Page 524 - Adams and Stashak's Lameness in Horses, 7th Edition

490 Chapter 4

LAMINITIS

VetBooks.ir JaMEs BElknap, andy paRks, and katy dERn

OVERVIEW to more rapid advancements in our understanding of
systemic and local pathophysiologic mechanisms
The disease generally termed “laminitis” likely exists through the more accurate description of clinical cases
solely due to the unique mechanical demands placed and through the establishment of experimental models
upon the equine epidermal cell in the digital lamellae, for all three types of laminitis. Concomitant with our
namely, the suspension of the distal phalanx (and thus increased understanding of the pathophysiology of the
the entire weight of the horse) within the hoof wall. different types of laminitis, effective management and
These lamellar epidermal cells are not only responsible treatment tools are now being discovered.
for adhesion of the hoof wall to the underlying connec- This chapter will review the relevant anatomy, our
tive tissue of the basement membrane and dermis/ current understanding of pathophysiologic mechanisms
corium (which attaches to the distal phalanx); it is now occurring in the different types of laminitis, the clinical
clear that the cells must maintain their cellular structural presentations of the animals, and finally, a discussion of
integrity (i.e. “stiffness”) to resist the distracting forces current diagnostic and therapeutic procedures for the
normally placed upon them in their function of suspend- different types of laminitis.
ing the distal phalanx—and therefore the entire muscu-
loskeletal system—of the horse. Although dysadhesion
of the epidermal lamellae from the dermal lamellae RELEVANT ANATOMY
(leading to lamellar separation) was traditionally
described as the primary cause of lamellar failure, The distal phalanx of the equine digit is surrounded by
stretching of the epidermal lamellae through loss of the hoof, consisting—similar to the skin—of the deeper
structural integrity of the lamellar epidermal cells is now dermal tissues and more superficial epidermal tissues. The
realized to be a central component to lamellar failure. dermal tissue surrounds and attaches to the outer cortex
When either the structural integrity of the epidermal of the distal phalanx through components of the extracel-
cells or the adhesion of these cells to the underlying der- lular matrix (e.g. collagen fibers). In addition to providing
mis is compromised, the resulting loss of structural important structural functions through the connective tis-
strength of the digital lamellae, combined with the forces sue attachments to the distal phalanx, the dermal tissue
on the distal limb, leads to a displacement of the distal also contains all of the vasculature and innervation of the
phalanx. Three types of displacement of the distal pha- hoof. The dermis of the hoof wall consists of the subla-
lanx are described: (1) symmetrical distal displacement mellar dermis (also described in the literature as “deep
in which the entire distal phalanx displaces distally, (2) dermis,” which attaches to the distal phalanx axially and
asymmetrical distal displacement in which the medial continues abaxially as the dermal lamellae [containing
(most common) or lateral aspect of the distal phalanx the microvasculature upon which the epidermal lamellae
displaces distally, and (3) dorsal rotation in which the depend for nutrition]). Importantly, the deep dermis also
dorsodistal tip of the distal phalanx displaces in palmar- extends on the palmar/ground surface of the distal pha-
odistal direction away from the hoof wall (discussed in lanx, providing the vasculature and innervation upon
detail later in the chapter). Throughout this discussion, which the solar epidermis is dependent.
the term palmar used for the forelimbs is intended to be Similar to the skin, the epidermal component of the
inclusive of plantar for the hindlimbs. hoof consists of both keratinized nonviable components
The term laminitis has been used to describe all clini- externally and live epidermal cells internally. The outer
cal presentations of equids in which structural failure of hoof wall, consisting of the stratum externum and stra-
the lamellae occurs. As laminitis occurs as a sequela to tum medium, provides both protection to the digit and a
other diseases, the types of laminitis are categorized critical support function due to fact that, through the
according to the general predisposing disease states lamellar attachments to the distal phalanx, the hoof wall
associated with lamellar failure: systemic sepsis/ bears the majority of weight of the horse normally via
endotoxemia (sepsis‐related laminitis [SRL]), systemic the hoof wall’s contact with the ground. The epidermal
endocrinopathies (endocrinopathic laminitis), or a dis- lamellae, part of the stratum internum of the hoof wall,
ease or injury to one limb that necessitates preferential extend vertically from the coronary groove of the proxi-
weight‐bearing on the opposite limb (supporting limb mal hoof wall distad to merge with the sole; they are
laminitis [SLL]). Although the predisposing disease placed around the periphery of the distal phalanx, inter-
states are readily recognized, there has been a great deal digitating with the same number of dermal lamellae.
of controversy for decades regarding the specific patho- There are approximately 550–600 primary epidermal
physiologic mechanisms leading to lamellar failure in lamellae, each of which consists of a keratinized, acellu-
each type of laminitis, and, as therapies for diseases are lar core (continuous with the keratinized hoof wall por-
based on the pathophysiology known for that disease, tion of the stratum internum). Approximately 150–200
disagreement about the cascade of events leading to secondary epidermal lamellae emanate from each pri-
lamellar failure has led to confusion regarding therapeu- mary epidermal lamella, interdigitating with secondary
tic regimens for the disease. The somewhat recent estab- dermal lamellae. The secondary epidermal lamellae
lishment of the three primary types of laminitis has led are made up of two types of live epithelial cells, basal