Figure 1 shows the measurement protocol for hoof abnormalities. A rotational laminitic case (as opposed to fatal sinker syndrome)
has an increased horn-laminar zone (HLZ), an increased positive palmar angle (PA), an increased extensor process-coronary
band (EP/CB) as the extensor process moves downward in the hoof capsule (as opposed to the entire distal displacement of the
third phalanx [P3] that occurs in fatal sinker syndrome), a decreased sole depth under the tip of P3 (SDT) and an increased
sole depth at the wing of P3 (SDW).
Figure 1: The measurement protocol used to measure hoof abnormalities: HLZ (horn-laminar zone), PA (palmar angle), EP/CB (extensor
process-coronary band), SDT (sole depth under the tip of P3), SDW (sole depth at the wing of P3) and W/B (wing-to-bulb measurement).
(Photos courtesy of Andrea Floyd, DVM)
As you obtain the recommended weekly lateral radiographs of your laminitic cases, you will be observing the changes that take
place over the four- to six-week span of the degenerating progression of the disease. Ideally you have jumped on the case
at its onset and biomechanically halted the pull of the deep flexor on the P3. Typically over the four- to six-week period,
improvements in the SDT, the PA, the heart rate and the horse's pain levels are noted.
In horses with fatal sinker syndrome, however, the values will change. The heart rate will increase (80 to 100 beats/min),
and the horse will become recumbent (Figure 2) unless it has a foal by its side, in which case the mare may not lie down until
it is too late.
Figure 2: The typical recumbent pose of a horse with fatal sinker syndrome, an uncommon form of Grade IV laminitis that can
Measurements will also change strikingly. Radiographically, the P3 may appear to be going back to its normal position within
the hoof capsule. In actuality, you may be witnessing the loss of all soft tissue architecture and the collapse of the P3
within the soft tissue (lamina and bulbar tissue). A halo or lucency throughout the coronary band may be observed on radiographs
(Figure 3). A ledge above the coronary band may also be seen (Figure 4).
Figure 3: A lateral radiograph showing a halo and ledge at the coronary region (white arrow).
When all these changes occur, the SDT decreases, the SDW decreases, the EP/CB increases, the HLZ increases and the PA decreases
as the P3 starts its slide down and back within the hoof capsule. The SDT and SDW may appear nearly the same. Venography will
likely reveal proximal displacement of the circumflex vessels and leakage of contrast material into the surrounding lamina.
Accurate records of your measurements are therefore critical for prognostic purposes.
Figure 4: A swollen coronary band before hoof wall avulsion becomes obvious.
The wing-to-bulb measurement (W/B) is another measurement I add to my protocol. This measurement is taken from the most palmar
aspect of the P3 wing in a straight line to the bulb of the heel (Figure 1). Be sure you use the same points for measurement
on each radiograph or you may misinterpret your findings. In the course of your evaluations, if this measurement decreases,
you may need to obtain radiographs repeatedly at 24-hour intervals to see if there has been further decrease. Fatal sinker
syndrome cases typically slide back within the hoof capsule, and the W/B measurement will show this impending problem.
I watch all parameters carefully for the first four to six weeks of new case admissions. If a horse presents with or develops
swelling around the entire circumference of the coronary band or starts to swell in the pastern region, immediately repeat
venography and start sequential cryocompression therapy. Once the swelling has stopped, perform another venographic study
to determine whether normal blood flow has returned. If blood flow is still compromised, then it is time to discuss Plan A
or Plan B with your client (see below).
Note that all venograms should be obtained on a standing horse that is loading the foot being radiographed.