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The purpose of the study was to compare radiographic and histopathologic findings …

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- Calcification of intervertebral discs in the dachshund: a radiographic and histopathologic study of 20 dogs

The present study shows that a significantly higher number (45.7% versus 27.4%) of calcified discs are identified by a histopathologic examination compared to a radiographic examination of intervertebral discs in dachshunds. This finding verifies the hypothesis presented at the beginning of the present paper.

At the radiographic examination, a total of 148 calcified discs were identified in 18 dogs. This makes a mean of 8.2 calcified discs in each affected individual. Both the percentage number of dogs with calcified discs (90.0%) and the mean number of such discs in each affected dog are higher than values for corresponding parameters found in previous radiographic studies [6,7,12]. The fact that the present study is related to clinical cases, whereas the previous radiographic studies were related to clinically normal dachshunds, could be the reason for this disparity.

In dogs no. 2 and 3, no calcified discs were found at the radiographic examination, but at the histopathologic examination a slight degree of calcification was found in five and 15 discs, respectively. This finding shows that a total absence of calcified discs visible on radiographs of a dachshund is no guarantee for the dog not having extensive calcification at histopathologic examination.

Of the 92 discs that were found to be calcified only at the histopathologic examination, 84 (91.3%) were noted to have a 'slight' degree, eight a 'moderate' degree and none a 'severe' degree of calcification. This agrees with the general understanding that an ordinary radiographic examination of the vertebral column in dogs is not very sensitive at detecting minor calcifications of intervertebral discs.

The radiographic examination in the present study was done on vertebral columns separated from the skull, ribs and pelvis and freed of soft tissue. This meant that superimposition of tissue adjacent to the discs (e.g. caput costae and os ilium) was avoided on the radiographs. Also motion blurring was not a problem with the examination of dead specimens. These factors contributed to high technical quality of the radiographs. Therefore, the sensitivity of the radiographic examination in the present study is likely higher than could be expected in similar studies on live dogs. As a consequence, the presented results concerning total number of calcified discs and degree of calcification of individual discs, cannot directly be compared to results from previous radiographic studies [5-7,9,12] on live dogs.

The 20 dachshunds included in the present study were euthanased up to 24 hours before being available for the radiographic examination. As a result, rigor mortis was present in several dogs and for these the standard procedure for spinal radiography of live dogs [13,16] was inappropriate. Two dogs (no. 3 and 18) had been subjected to post-mortem examinations immediately after euthanasia and their vertebral columns were thereafter included in the study. To achieve radiographs of high technical quality from more dogs, separated vertebral columns, instead of vertebral columns "in situ", were chosen for radiography. Diagnostic imaging of separated vertebral columns is previously reported in radiographic [17,18] and MR [10] examinations in dogs.

Radiographs and histological sections were each evaluated by one person, which means that both the radiographic and the histopathologic examination were subject to significant interobserver variation. With two or more persons evaluating the radiographs and the histological sections respectively, the interobserver variation and consequently the standard deviations of the observations presented in Table 2, would decrease. Consequently, the calculated sensitivity of 0.6 represents an underestimate.

At the radiographic examination, additional radiographs were taken if there was any doubt about calcification. At the histopathologic examination, at least two complete histological sections of each disc were examined to remove any doubt about calcification. By this, the present study should not be encumbered with significant intraobserver variation.

A specificity of 1.0 indicates that no false-positive errors were made at the radiographic examination. The high technical quality of the radiographs is probably a significant reason for this. Another reason could be a reserved attitude by the reader of the radiographs, evaluating discs with uncertain calcification as negative.

Von Kossa, a special stain for identification of calcium, was included in the histopathologic examination of all intervertebral discs. In previous histopathologic examinations of disc degeneration in dogs [4,10], this stain was not used. In the study by Seiler et al. [10], decalcification was also part of the preparation for histopathologic examination. A discrepancy in results can easily arise when different studies adopt different protocols.

In the present study, histopathology was found considerably more sensitive than radiography in identifying calcified discs. Nevertheless, in ten discs the degree of calcification was found to be higher in radiography than in histopathology. A histopathologic examination of non representative sections could be an explanation for the different findings in these ten discs.

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