Deborah A. Grosenbaugh

Equine Laminitis: A Potential Model of Raynaud's Phenomenon

Raynauds phenomenon (RP) and equine laminitis in the horse are medical enigmas. Clinical and scientific data were compared to evaluate the degree of similarity that exists between these two peripheral vascular diseases. Data indicate that certain pathologic and pharmacologic aspects seem to have common features. Some of the correlations maybe due simply to both diseases having ischemia of the distal digits as a pathologic component. The exact etiology of the ischemia is not known for either disease. The results of this study suggest the hypothesis that RP and laminitis are the same disease in different species. This hypothesis can be tested more efficiently when the pathophysiology of both conditions is better documented. It is possible that comparative studies will promote advances in the understanding of both RP and laminitis. The fact that equine laminitis can be experimentally induced is of potential value in such future studies.

The Pathophysiology of Chronic Laminitis: Pain and Anatomic Pathology

The potential pathologic manifestations of chronic laminitis are just as varied, and possibly more so, than the list of possible inciting agents of the disease itself. The extent to which rehabilitation and return to normal function can be attained, cannot always be accurately determined by physical examination. It should be remembered that significant physiologic and pathologic alterations occur in chronic laminitis; thus, even if radiographically the patient returns to a normal appearance, residual morphologic and structural defects are likely to remain.

The Digital Pathologies of Chronic Laminitis

This review indicates that the patient-to-patient uniqueness commonly seen in chronic laminitis represents the variable presence of the digital pathologies. Although some degree of mechanical failure is always present, the secondary metabolic and growth dysplasias, vascular pathologies, and sepsis may or may not be evident. The presence and severity of these pathologies appear to have a more significant impact on the prognosis of individual cases than does the displacement of the distal phalanx. It should be reiterated that it is often the combined presence of these individual pathologies that gives rise to the patient that is totally refractory to treatment. In the absence of these pathologies, many horses with significant displacement of the distal phalanx are not in pain and are not in need of treatment. It thus follows that a key to the improved rehabilitation of difficult patients is focusing research on the physiopathology and diagnosis of these nonmechanical problems.

EGF receptor-binding activity in the urine of normal horses and horses affected by chronic laminitis.

A heterologous radioreceptor binding assay (RRA) has been developed capable of detecting nanogram amounts of epidermal growth factor (EGF) receptor-binding activity in equine urine. The binding parameters of [125I]mEGF (murine EGF) to EGF receptors on equine plasma membranes are in good agreement with values from other EGF-RRA systems. The dissociation constant estimated from equilibrium methods (KD = 4 X 10(-10) M) is in reasonable agreement with that determined from the rate constants (KD = 6 X 10(-10) M) and is in good agreement with values determined in other species. The assay is specific for equine EGF (eEGF) receptor-binding activity and capable of detecting less than 0.34 nM eEGF receptor-binding activity in urine. Equine EGF receptor-binding activity in equine urine form adult horses varied widely between samples (8.5 +/- 6.5 nM). This variability was somewhat reduced when values were adjusted for dilutional effects using urine creatinine as an indicator (3.6 +/- 2.0 nanomoles/g creatinine). No significant differences were demonstrated between the means of EGF binding activity concentrations in clinically normal horses and horses affected by chronic laminitis.