Adrenal gland shape and size are routinely assessed during comprehensive abdominal ultrasonographic examinations in dogs, and various features of adrenal gland size (ie, width, thickness, and height) have been described.1–14 When measurements are made in the transverse plane, LAT and RAT reportedly should not exceed 0.74 and 0.81 cm, respectively, in healthy dogs. However, measurements of adrenal gland dimensions in healthy small-breed dogs have challenged the validity of these reference values.13,14 For example, in 1 study,13 median adrenal gland width was reported to range from 0.39 to 0.45 cm in healthy Yorkshire Terriers, Shih Tzus, Maltese, Miniature Schnauzers, and Poodles with median body weights ranging from 2.9 to 8.0 kg (6.4 to 17.6 lb). Another recent study1 recommended modifying the upper limit for the longitudinal axis of the caudal pole of the adrenal gland in Labrador Retrievers (left adrenal gland, 0.79 cm; right adrenal gland, 0.95 cm) and Yorkshire Terriers (left adrenal gland, 0.54 cm; right adrenal gland, 0.67 cm). A third study14 evaluated the reproducibility of measurements of adrenal gland thickness in healthy dogs arbitrarily stratified into 3 weight groups (< 10 kg, 10 to 30 kg, and > 30 kg [< 22 lb, 22 to 66 lb, and > 66 lb]) and found interobserver coefficients of variation as high as 18%, with the highest coefficients in dogs weighing < 10 kg. In that study, the upper limits of the 95% CIs for thickness of the caudal pole of the adrenal gland in healthy dogs weighing < 10 kg, 10 to 30 kg, and > 30 kg (n = 15/group) were 0.54, 0.64, and 0.74 cm, respectively.
When measuring adrenal gland size, a salient clinical concern is the influence of non–adrenal gland illness on those measurements. The considerable overlap in adrenal gland size between healthy dogs, dogs with adrenal gland illness, and dogs with non–adrenal gland illness makes definitive recognition of adrenomegaly associated with specific adrenal gland diseases such as hyperadrenocorticism difficult.1–4,11,13,14 Although a positive correlation between age and adrenal gland size in dogs has been reported, contradictory findings regarding the effects of age and body weight on adrenal gland dimensions also exist.1–3,7,13,14
Given that adrenal gland size may influence the clinical decision to pursue costly adrenal function testing, the present study was designed to evaluate ultrasonographically measured adrenal gland thickness in a large population of dogs with non–adrenal gland illness. Specifically, the purpose of the study reported here was to determine whether body weight, age, or sex was associated with ultrasonographically determined adrenal gland thickness (specifically, thickness of the caudal pole of the adrenal gland) in dogs with non–adrenal gland illness. Thickness of the caudal pole of the adrenal gland was evaluated because in previous studies,2–4,14–16 this measurement was found to have the smallest interobserver variability attributable to anatomic differences between the right and left adrenal glands.
Materials and Methods
Study design and case selection criteria
The study was designed as a retrospective cross-sectional study. To identify dogs eligible for inclusion in the study, electronic databases of the Cornell University Hospital for Animals were searched for dogs ≥ 1 year of age that had undergone a comprehensive abdominal ultrasonographic examination between 2007 and 2013.
Dogs were eligible for inclusion in the study if any non–adrenal gland illness had been diagnosed, static longitudinal images of the caudal poles of both adrenal glands were available for review, and no nodules or masses were seen in either adrenal gland. Dogs were excluded if they were healthy, a diagnosis of adrenal gland disease had been made, or the history or pharmacy records documented administration of any steroid medication or herbal supplement at any time before abdominal ultrasonography. In addition, dogs suspected to have non–adrenal gland illness in which a definitive diagnosis had not been made were excluded if they had any historical, clinical, or clinicopathologic abnormalities suggestive of hyper- or hypoadrenocorticism. Specifically, features considered suggestive of hyperadrenocorticism in the absence of an alternative cause included polyuria, polydipsia, polyphagia, bilaterally symmetrical truncal alopecia, pot-belly appearance, hepatomegaly, erythrocytosis, stress leukogram, thrombocytosis, high serum alkaline phosphatase activity, hypercholesterolemia, and dilute urine.17 Features considered suggestive of hypoadrenocorticism in the absence of an alternative cause included vomiting, anorexia, lethargy, bradycardia, non-regenerative anemia, reverse stress leukogram, hyponatremia, hyperkalemia, hypercalcemia, and hypocholesterolemia.18
In selecting dogs for inclusion in the study, common breeds in the hospital population were prioritized in an attempt to evaluate dogs with similar body conformation within body weight categories. Dogs were arbitrarily stratified into 5 body weight categories (≤ 6 kg, > 6 to ≤ 12 kg, > 12 to ≤ 20 kg, > 20 to ≤ 30 kg, and > 30 kg [≤ 13.2 lb, > 13.2 to ≤ 26.4 lb, > 26.4 to ≤ 44 lb, > 44 to ≤ 66 lb, and > 66 lb]) and 3 age groups (< 4 years, 4 to 8 years, and > 8 years) within each body weight category. On the basis of subjective clinical observations of adrenal gland size differences among dogs in the various body weight categories, a priori power analysis predicted that 17 dogs would be needed in each age group to achieve a 90% power to reject the null hypothesis of no effect of age on adrenal gland thickness.
Data collection and evaluation
For dogs included in the study, medical records were searched to obtain information on signalment (ie, breed, sex, age, and body weight), history, results of clinicopathologic testing (eg, CBCs, serum biochemical analyses, and urinalyses), and definitive diagnoses. Specific hematologic test results that were recorded included Hct, RBC count, mean corpuscular volume, mean corpuscular hemoglobin concentration, total and differential WBC counts, and platelet count. Specific serum biochemical test results that were recorded included sodium, potassium, chloride, calcium, phosphorus, urea nitrogen, creatinine, glucose, total protein, albumin, globulin, cholesterol, and total bilirubin concentrations and alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, γ-glutamyltransferase, and creatine kinase activities. Specific urinalysis results that were recorded included specific gravity, results of tests for the presence of protein and glucose, and results of analysis of the urine sediment (ie, presence of cylindruria, crystalluria, pyuria, hematuria, or bacteriuria).
To verify final diagnoses, all cases were reviewed by an internal medicine resident (PLB) and 2 individuals (SAC and JFR) who were board certified by the American College of Veterinary Internal Medicine. Reviewers considered all clinical findings; results of all routine clinicopathologic testing; results of any disease-specific diagnostic testing, diagnostic imaging (eg, thoracic or abdominal radiography, esophageal or tracheal fluoroscopy, echocardiography, CT, MRI, or colorectal scintigraphy), cytologic and histopathologic examinations, endoscopic examinations (including cystoscopy and laparoscopy), exploratory surgical procedures, tests of endocrinologic function (eg, thyroid and adrenal gland function testing), bacteriologic culture of urine samples, Coombs testing, antinuclear antibody testing, serologic testing for infectious disorders, serum protein electrophoresis, coagulation testing, arterial blood gas analyses, and necropsy; and serum bile acids concentration, serum protein C activity, and urine protein-to-creatinine concentration ratio.
Ultrasonographic evaluations
All ultrasonographic examinations had been performed by individuals who were board certified by the American College of Veterinary Radiologists or by imaging residents trained in standard adrenal gland ultrasound technique by one of the authors (AEY). Abdominal ultrasonography was performed with an 8- or 5-MHz curved-array transducera with the dogs positioned in dorsal, oblique dorsal, right lateral, or left lateral recumbency depending on dog anatomy, dog temperament, and the location of intestinal gas interposed between the transducer and adrenal gland. Generally, the adrenal glands were imaged with the transducer positioned in the subcostal region. Angle of the transducer and positioning of the transducer toward or away from the ventral midline were varied as needed to obtain the optimal longitudinal image of each adrenal gland. Most measurements of adrenal gland thickness (ie, largest dimension of the caudal pole perpendicular to the longitudinal axis) were recorded at the time of the original ultrasound examination by sonographers who were not blinded to the patient's clinical information but were unaware of the present study. Any unrecorded measurements were finalized retrospectively on static images by a single observer (PLB) trained by one of the authors (AEY) for this purpose and blinded to patient signalment.
Statistical analysis
All data were evaluated by means of box-and-whisker plots, the Kolmogorov-Smirnov test, or histograms to determine whether they were normally distributed. Age was found to be normally distributed within body weight categories, with relatively equal numbers of males and females within each body weight category. However, relevant hematologic and serum biochemical data were found to have nonparametric distributions. Therefore, differences in clinicopathologic variables between dogs stratified by 3 age groups (all pairwise comparisons), 5 body weight categories (all pairwise comparisons), and sex (male vs female) were evaluated by use of the Wilcoxon rank-sum test. Bonferroni corrections were used to identify significant differences (ie, for the hematologic data, values of P < 0.006 were considered significant, and for the serum biochemical data, values of P < 0.003 were considered significant).
Data for LAT and RAT were analyzed separately. Values for LAT and RAT when dogs were stratified into the 5 body weight categories were inconsistently normally distributed. Therefore, the Pearson method was used to test for correlations between LAT and body weight and between RAT and body weight for all dogs combined, for dogs categorized into the initial 5 body weight categories, and for dogs categorized into 2 condensed body weight categories (≤ 12 kg and > 12 kg). Correlations were ranked as mild (r = 0 to 0.30), moderate (r = 0.30 to 0.70), or strong (r > 0.70). A general linear mixed model with possible interactions between independent variables was used to test whether age, body weight, and sex were associated with LAT and RAT. Linear regression was used to investigate potential relationship between LAT and age and between RAT and age for dogs in the 2 condensed body weight categories (≤ 12 kg and > 12 kg).
For dogs in the 2 condensed body weight categories, the lower (2.5th percentile) and upper (97.5th percentile) limits of the central 95% of the data for LAT and RAT were established. Subsequently, the Horn algorithm with Tukey interquartile fences was used to identify and eliminate outliers,19,20 and the 95% CIs for the upper and lower limits were determined by use of a bootstrap-based procedure21 to suggest upper and lower limits for detection of adrenomegaly in dogs with non–adrenal gland illness. Methods used to establish clinical reference intervals were compliant with guidelines established by the American Society for Veterinary Clinical Pathology for determination of reference intervals.22
All analyses were performed with standard software.b–d Unless otherwise stated, values of P < 0.05 were considered significant.
Results
A total of 266 dogs representing 12 breeds met the criteria for inclusion in the study (Table 1). Small dogs were represented by 3 breeds (Chihuahua, Maltese, and Yorkshire Terrier), medium dogs were represented by 6 breeds (American Cocker Spaniel, Australian Cattle Dog, Beagle, Border Collie, Pug, and West Highland White Terrier), and large dogs were represented by 3 breeds (Golden Retriever, Labrador Retriever, and Staffordshire Terrier). Of the 266 dogs, 57 weighed ≤ 6 kg, 61 weighed > 6 to ≤ 12 kg, 50 weighed > 12 to ≤ 20 kg, 52 weighed > 20 to ≤ 30 kg, and 46 weighed > 30 kg. Forty-four dogs were < 4 years old, 108 were 4 to 8 years old, and 114 were > 8 years old. There were 22 sexually intact and 119 castrated males and 19 sexually intact and 106 spayed females.
Left adrenal gland thickness and RAT (largest dimension of the caudal pole perpendicular to the longitudinal axis) measured ultrasonographically in 266 dogs with non–adrenal gland illness.
| Breed | No. of dogs | Age (y) | Weight (kg) | LAT (cm) | RAT (cm) |
|---|---|---|---|---|---|
| Chihuahua | 17 | 7.2 ± 3.7 | 4.3 ± 1.3 | 0.45 (0.30–0.62) | 0.43 (0.30–0.59) |
| Maltese | 17 | 8.3 ± 3.4 | 4.5 ± 1.9 | 0.40 (0.31–0.56) | 0.42 (0.28–0.54) |
| Yorkshire Terrier | 31 | 7.4 ± 3.3 | 3.0 ± 1.3 | 0.40 (0.22–0.61) | 0.42 (0.22–0.58) |
| Australian Cattle Dog | 3 | 7.0 ± 3.4 | 22.3 ± 2.7 | 0.50 (0.49–0.61) | 0.48 (0.33–0.53) |
| Beagle | 21 | 6.7 ± 2.8 | 15.1 ± 2.2 | 0.47 (0.30–0.70) | 0.47 (0.30–0.76) |
| Border Collie | 15 | 9.8 ± 3.4 | 21.1 ± 4.1 | 0.50 (0.38–0.60) | 0.47 (0.40–0.63) |
| American Cocker Spaniel | 36 | 8.5 ± 3.1 | 12.9 ± 3.3 | 0.48 (0.36–0.69) | 0.51 (0.34–0.69) |
| Pug | 32 | 7.5 ± 3.0 | 9.3 ± 1.9 | 0.39 (0.29–0.57) | 0.41 (0.30–0.64) |
| West Highland White Terrier | 6 | 5.5 ± 2.1 | 9.4 ± 1.6 | 0.45 (0.27–0.53) | 0.43 (0.36–0.65) |
| Golden Retriever | 39 | 6.6 ± 3.0 | 32.3 ± 5.3 | 0.49 (0.34–0.80) | 0.50 (0.39–0.68) |
| Labrador Retriever | 44 | 7.0 ± 3.3 | 32.6 ± 6.1 | 0.51 (0.28–0.71) | 0.53 (0.34–0.70) |
| Staffordshire Terrier | 5 | 3.0 ± 1.9 | 23.6 ± 2.4 | 0.54 (0.43–0.64) | 0.54 (0.40–0.62) |
Data are given as mean ± SD for age and weight and as median (range) for LAT and RAT.
Definitive diagnoses included neoplasia (n = 82); gastrointestinal, pancreatic, or esophageal disease (50), urinary tract disease (protein-losing nephropathy, chronic kidney disease, pyelonephritis, or urolithiasis; 26); immune-mediated disease (immune-mediated hemolytic anemia, immune-mediated thrombocytopenia, or vasculitis; 21); portosystemic vascular abnormality, microvascular dysplasia, or acquired portosystemic shunting (14); hepatitis, cholecystitis, or gallbladder mucocele (14); neurologic disease (13); endocrine disease (diabetes mellitus or hypothyroidism; 8); cardiovascular disease (7); respiratory disease (4); multiorgan disease (3); and orthopedic disease (2). An additional 22 dogs had miscellaneous diseases, including dermatologic disease (n = 3), ocular disease (2), unexplained high hepatic enzyme activities that resolved without treatment (2), lumbosacral pain (2), psychogenic polydipsia (2), fever (1), trauma (1), mesenteric cyst (1), septic peritonitis (1), splenic hematoma (1), hypercalcemia (1), hypoglycemia (1), hyperglobulinemia (1), lethargy (1), self-resolved regenerative anemia (1), and mushroom toxicosis (1). Disorders in the 3 dogs with multiorgan disease included diabetes mellitus, pancreatitis, and meningoencephalomyelitis (n = 1); thromboembolic disease and myocarditis (1); and seizures and sepsis (1).
No significant differences among groups were identified when results of hematologic testing, serum biochemical testing, and urinalysis were compared among dogs stratified into the 3 age groups, the 5 body weight categories, or the 2 sex categories, except that lymphocyte counts were significantly (P < 0.001) higher in dogs that were < 4 years old than in dogs that were 4 to 8 years old and in dogs that were > 8 years old.
For all dogs, regardless of body weight, mean ± SD LAT was 0.44 ± 0.09 cm in dogs < 4 years old, 0.45 ± 0.09 cm in dogs 4 to 8 years old, and 0.51 ± 0.10 cm in dogs > 8 years old (Table 2), and mean ± SD RAT was 0.46 ± 0.09 cm in dogs < 4 years old, 0.47 ± 0.09 cm in dogs 4 to 8 years old, and 0.49 ± 0.10 cm in dogs > 8 years old (Table 3). Mean LAT (P ≤ 0.001) and RAT (P = 0.04) in dogs that were > 8 years old were significantly higher than values for dogs that were < 4 years old. Mean LAT in dogs that were > 8 years old was significantly (P < 0.001) higher than mean LAT for dogs that were 4 to 8 years old, but mean RAT was not (P = 0.12). Mean LAT and RAT were not significantly (P = 0.29 and 0.22, respectively) different between dogs that were < 4 years old and dogs that were 4 to 8 years old.
Mean ± SD LAT (cm) for the dogs in Table 1 categorized according to body weight and age.
| Age (y) | |||
|---|---|---|---|
| Body weight (kg) | < 4 | 4 to 8 | » 8 |
| ≤ 6 | 0.36 ± 0.09 (10) | 0.42 ± 0.09 (25) | 0.43 ± 0.08 (22) |
| > 6 to ≤ 12 | 0.43 ± 0.05 (11) | 0.41 ± 0.08 (24) | 0.44 ± 0.08 (26) |
| > 12 to ≤ 20 | 0.46 ± 0.02 (5) | 0.46 ± 0.08 (18) | 0.54 ± 0.09 (27) |
| > 20 to ≤ 30 | 0.48 ± 0.10 (9) | 0.49 ± 0.10 (20) | 0.54 ± 0.08 (23) |
| > 30 | 0.47 ± 0.10 (9) | 0.49 ± 0.08 (21) | 0.61 ± 0.07 (16) |
| All dogs | 0.44 ± 0.09 (44) | 0.45 ± 0.09 (108) | 0.51 ± 0.10 (ll4) |
Numbers in parentheses represent number of dogs in each group.
Mean ± SD RAT (cm) for the dogs in Table 1 categorized according to body weight and age.
| Age (y) | |||
|---|---|---|---|
| Body weight (kg) | < 4 | 4 to 8 | > 8 |
| < 6 | 0.38 ± 0.05 (10) | 0.43 ± 0.08 (25) | 0.42 ± 0.10 (22) |
| > 6 to ≤ 12 | 0.47 ± 0.08 (ll) | 0.44 ± 0.08 (24) | 0.44 ± 0.08 (26) |
| > 12 to ≤ 20 | 0.46 ± 0.13 (5) | 0.47 ± 0.10 (l8) | 0.55 ± 0.11 (27) |
| > 20 to ≤ 30 | 0.49 ± 0.06 (9) | 0.52 ± 0.09 (20) | 0.52 ± 0.08 (23) |
| > 30 | 0.50 ± 0.08 (9) | 0.52 ± 0.09 (21) | 0.56 ± 0.06 (l6) |
| All dogs | 0.46 ± 0.09 (44) | 0.47 ± 0.09 (l08) | 0.49 ± 0.10 (114) |
Numbers in parentheses represent number of dogs in each group.
Histograms of LAT and RAT for dogs grouped on the basis of the 5 arbitrary body weight categories were constructed (Figure 1). When mean values and 95% CIs were evaluated for dogs grouped on the basis of the 5 body weight categories, there was a distinct difference in LAT, but not RAT, between dogs that weighed ≤ 12 kg and dogs that weighed > 12 kg (Figure 2). In addition, mean LAT and RAT did not differ significantly between dogs that weighed ≤ 6 kg and dogs that weighed > 6 but ≤ 12 kg, or among dogs that weighed > 12 but ≤ 20 kg, > 20 but ≤ 30 kg, and > 30 kg. Therefore, body weight categories were condensed into 2 categories (≤ 12 kg vs > 12 kg) for further analyses. Mean ± SD LAT and RAT were significantly (P < 0.001 and P < 0.001, respectively) smaller in dogs that weighed ≤ 12 kg (LAT, 0.42 ± 0.08 cm; RAT, 0.43 ± 0.08 cm) than in dogs that weighed > 12 kg (LAT, 0.51 ± 0.10 cm; RAT, 0.52 ± 0.09 cm).
Histograms of LAT and RAT (largest dimension of the caudal pole perpendicular to the longitudinal axis) measured ultrasonographically in 266 dogs with non-adrenal gland illness that weighed ≤ 6 kg (13.2 lb; n = 57; first row), > 6 to ≤ 12 kg (> 13.2 to ≤ 26.4 lb; 61; second row), > 12 to ≤ 20 kg (> 26.4 to ≤ 44 lb; 50; third row), > 20 to ≤ 30 kg (> 44 to ≤ 66 lb; 52; fourth row), or > 30 kg (66 lb; 46; fifth row). In each graph, the normal distribution is indicated by a red line.
Citation: Journal of the American Veterinary Medical Association 248, 6; 10.2460/javma.248.6.652
Mean LAT in the dogs in Figure 1 grouped according to body weight category (1 = ≤ 6 kg; 2 = > 6 to ≤ 12 kg; 3 = > 12 to ≤ 20 kg; 4 = > 20 to ≤ 30 kg; 5 = > 30 kg). Error bars represent 95% CI of the mean.
Citation: Journal of the American Veterinary Medical Association 248, 6; 10.2460/javma.248.6.652
Correlations between LAT and body weight and between RAT and body weight for all dogs combined, for dogs categorized into the initial 5 body weight categories, and for dogs categorized into the 2 condensed body weight categories were inconsistent (Table 4), with Pearson correlation coefficients generally higher for LAT versus body weight than for RAT versus body weight. None of the correlations between body weight and adrenal gland thickness for the initial 5 body weight categories were strong, 7 were moderate (5 for LAT and 2 for RAT), and 3 were weak (1 for LAT and 2 for RAT).
Correlation between body weight and LAT and between body weight and RAT for all dogs in Table 1 combined and for dogs categorized into the initial 5 body weight categories and into 2 condensed body weight categories (≤ 12 kg and > 12 kg).
| LAT | RAT | ||||
|---|---|---|---|---|---|
| Body weight (kg) | No. of dogs | Coefficient* | P value | Coefficient* | P value |
| ≤ 6 | 57 | 0.35 | 0.008 | 0.18 | 0.19 |
| > 6 to ≤ 12 | 61 | 0.10 | 0.44 | –0.18 | 0.15 |
| > 12 to ≤ 20 | 50 | 0.37 | < 0.01 | 0.30 | 0.04 |
| > 20 to ≤ 30 | 52 | 0.21 | 0.13 | 0.10 | 0.46 |
| > 30 | 46 | 0.64 | < 0.001 | 0.31 | 0.03 |
| < 12 | 118 | 0.19 | 0.04 | –0.06 | 0.53 |
| > 12 | 148 | 0.39 | < 0.001 | 0.21 | 0.02 |
| All dogs | 266 | 0.32 | < 0.001 | 0.14 | 0.03 |
Pearson correlation coefficient.
General linear mixed model analysis revealed significant associations between body weight and adrenal gland thickness (LAT, P < 0.001; RAT, P < 0.001), between age and adrenal gland thickness (LAT, P < 0.001; RAT, P = 0.009), and between sex and LAT (P = 0.02; Table 5). Both LAT and RAT significantly increased with body weight and age, and LAT was higher in males than in females. When analyses were repeated with values for dogs in the 2 condensed body weight categories considered separately, significant associations between LAT and age (P < 0.001) and between RAT and age (P = 0.01) were found for dogs that weighed > 12 kg; however, for dogs that weighed ≤ 12 kg, a significant (P = 0.01) association with age was found only for LAT. There were no significant differences in LAT or RAT between males and females when specific age groups were compared. Males weighing > 12 to ≤ 20 and > 20 to ≤ 30 kg had significantly (P < 0.03) larger adrenal gland thickness (both adrenal glands) and LAT, respectively, than did females.
Results of general linear mixed model analysis of whether body weight (kg), age (y), and sex (male vs female) were associated with LAT and RAT for the dogs in Table 1.
| Variable | Sum of squares | F ratio | P value |
|---|---|---|---|
| LAT | |||
| Body weight (kg) | 0.4990 | 68.51 | < 0.001 |
| Age (y) | 0.2306 | 31.66 | < 0.001 |
| Body weight × age | 0.0214 | 2.93 | 0.09 |
| Sex (male vs female) | 0.0384 | 5.27 | 0.02 |
| Body weight × sex | 0.0079 | 1.08 | 0.30 |
| Sex × age | 0.0006 | 0.08 | 0.77 |
| Body weight × sex × age | 0.0008 | 0.11 | 0.75 |
| RAT | |||
| Body weight (kg) | 0.5029 | 64.91 | < 0.001 |
| Age (y) | 0.0536 | 6.92 | 0.009 |
| Body weight × age | 0.0158 | 2.04 | 0.15 |
| Sex (male vs female) | 0.0001 | 0.01 | 0.90 |
| Body weight × sex | 0.0007 | 0.09 | 0.76 |
| Sex × age | 0.0015 | 0.19 | 0.66 |
| Body weight × sex × age | 0.0000 | 0.00 | 0.99 |
All possible interactions between independent variables were offered to the model.
After outliers were eliminated (Figure 3), 95% reference intervals with 95% CIs for the upper and lower limits of the reference intervals were calculated for LAT and RAT for dogs that weighed ≤ 12 kg and for dogs that weighed > 12 kg (Table 6).
Scatterplots of LAT (A) and RAT (B) for the dogs in Figure 1 grouped into 2 condensed body weight categories (≤ 12 kg and > 12 kg) and into 3 age categories within each body weight category. For each box plot, the solid line at the waist of the box represents the median, the notch indicates the 95% CI, the box represents the interquartile range (ie, 25th to 75th percentiles), and the whiskers extend to the furthest observation within 1.5 interquartile ranges of the 25th or 75th percentile. Outliers are denoted by an asterisk.
Citation: Journal of the American Veterinary Medical Association 248, 6; 10.2460/javma.248.6.652
The 95% reference limits and 95% CIs for the lower (2.5th percentile) and upper (97.5th percentile) reference limits for LAT and RAT (cm) for the dogs in Table 1 categorized on the basis of body weight (≤ 12 kg and > 12 kg) and age (< 4, 4 to 8, and > 8 years) within body weight category.
| Reference limit (cm) | |||||
|---|---|---|---|---|---|
| Adrenal | Body weight (kg) | Age (y) | No. of dogs | Lower (95% CI) | Upper (95% CI) |
| Left | |||||
| ≤ 12 | |||||
| < 4 | 21 | 0.22 (0.22–0.30) | 0.51 (0.48–0.51) | ||
| 4–8 | 49 | 0.30 (0.27–0.31) | 0.60 (0.56–0.62) | ||
| > 8 | 48 | 0.32 (0.30–0.35) | 0.60 (0.58–0.61) | ||
| All | 118 | 0.27 (0.22–0.30) | 0.60 (0.56–0.62) | ||
| < 4 | 22 | 0.30 (0.28–0.33) | 0.64 (0.60–0.64) | ||
| > 12 | |||||
| 4–8 | 59 | 0.34 (0.32–0.36) | 0.68 (0.64–0.71) | ||
| > 8 | 65 | 0.39 (0.38–0.41) | 0.70 (0.67–0.70) | ||
| All | 146 | 0.34 (0.28–0.36) | 0.70 (0.66–0.70) | ||
| Right | |||||
| < 12 | |||||
| < 4 | 20 | 0.30 (0.30–0.32) | 0.56 (0.49–0.56) | ||
| 4–8 | 47 | 0.29 (0.28–0.36) | 0.61 (0.55–0.64) | ||
| > 8 | 48 | 0.26 (0.23–0.30) | 0.58 (0.54–0.58) | ||
| All | 115 | 0.28 (0.23–0.30) | 0.58 (0.50–0.60) | ||
| > 12 | |||||
| < 4 | 23 | 0.39 (0.39–0.40) | 0.69 (0.60–0.69) | ||
| 4–8 | 59 | 0.33 (0.30–0.36) | 0.68 (0.65–0.70) | ||
| > 8 | 66 | 0.39 (0.34–0.40) | 0.72 (0.65–0.76) | ||
| All | 148 | 0.34 (0.30–0.39) | 0.69 (0.67–0.72) | ||
Discussion
Results of the present study suggested that in dogs with non–adrenal gland illness, body weight, age, and sex were significantly associated with adrenal gland thickness (measured ultrasonographically as the largest dimension of the caudal pole perpendicular to the longitudinal axis of the gland), indicating that these variables should be considered when evaluating adrenal gland thickness in dogs with non–adrenal gland illness and when developing reference intervals for dogs. Specifically, adrenal gland thickness was lower in dogs that weighed ≤ 12 kg than in dogs that weighed > 12 kg and, regardless of body weight, LAT increased with age. Both LAT and RAT were larger in male than in female dogs that weighed > 12 to ≤ 20 kg, and LAT was larger in male than in female dogs that weighed > 20 to ≤ 30 kg.
The paucity of data validating reference intervals for adrenal gland size in dogs of various ages and body weights with non–adrenal gland illness complicates ultrasonographic identification of adrenomegaly. In the present study, we found that LAT increased with advancing age and was significantly higher in dogs that weighed > 12 kg than in dogs that weighed ≤ 12 kg. These 2 observations argue against application of a single threshold for detection of adrenomegaly in dogs with non–adrenal gland disease. The largest dimension of the caudal pole perpendicular to the longitudinal axis of the gland was used in the present study because Barberet et al11 showed that this measurement had the lowest intra- and interobserver variability. Recently, this relationship was confirmed in another study14 involving healthy dogs.
In the present study, results of hematologic testing, serum biochemical testing, and urinalysis were compared among dogs stratified into 3 age groups, 5 body weight categories, and 2 sex categories to detect any pattern of clinicopathologic abnormalities that might have suggested increased or decreased adrenal gland function or the presence of specific diseases in any age or body weight group or either sex. The only significant difference identified was significantly higher lymphocyte counts in dogs that were < 4 years old than in dogs that were 4 to 8 years old or > 8 years old. Although it is well accepted that puppies have high lymphocyte counts that normalize with maturation,23 dogs < 1 year of age were excluded from the present study.
Controversy regarding reference intervals for healthy dogs and dogs with non–adrenal gland illness have limited the clinical utility of ultrasonographic measurement of adrenal gland size.1–4,8,11,13 Historically, in healthy dogs, the left adrenal gland was reported to range from 0.51 to 0.74 cm in size and the right adrenal gland was reported to range from 0.36 to 0.81 cm in size.2 In dogs with non–adrenal gland illness, the left adrenal gland was reported to range from 0.38 to 1.06 cm in size and the right adrenal gland was reported to range from 0.38 to 1.1 cm in size.2 However, these measurements often reflected the largest cranial or caudal pole measurement for each gland.2 Recently, published ranges for height of the caudal pole (measured in the longitudinal plane) were 0.28 to 0.54 cm for the left and 0.17 to 0.67 cm for the right adrenal gland in healthy Yorkshire Terriers (n = 24) and 0.32 to 0.79 cm for the left and 0.46 to 0.95 cm for the right adrenal gland in healthy Labrador Retrievers (17).1 Despite being obtained in healthy dogs, some of these ranges are more liberal than values obtained in the present study with larger numbers of Yorkshire Terriers (31) and Labrador Retrievers (44) with non–adrenal gland illness.
Measurements of adrenal gland thickness in the present study were similar to or smaller than dimensions previously reported for healthy dogs and dogs with non–adrenal gland illness.1–4 Our strict criteria to exclude dogs with possible hyperadrenocorticism might account for this difference. We propose that limits for LAT and RAT determined herein more accurately depicted adrenal gland size in dogs with non–adrenal gland illness likely to undergo abdominal ultrasonography.
There are conflicting conclusions in the literature regarding the influence of age and body weight on adrenal gland size in dogs.1–3,7,13 In 1 study,1 advancing age was associated with an increase in the height of the caudal pole of the adrenal gland in the longitudinal and transverse planes, but in another study,2 no relationship was found. In yet other studies,2,3,7 only length or width of the left adrenal gland had a weak positive association with age. In a study by Choi et al,13 no significant correlation was demonstrated between adrenal gland thickness and body weight in dogs that weighed < 10 kg. However, in a more recent study,1 significantly larger upper limits for adrenal gland thickness were reported for healthy Labrador Retrievers than for Yorkshire Terriers. In the present study, we detected significant associations of age and body weight with LAT and RAT by means of general linear mixed model analysis. Although no significant difference in adrenal gland thickness was found between dogs that weighed ≤ 6 kg and dogs that weighed > 6 kg but ≤ 12 kg or among dogs that weighed > 12 kg but ≤ 20 kg, > 20 kg but ≤ 30 kg, and > 30 kg, there was a distinct difference in LAT, but not RAT, between dogs that weighed ≤ 12 kg and dogs that weighed > 12 kg. Linear regression analysis indicated that adrenal gland thickness was significantly smaller in dogs that weighed ≤ 12 kg than in dogs that weighed > 12 kg.
Contradictory evidence also exists in the literature regarding the association between sex and adrenal gland size. In 1 study,1 sex had only a limited association with adrenal gland size, whereas in another study,7 adrenal gland thickness was larger in males and adrenal gland length was larger in females. However, the study by DeChalus et al1 had more females (27 sexually intact and 13 spayed) than males (12 sexually intact and 1 castrated), whereas the study by Mogicato et al7 had more males (84 sexually intact and 3 castrated) than females (50 sexually intact and 9 spayed). In the present study, sex distribution (19 sexually intact and 106 spayed females and 22 sexually intact and 119 castrated males) was more balanced. We found no significant difference in adrenal gland thickness between males and females when all body weight categories were combined. However, when individual body weight categories were analyzed, among dogs that weighed > 12 to ≤ 20 kg, both LAT and RAT were significantly larger in males than in females, and among dogs that weighed > 20 to ≤ 30 kg, LAT was significantly larger in males than in females. These findings were substantiated by results of our linear mixed model analysis, which found that sex was significantly associated with LAT.
The present study had some limitations inherent to any retrospective study. Most importantly, we could not be certain that all dogs with atypical hypoadrenocorticism were excluded, because cortisol concentrations measured during ACTH stimulation testing were not required as an inclusion criterion. Similarly, dogs with occult or early hyperadrenocorticism might have been included. To limit the possibility of erroneously including dogs with hyperadrenocorticism, we only selected dogs with a definitive diagnosis of non–adrenal gland disease and dogs in which clinical signs resolved with treatment and without documented relapses.
Intra- and interobserver variability in ultrasonographic measurements of adrenal gland thickness may have been a confounding factor in the present study. To minimize interobserver variability, a method documented to provide the most consistent measurements of adrenal gland thickness was used.11 Nevertheless, because ultrasonographic measurements of adrenal gland thickness are routinely obtained in clinical practice by primary care veterinarians with various degrees of training and experience, interobserver variability should be taken into consideration when applying limits for adrenal gland thickness obtained in this or any other study.
Finally, if breed-specific differences in adrenal gland size exist in dogs, then inclusion of specific breeds in the present study may have limited the applicability of our findings to dogs of other breeds and dogs of mixed breeding. Studies of adrenal gland thickness in breeds of dogs not included in the present study may provide further useful information.
In conclusion, our findings suggested that use of a single threshold for adrenal gland size in dogs regardless of body weight is inappropriate and may lead to under-recognition of adrenomegaly in small dogs. On the basis of the large population of dogs with non–adrenal gland illness in the present study, we concluded that regardless of age, dogs that weighed ≤ 12 kg should have an adrenal gland thickness no greater than 0.62 cm whereas dogs that weighed > 12 kg should have an adrenal gland thickness no greater than 0.72 cm. Additional studies are necessary to further qualify these findings and to contrast these limits with measurements of adrenal gland thickness in dogs with hyperadrenocorticism.
Acknowledgments
No third-party funding or support was received in connection with this study or the writing or publication of the manuscript.
Presented as a poster at the American College of Veterinary Internal Medicine Forum, Nashville, Tenn, June 2014.
ABBREVIATIONS
| CI | Confidence interval |
| LAT | Left adrenal gland thickness |
| RAT | Right adrenal gland thickness |
Footnotes
Phillips IU22 and Phillips HD5000, Phillips, Andover, Mass.
Statistix 9, version 9.0, Analytical Software, Tallahassee, Fla.
JMP, version 11 for Windows, SAS Institute Inc, Cary, NC.
Analyse-It Add-In for Microsoft Excel 3.90.6, Analyse-it Software Ltd, Leeds, West Yorkshire, England.
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