (-plasia).
Here, we are talking mostly about bone (osteo)
and cartilage (chondro-), and mostly about
those tissues in joints. But just as the poet says, “no man is an island”, and
genes that cause one thing can sometimes also cause something else. Some times
it is a very obvious double influence, such as the gene that causes both
deafness and white coat phenotypes in some breeds, or dwarfism and blood
disorder in the Malamute. Most of the time, the influence of one gene or set of
nucleotides is less obvious.
A
couple more definitions would be helpful at this point, although you should
realize that there are sometimes loose adherence to
strict interpretation of such definitions:
· chondrodysplasia: any growth plate (cartilage) disturbance resulting in canine dwarfism; in human pathology, it has a different meaning: enchondromatosis, a rare disorder marked by enlarged cartilage and tumors in joints.
·
chondrodystrophic: semantically similar
to the above, but while –plasia refers to
changeable shape, –trophic refers to growth. Thus, an abnormal cartilage growth pattern.
·
achondroplasia: that
type which results in an individual with extremities shorter than the trunk.
Examples in dogs include Basset Hound, Shih-Tsu, and others
mentioned elsewhere. In humans, it usually is marked by stubby hands, large
head with sunken nasal bridge and, frequently, spinal column deformities.
CHONDRODYSTROPHY
Several breeds are of a body type we call chondrodystrophic,
such as the Dachshund and Corgi. They have shorter legs (often bowed) and other
dwarf characteristics in parts of the body.
Frequently, these breeds also have a shorter vertebral arch that tends
to produce a smaller vertebral canal. The vertebral body centers of ossification
unite with the arch prematurely, with the same type of dystrophic bone growth
pattern that causes shorter “long bones” in those breeds. See Chapter 16 in my
2004 orthopedics book for more illustrations and discussion on dwarfism. In
some chondrodystrophoid breeds such as the Basset, a
premature closure of the distal metaphyseal plate of
the ulna (near the wrist) was thought by Herron, Grüll,
Henschel, and von Hitz to
cause fracture of an already closed anconeal process
at the other end of that limb. Kasström and
colleagues (and later, Wind) thought that this condition in certain dwarfed
breeds “was the result of an abnormal pressure on the anconeal
process... by the shortened ulna.” This anomaly in the anconeal
process is not the same as the failure to unite, and obviously has a different
genetic origin. Dr. Wind, the eminent expert on elbows, has observed that
many cases of elbow dysplasias include subluxations associated with dwarfism
There are many types of dwarfing related to slow endochondral bone formation (at the ends). Dwarfism can be
proportionate or disproportionate, depending on the specific gene defects.
Examples of the former include the pathological pituitary dwarfism of the
German Shepherd Dog and the related Karelian Bear
Dog. You can see GSD pituitary dwarfism in “The Total German Shepherd Dog”
(www.Hoflin.com). The non-pathologic selective miniaturization seen in Shar-Pei, Bull Terriers, Australian Shepherds, Poodles, and
numerous other miniature and toy breeds and varieties probably should not be
included in a discussion of dwarfism. Disproportionate dwarfs include Bulldogs,
Basset Hounds, Pekingese, Dachshunds, Corgis, and many more that we see only in
this form. While some would object to inclusion of their favorite breed here,
it is still true that these are results of genetic defects. There are also
“unnatural” occurrences of disproportionate dwarfism in breeds where you might
not expect it: Malamutes, GSDs, and a few other
“accidentals”. Of course, if one were to deliberately linebreed
on these defects, a sub-population of short-legged representatives would be
more common. That is what happened with a branch of the Parson Jack Russell
Terrier, now known by several similar names in various show registries.
Some of these defects involve irregularities in the construction of
the hypophysis (pituitary gland), as I have said;
others may involve primary proteoglycans degradation
that results in mucopolysaccharidoses, or other
metabolic abnormalities. The various types of mucopolysaccharidoses
involve enzyme deficiencies, incomplete fusion of the sacrum, incompletely
developed vertebral end plates, short limbs, abnormal joint mobility, and other
signs and deformities. Some osteochondrodysplasias
(you now know how to break that word into segments, and what they mean) can
be recognized at birth, others not until skeletal maturity approaches. The most
common seen at birth is the achondroplasias of
rabbits, mice, and humans, although some (erroneously?) apply that term to
conditions in dogs, too. Some achondroplasia is
from sporadic mutation, and most seem to be transmitted by a dominant gene. A
few authorities have claimed that achondroplasia is
not seen in the canine, but Aegerter and Kirkpatrick
describe it as a genetic chondroblast (cartilage cell)
disturbance in the epiphysis. Betts calls it “a symptom rather than a disease”
and does not hesitate to apply the word to the “normal” condition seen in the
Dachshund, Beagle, Basset Hound, French Bulldog,
Pekingese, and similar breeds. He properly excludes pituitary dwarfs,
miniaturized but proportionate breeds, Malamute dwarfism, and the dwarfism of
Labs with retinal dysplasia. Various forms of chondrodysplasia affect Cocker Spaniels and German
Shorthaired Pointers.
Miniature Poodles are occasionally found with a form of osteochondrodysplasia that has sometimes but properly
erroneously referred to as “achondrodysplasia”. An
increased collagen concentration and RNA content is often found in affected
cartilage of such dwarfs, though DNA content is normal. There are differences
in appearance between individuals because of modifier genes as well as
environmental forces. Miniature Poodles with inherited epiphyseal
chondrodysplasia are rhizomelic
(it seems the shortness of their limbs comes primarily from the retardation of
growth nearest the hips and shoulders) and often have ventrodorsal
compression of the chest and enlarged joints. Occasionally a spontaneous
mutation will produce symptoms similar to congenital spondyloepiphyseal
dysplasia and “achondrodysplasia”,
and variations on these are seen in many breeds, including multiple epiphyseal dysplasia
in Miniature Poodles and Beagles.
Asynchronous (unequal) growth of the radius and ulna, that
is, when these two bones’ growth rates are not coordinated, is found in
non-dwarfs in a wide continuous “spectrum” of severity, often unnoticed by the
average dog observer (and many a licensed dog-show judge!). Dwarfism is often
an accompaniment to that asynchronous growth of the “double bones” in either
front or rear limbs. Such dogs also have nearby limb segments shortened, such
as the humerus or femur. The more extreme
end of that range is considered by some to be “normal” in some breeds (to name
some more: Corgis, Basset, Lhasa Apso, and Pekingese). However, elbow subluxation accompanied by pain has been reported in these
dwarf breeds, and probably has a causal connection. While carpal valgus (turning out at the pasterns) and external rotation
of the foot are “acceptable” within the descriptions of “breed type”, the
occasional or perhaps frequent abnormalities of the ligaments and joints in the
elbow that accompany this should be avoided or treated.
OTHER SIGNS
Chondrodysplasia in
the most general sense is sometimes called a syndrome, other times part of
a syndrome, the definition of that word being a collection of interconnected
symptoms. Some dogs may have many, some
a few, and others no readily observable symptoms. The
clinical signs may be very mild, from almost undetectable bowing or shortening
of the legs, to obvious skeletal deformity and the presence of several health
problems. Chondrodysplasia Punctata is one name applied to a syndrome of
multi-systemic disorders, and is so-called because of the “dots” of calcium
phosphate deposits in the softer cartilage. It reminds me of the school of art
made popular by Georges Seurat called “Pointillism”.
This genetic-metabolic problem has various skeletal expressions. Depending on
the particular variety, the mode of inheritance could be autosomal
recessive or dominant, or X-chromosome-linked recessive or dominant, some with
full penetrance, and some not.
Besides skeletal indications, there are eye disorders such as microphthalmia (smaller eyes than they should be), lens
detachments, cataracts, glaucoma, retinal defects, and nystagmus
(jerking or twitching of the eyeballs). Other occasionally reported symptoms
are problems with internal organs, head and neck bone defects, partial
deafness, alopecia, and luxated patellas
(for more on this stifle problem, see my upcoming orthopedics book or some of
the websites that carry my articles).
PREMATURE PHYSIS CLOSURE
Premature closure of growth plates happens because, in some
etiology (manner), the ossification process of endochondral
cartilage is disturbed. Overfeeding and mineral supplementation are definitely
contributors, but genetic susceptibility has to be taken into account, as well
— probably much more. Ettinger mentions that “the
most common cause of premature growth interference has been direct trauma to
the growth plate area”, though HOD and achondroplasia
have also been reported in association with it. But he and his sources may have
been giving too much credit/blame to physical or mechanical damage. The distal
(furthest part) radius and ulna seem to be the most frequently involved sites
for these disturbances.
Growth disturbances in the radius and ulna can be related to an
outward twisting of the top of the ulna away from a good fit with the humeral condyles, enough so that subluxation
or even luxation takes place. This lateral rotation
may also exist independently, with no observed growth plate disruptions
in those bones. The radius head can also dislocate, and both may occur at the
same time, so there is quite a variety of changes
possible, although the disorder is rare.
If the dislocations are not accompanied by (or secondary to) such
asynchronous growth manifestations as seen in the ulna and/or radius, they are
called congenital elbow dislocations. The premature closure of physes in ulna or radius, retained cartilage, chondrodysplasia or achondrodysplasia,
and synostosis are separate problems. A condition of
missing digits called ectrodactyly and another
abnormality called cleft hand deformity have been seen in conjunction with
congenital luxations and subluxations
(also called arthrodysplasia) in the elbow.
ROOTS OF MANY OF THESE DEFECTS
Cholesterol has had a bad name among
fad-diet promoters and people too lazy or busy to physically work off their
calories. It is a product of the liver, necessary for the synthesis of Vitamin
D as well as the assimilation of it, essential fatty acids, and Vitamins A, E,
and K, but in sedentary people and those with genetic inability to metabolize
it correctly, it can build up in the blood vessels and contribute to heart
disease and stroke risk. At least some chondrodysplasias
involve an error in the coding for biosynthesis of cholesterol. Since in the Havanese, a miniature Cuban breed, those who evidenced this
dwarfism tested as having abnormal levels of several cholesterol-related
sterols, a program of blood serum testing was undertaken. It was found that Havanese with normal straight legs had no such metabolic
abnormality. in the body. These vitamins are needed
for calcium utilization, bone development, and healthy eyes. The appearance of
congenital defects, including osteochondrodysplasias,
can often be blamed on inability to use these chemicals. Tracing a structural
defect to its headwaters of a genetic defect expressed in a metabolic disorder
along the route, is akin to finding the source of the
PLEIOTROPIC DEFECTS
Pleiotropy is
the phenomenon of having more than one phenotypic expression (often in grossly
different manifestations) caused by the same gene — the same genetic defect.
Alaskan Malamutes’ dwarfism is a pleiotropic genetic
defect that shows up as both dwarfism of their particular type and
a blood disorder. It has been fairly extensively studied, and while one dog may
vary in appearance considerably from the other, the disorder is a simple autosomal (not sex-linked) recessive trait with complete penetrance. Asynchronous growth of the radius and ulna (one
at a different rate or completion than the other, remember) is part of the
deformity in this breed. The chondrodysplasia in this
breed has at times been mistaken for the Vitamin D deficiency called rickets,
but only the tubular bones are affected, other than retarded ossification of
the lateral tarsal (cuboid) bone. The head, spine,
and other bones are not stunted or changed, and body length is normal. The gene
that causes this chondrodysplasia also creates a macrocytic hypochromic anemia;
the discovery of this being indicative of the way carriers may be found. A
third effect of this one gene, by the way, is a different ability to bind
certain trace minerals in the liver.
While on a judging assignment in
A few other problems are similar, in that one gene (or gene pair,
really) can cause ocular-skeletal dysplasia in
Labrador Retrievers and possibly Samoyeds, for example. In this disorder,
several defects in the eyeball, iris, and arteries serving the eyes are found
in the same dogs that have short, thick leg bones (micromelia),
prominent carpi (wrists) and elbows, and east-west
stance in front. Hind legs usually are hyperextended
(straight in stifle) yet still very short.
Great
Norwegian Elkhound chondrodysplasia
is similar to the other canine dwarfisms as well as to human spondylometaphyseal dysplasia; it
is widespread in the breed, and may be associated with glycosuria
(sugar in the blood), although in one study this was not found. Some curvature of the front legs may be
noticed as early as 5 weeks of age, and all limbs are short in proportion to
the body. It is also a simple recessive trait.
A disorder almost identical to the chondrodysplasia
in two of the above-named breeds has recently been found in Akitas. Knowing how such reports usually lead to the identification
of the same disease in other breeds (as has happened in panosteitis,
GSD myelopathy, etc.), it is not very risky to
predict that more will be added to this list in the future, though not at a
high rate, given the very obvious nature of dwarfism and most breeders’ desires
to sweep it under the rug or eliminate it.
On an excellent website called Rhosyn German Shepherd Dogs,
there was a good description and illustrations of dwarfism in the Havanese breed, and an ancillary discussion of dwarfism in
the GSD, even though the site owner did not want to use that word for the
condition. Havanese with dwarfism display ocular
abnormalities, as do a few other breeds. The front legs grow crooked or bowed,
and all four legs are shortened, giving the height-to-length ratio an
undesirably short aspect. Havanese breeders
have reported that all cases of early-onset cataracts leading to premature
blindness, and nearly all “other serious health problems reported in Havanese within the past few years, have been in dogs that
also exhibit the symptoms [of chondrodysplasia]”. In Havanese, it also has been noted that some dogs have such
subtle signs that they appear to have a straight leg on one side but not the
other — asymmetric.
Furthermore, that such asymmetrical dogs, if they are
also diagnosed as having cataracts, will have the cataract in the eye that is
on the same side as the crooked leg! The Rhosyn
website mentioned above says, “To date, no Havanese
with straight legs have been diagnosed with early
onset cataracts!”
Many other dysplasias considered as a
subcategory under osteochondrodysplasias result from
disturbed ossification along the periphery (outer edge) of the growth plates in
various bones such as the ribs, vertebral processes, skull, and elsewhere.
Certain dwarfism characteristics have been made part of the breeds’ show
standards and are not much covered here, but even some breeds that are not
normal skeletally, anyway, such as Dachshunds and French Bulldogs, sometimes
are even more afflicted with chondrodysplasia than
their artificially-considered “normal” compatriots. In many cases, dwarfisms
with partial penetrance or expression may go
unrecognized, with the breeder considering the mildly affected pup to be simply
a “runt”.
Recently, some cases of chondrodysplasia/chondrodystrophy
of the sort found in the Corgi have been reported in German Shepherd Dogs in
Illustrations
either accompany this article or are available.
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Figure 1. Normal proportions and leg length in a Havanese, with about as much chest depth as the distance from elbow to floor. |
Figure 2. Chondrodysplastic dwarf “Hav” with bottom
of chest well below the elbow, & with shortened limbs. |
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Credits:
photographs graciously supplied by Rhosyn kennels in Australia. photos
copyrighted © 2000 – 2003 by Rhosyn: http://www.rhosyngsd.com/ |
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Figure 5. “Dusty”,
side view, 9 months old |
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