 If you’ve been around horses for any
length of time, you probably know of some
unlucky equine who has been diagnosed
with navicular disease (or navicular syndrome).
Navicular is the most common
source of performance limiting lameness
in the front legs of horses today. Sadly,
the actual cause of this lameness is poorly
understood, probably because it’s difficult
to pinpoint the true cause of foot pain.
Disease or Syndrome?
Over the past decade, I’ve seen veterinarians
diagnosing fewer and fewer cases
of true navicular disease. Instead many
elect to simply classify any horse with
navicular region pain as a navicular
syndrome horse.
So what is the difference between a
disease and a syndrome? The truth is,
not very much. Disease is defined as
a pathological condition of a part, organ,
or system of an organism resulting from
various causes, such as infection, genetic
defect, or environmental stress, and
characterized by an identifiable group of
signs or symptoms.
A syndrome is more generally defined as a
group of symptoms that collectively indicate
or characterize a disease, psychological
disorder, or other abnormal condition.
By using the term syndrome, a veterinarian
may feel he has left his options
somewhat open. He doesn’t have to deal
with the stigma that surrounds a diagnosis
of true navicular disease, where the
prognosis is seen as very poor. The term“disease” implies a known cause and
a specific treatment1 but by calling it a
syndrome, prognosis is left to the discretion
of the attending veterinarian.
The Conventional View
Conventional veterinary medicine views
navicular disease as chronic forelimb
lameness associated with pain originating
from the distal sesamoid (navicular
bone) and its closely related structures,
including the distal impar ligament, collateral
ligaments of the navicular bone,
the navicular bursa, and the deep digital
flexor tendon.2 These collective structures
are sometimes referred to as the navicular
apparatus. Navicular disease is considered
degenerative in nature, resulting in
progressively worsening lameness.
Conventional veterinary medicine
defines navicular disease as a single
disease. Given the variety of symptoms
that manifest in lameness of the fore
foot, however, it is likely that several
different conditions, with different origins,
are responsible for pain associated with
the navicular region. MRI evidence
confirms that many other problems in
horses cause the same clinical signs as
those in horses diagnosed with navicular
disease. This has lead some researchers
to question the term “navicular disease”,
feeling that it no longer applies
to many of the horses being evaluated
for foot lameness problems.
Clearing Up
the Confusion
Researchers have been unable to reproduce
navicular disease in experiments,
so they can only speculate about what
causes it. In fact, today’s veterinarian
can subscribe to several theories on
how the condition occurs3 and this will
determine how he decides to treat it.
One theory suggests vascular problems
as the cause of navicular disease.
Researchers reportedly observed thrombosis
(clotting) and arteriosclerosis
(thickening arterial walls), leading to
ischemia (insufficient blood supply)
within the navicular bone in horses
diagnosed with navicular disease. This
theory, however, has been largely rejected
because of a failure to reproduce clinical
signs or pathological changes when
researchers reduce blood supply to the
navicular bone of horses in clinical studies.
A second theory, which focuses on
postmortem studies of horses with longterm
chronic lameness and radiograph
abnormalities, suggests that biomechanical
factors may promote this degenerative
disease. Proponents of biomechanics
as a cause define navicular disease as
pathological changes of the soft tissue
of the navicular apparatus – the navicular
bursa, and the articular cartilage of the
joint.4 They propose that the pathological
changes are the result of inflammation
caused by vibration and friction.
In other words, this theory suggests
that environmental influences can result
in stress on the navicular area during
movement. Horses that work over hard
surfaces, for example, experience excessive
vibrations that result in changes
to the mechanics of joint movement.
This leads to extreme compression of
the navicular bone by the deep digital
flexor tendon. Toe first landing and foot
imbalance are other examples of negative
influences that can adversely affect
the biomechanics of joint movement.
How to Know if Your Horse has Navicular
Most people follow a more conventional, reactive way of thinking,
so they consult with their veterinarian only after a steady loss of
performance from their horse. The horse may have exhibited a
shortened stride, with forelimb stiffness, intermittent shifting of
weight from one forelimb to the other, or pointing of intermittent
limbs. The observant horse owner may even recall that the horse
had previously warmed out of his lameness, but the warm-up took
progressively longer, until the horse no longer worked out of the
lameness. In advanced cases, she may have observed that the horse
had packed mounds of bedding beneath his heels, or that he rested
his hindquarters on a manger or fence rail.
If you follow Applied Equine Podiatry or a similar proactive approach,
you’ll understand that even a slight loss of performance over a
short period, coupled with the occurrence of mild hoof deformity
(flare, imbalance, increased asymmetry), could lead to pain within
the Internal Arch Apparatus, and a diagnosis of navicular
syndrome/disease.
Learning about proper foot structure will help you become proactive.
Observe your horse when he is sound; watch him move under saddle
both in a straight line and circling. Make a mental picture of your
horse’s movements. Have x-rays taken when your horse is sound,
and know what a good foot should look like. |
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Published in the January/February 2008 issue of Equine Wellness Magazine |
