RADIOLOGY CORNER-ANAL SAC GAS APPEARING AS AN OSTEOLYTIC PELVIC LESION

RUTH DENNIS, MRCVS, JACQUES PENDERIS, MVM, MRCVS Veterinurji Radiologj~ & Ultrasound, Vol. 43, No. 6, 2002, p p 552-553.

HE ANAL SACS NORMALLY contain a viscous, putrescent T material used in scent marking. The paired sacs lie on either side of the anal canal between the internal and exter- nal anal sphincter muscles, opening onto the lateral margin of the anus by a single duct.’ The sacs act as reservoirs for secretions from apocrine and sebaceous glands and, in the dog, are normally spherical and approximately 1 cm in di- ameter. The anal sacs occasionally contain gas rather than liquid or paste-like material, and this may be visible radio- graphically, especially on a ventrodorsal projection of the area. If superimposed over the ischium, a gas-filled anal sac may mimic an osteolytic lesion.

An 8-year-old male cocker spaniel suffered from progres- sive lumbar pain for 6 weeks with recent severe deteriora- tion and a right pelvic limb lameness. In radiographs sub- mitted by the referring veterinarian, which had been ob- tained 8 days previously, there appeared to be bilateral, well-circumscribed osteolytic lesions in the ischium, larger on the right side. In radiographs performed under general anesthesia at the time of referral, there was a 15-mm diam- eter right-sided radiolucent region in or over the ischium, but the one on the left was no longer visible (Fig. 1). Be- cause a bone lesion in this location did not explain the dog’s clinical signs, anal sac gas was suspected, and further ra- diographs were made to confirm this before proceeding to myelography. An oblique ventrodorsal radiograph tilted so that the right side of the pelvis was closer to the table caused the lucent area to be displaced to the opposite side, indicat- ing that it was dorsal to the pelvis (Fig. 2). In a slightly underexposed lateral radiograph examined using a bright light, the anal location of the radiolucency was confirmed. Following manual expression of the gland, the radiolucent region was no longer visible.

To establish the incidence of anal sac gas in dogs, one of the authors (RD) assessed ventrodorsal pelvic radiographs for the presence of gas while reading hip radiographs for the British Veterinary Association’s Hip Dysplasia Scoring Scheme. This population consists mostly of intact dogs of

~~

From the Centre for Small Animal Studies, Animal Health Trust, Lan- wades Park, Kentford, Newmarket, Suffolk, UK.

Address correxpondence and reprint requests to Mrs. R. Dennis, the Centre for Small Animal Studies, Animal Health Trust, Lanwades Park, Kentford. Newmarket, Suffolk CB8 7UU, UK.

Received March 4, 2002.

medium and larger breeds, and radiography has usually been performed under sedation or general anesthesia to avoid the need for manual restraint, which is strongly dis- couraged in the U.K. Of 2,137 radiographs, anal sac gas was seen in 4.9% of females and 6.2% of males. An erroneous impression of increased incidence in females was because three times as many females were assessed under the Scheme. Anal sac gas was most often unilateral (4.0% of females and 1.7% of males) but was occasionally bilateral (0.9% of females and 1.7% of males) (Fig. 3). If bilateral, the two sacs were often unequal in size; overall, the diam- eter seen ranged from 3-15 mm. Tail docking had no no- ticeable effect on the incidence of anal sac gas.

The location of the sacs varied markedly both in a cra- niocaudal and mediolateral direction, presumably because of slight anatomic variation as well as differences in posi- tioning and centering between radiographs; more cranial centering of the x-ray beam will result in a more cranial

FIG. 1 . Ventrodorsal pelvic radiograph of an 8-year-old cocker spaniel, showing a round, well-defined radiolucent area IS miii in diameter within or superimposed over the right ischium (arrow).

552

VOL. 43, No. 6 ANAL SACS 553

FIG. 3. Ventrodorsal pelvic radiograph of a 6-year-old male golden re- triever showing hilaterdl anal sac gas.

FIG. 2. Left ventral-right dorsal oblique pelvic radiograph of the same dog as in Fig. 1 , showing displacement of the radiolncent structure to the left sidc of the pubic xymphysis (arrow), indicating that it lies dorsal to the bone.

radiographic location of the anal sacs relative to the pelvis. In most animals, the gas was partly or completely superim- posed over the ischial bone, but a more caudal or cranial location was not unusual, and in one dog, the anal sac was close to the hip joint (Fig. 4). Although anal sac gas can be mimicked by the thin bony plate of the ischiatic table and by fecal gas, these were usually easily recognized.

Anal sac gas can clearly be persistent or recurrent in the same gland, as shown by this example in which an identical appearance to the right gland was seen on radiographs made 8 days apart. It may also arise or resolve quickly, because one dog in the hip dysplasia study (a Tibetan spaniel) had anal sac gas in only one of two submitted radiographs, although they had been obtained during the same procedure. Anal sacs have also been seen to fill with barium during a barium enema study (personal communication, Professor Martin Sullivan, University of Glasgow Veterinary School).

Although the clinical significance of anal sac gas remains unknown, it is important to recognize it to prevent an erro- neous diagnosis of a discrete, osteolytic lesion, which could have important implications for the patient.

REFERENCE I. Al-Bagdacli F. The integument. In: Evans HE (ed): Miller’s Anatomy FIG. 4. Schematic representation of the variable location of anal sac gas

of the Dog. 3rd Ed. Philadelphia, WB Saunders Co., 1993, 115. in the dog.