Multidetector CT findings of splenic artery aneurysm in children with chronic liver disease

BACKGROUND

Splenic artery aneurysm (SAA) is a well-known complication of chronic liver disease and portal hypertension in adults. The incidence of SAA in children undergoing selective hepatic angiography prior to liver transplantation is reported as 4%, but there are few systematic studies.

OBJECTIVE

To investigate the SAAs detected by multidetector CT angiography (MDCTA) among children with chronic liver disease.

MATERIALS AND METHODS

A total of 124 children (71 girls, 53 boys; mean age 118 months; age range 5 days to 204 months) with chronic liver disease underwent MDCTA to display the vascular anatomy and any vascular complications during the pretransplantation period. Of these children, 23 also underwent coeliac angiography. The digital subtraction angiography (DSA) and MDCTA findings were compared.

RESULTS

SAAs were detected in 13 children (10.4%); none was detectable by US. All patients had more than one aneurysm; ten patients had more than three. In all except one patient, the SAAs were located only in the intraparenchymal branches of the splenic artery; in one patient they were located in the intraparenchymal segment and in the distal third of the splenic artery. The mean size of the aneurysms was 6.5 mm (range 2.5-18 mm). All patients with aneurysms had splenomegaly and vascular collaterals. Nine of the children with SAAs had portal vein pathologies (two occlusions, two stenoses, five dilatations). A statistically significant difference existed with regard to the size of spleen (P < 0.05) and patient age (P < 0.05) between children with SAAs and children without SAAs. There was an increased risk of SAAs in patients with portal vein pathologies. In 19 patients without SAAs on MDCTA, no SAAs were seen on DSA.

CONCLUSIONS

It is likely that the incidence of SAA in children with chronic liver disease will increase with improved survival of children with long-standing portal hypertension and chronic liver disease. MDCTA with multiplanar reconstruction is a noninvasive and effective means of imaging paediatric patients with SAAs, especially during the peritransplantation period, which is considered to be a time of significant risk for SAA rupture in this patient population.

Endovascular therapy for thoracic and abdominal aortic emergencies

BACKGROUND

Emergent situations of both the thoracic and abdominal aortae are serious life-threatening situations. Endovascular stent graft repair offers an alternative to conventional operation for management of aortic diseases. Our aim was to report our experience with endovascular stent graft repair of thoracic and abdominal aortic emergencies.

METHODS

Between April 2004 and March 2007, endovascular stent graft repair was performed in 16 patients (13 males, 3 females; mean age 61.4+/-16.3; range 38 to 86 years). The emergent lesions were in the thoracic aorta in 8 patients and abdominal aorta in the remainder. The deployed stent graft systems were Talent-Medtronic (n=8) and Excluder-Gore (n=8).

RESULTS

Successful deployment of the stent grafts in the appropriate position was achieved in all patients. Hospital mortality occurred in 3 patients. There was no conversion to open surgery. The total number of endoleaks was 3 (18.75%).

CONCLUSION

Endovascular stent graft placement is a feasible and effective approach in the emergency treatment of patients with complicated emergent pathologies of both thoracic and abdominal aortae.

Endovascular stent placement in patients with hepatic artery stenoses or thromboses after liver transplant

Hepatic artery stenosis or thrombosis following liver transplant is a potentially life-threatening complication. Successful liver transplant depends on uncompromised hepatic arterial inflow. Early diagnosis and treatment of complications prolong graft survival. Interventional radiologic techniques are frequently used to treat hepatic artery complications. Twenty patients with hepatic artery stenoses (n = 11) or thromboses (n = 9) were included in this study. Eighteen of the 20 patients were successfully treated by stent placement. In 9 patients, early endovascular interventions were performed 1 to 7 days after surgery. Two patients were operated owing to the effects of dissection and bleeding from the hepatic artery. Repeat endovascular interventions were performed 10 times in 6 patients. Follow-up ranged from 5 months to 4.5 years. Nine patients with patent hepatic arteries died during follow-up owing to reasons unrelated to the hepatic artery interventions. In 3 patients, the stents became occluded at 3, 5, and 9 months after surgery but no clinical symptoms were present.

CT-guided cutting needle lung biopsy using modified coaxial technique: factors effecting risk of complications

PURPOSE

We present our 7-year experience with coaxial computed tomography (CT)-guided cutting needle lung biopsy and evaluate the factors affecting risk of complications.

MATERIAL AND METHOD

Between June 2000 and March 2007, we performed 225 CT-guided coaxial lung biopsies in 213 consecutive patients (161 men, 52 women). Lesion size, lesion depth, lesion location, needle-pleural angle, presence of pleural effusion, patient's position, and complications secondary to biopsy procedure (pneumothorax and bleeding) were noted. Pneumothorax was graded as mild, moderate, and severe. Bleeding complications were graded as mild, moderate, and severe.

RESULTS

Two hundred twenty-five biopsy procedures were performed in 213 patients. The mean diameter of the lung lesion was 41.3+/-20.1mm. The mean distance from the peripheral margin of the lesion to the pleura was 17.3+/-19.2mm. After 225 procedures, there were 42 mild (18.6%), 13 moderate (5.7%), and 4 severe (1.7%) pneumothoraxes. Small hemoptysis occurred in 27 patients (12%), and mild parenchymal hemorrhage occurred in 2 patients (0.8%). The overall complication rate was 39.1%. Although, a statistically significant correlation was found between female sex, presence of emphysema, lesion depth, and pneumothorax, none of these factors had a predictive value for pneumothorax. Although, statistically significant correlations were found between female sex, lesion size, and bleeding, only lesion size had a predictive value for bleeding.

CONCLUSION

The most frequent and important complications of this procedure are pneumothorax and bleeding. But any factor is the predictor of pneumothorax and lesion depth is a poor predictor of bleeding complication.

Perforated ileal diverticulitis: CT findings

Diverticulosis of the ileum is very uncommon. The differential diagnosis for any inflammatory process in the right lower quadrant of the abdomen includes appendicitis, Crohn's disease, and other infectious and inflammatory conditions of the terminal ileum and cecum. Diagnosis of small bowel diverticulitis is based on radiological findings, and computed tomography is the method of choice to demonstrate mural changes and the mesenteric extent of the inflammation. Preoperative knowledge of this condition may influence surgical and medical management.

Percutaneous management of anastomotic bile leaks following liver transplantation

PURPOSE

To review our experience with percutaneous internal-external biliary drainage in treating biliary anastomotic leaks following orthotopic liver transplantation.

MATERIALS AND METHODS

Between September 1997 and June 2006, 157 liver transplantations were performed in our hospital. Percutaneous transhepatic biliary drainage was performed in 10 patients (9 males, 1 female; mean age, 32.9 years; age range, 2-62 years) with patent hepatic arterial systems to treat clinically significant anastomotic bile leaks.

RESULTS

Bile leaks were resolved and anastomotic patency was restored in all patients. Massive hemobilia occurred in 1 patient due to arterial pseudoaneurysm and was treated with embolization. No major complications were seen in the other patients. During a mean follow-up of 19.5 months, anastomotic stricture occurred in 2 patients (1 in combination with a recurrent leak). Both patients were successfully treated with percutaneous methods. The remaining 8 patients had no biliary problems.

CONCLUSION

When treating anastomotic bile leaks in liver transplant patients, percutaneous procedures may be performed with high technical success and low complication rates.

Endovascular treatment of active bleeding after liver transplant

OBJECTIVES

To evaluate the incidence of active bleeding complications following transplant and the efficacy of interventional radiologic management.

MATERIALS AND METHODS

Between June 2000 and February 2007, 14 liver transplant patients with active bleeding were treated via endovascular techniques (coils, glue, or graft-covered stents). Active bleeding was spontaneous in 6 patients through the inferior epigastric artery (n=1), the inferior phrenic artery (n=1), the superior mesenteric artery (n=2), the internal mammary artery (n=1), and the hepatic artery (n=1). In 8 patients, active bleeding was due to transhepatic biliary or endovascular interventions. Hemobilia (n=2) due to pseudoaneurysm formation after transhepatic biliary interventions was embolized with coils. Hepatic artery rupture was observed in 6 patients during endovascular interventions performed on hepatic artery stenosis or thrombosis that had been treated with graft-covered stents. Technical success, clinical improvement, and complications were documented.

RESULTS

Active bleeding was stopped by endovascular intervention in 13 of 14 patients. Embolizations with coils or glue were successful. In 1 patient with hepatic artery bleeding, the graft-covered stent failed to seal the rupture site, and this patient underwent reoperation.

CONCLUSION

Arterial bleeding complications after liver transplant during the early and late postoperative period, due either to spontaneous active bleeding or to percutaneous or endovascular interventions, can be successfully managed with interventional radiologic techniques.

Percutaneous therapy of ureteral obstructions and leak after renal transplantation: long-term results

The purpose of this study was to evaluate the long-term outcome of percutaneous therapy of ureteral complications after renal transplantation. Between January 2000 and June 2006 we percutaneously treated 26 renal transplant patients with ureteral obstruction (n=19) and leak (n=7). Obstructions were classified as early (<2 months after transplantation) or late (>2 months). Patients with leak were treated with nephro-ureteral catheter placement and subsequent double-J stenting. Balloon dilatation, stent placement, and basket extraction were used to treat ureteral obstructions. Patients were followed with ultrasonography. No major procedure-related complication occurred. The mean follow-up time was 34.3 months (range: 6 to 74 months). Initial clinical success was achieved in all 19 patients with obstruction and 6 of 7 patients with leak. Four of 9 early obstructions and 4 of 10 late obstructions recurred during the follow-up. All recurrences were initially managed again with percutaneous methods, including cutting balloon technique and metallic stent placement. Although there was no recurrence in patients with successfully treated leak, stricture was seen at the previous leak site in two patients. These strictures were also successfully managed percutaneously. We conclude that in the treatment of ureteral obstruction and leak following renal transplantation, percutaneous therapy is an effective alternative to surgery. However, further interventions are usually needed to maintain long-term patency.

Multidetector Computed Tomographic Angiography Findings of Splenic Artery Steal Syndrome in Liver Transplantation

AIMS

Splenic artery steal syndrome, a common complication in liver transplantation, is diagnosed by conventional angiography showing an enlarged splenic artery and by dynamic findings. The aim of this study was to determine multidetector computed tomographic angiography (MDCTA) findings of splenic artery steal syndrome to develop diagnostic criteria.

MATERIALS AND METHODS

Ten patients were diagnosed as displaying splenic artery steal syndrome among 198 liver transplant patients. The diagnosis was confirmed by celiac angiography. In eight of them, MDCTA was performed. Axial and coronal maximum-intensity projection images were obtained in arterial and portal phases. We measured the diameter of the celiac trunk and of the splenic, left gastric, common hepatic, superior mesenteric artery, and transplant hepatic arteries. We also measured the diameter of the proximal and the distal segments of the abdominal aorta, along with the size of the spleen, the ratio of the splenic artery to the common hepatic artery, the ratio of splenic artery to transplant hepatic artery, the diameter of portal vein and superior mesenteric vein. The control group consisted of liver transplant patients with normal liver enzyme levels. We performed Student t test for statistical examination.

RESULTS

The diameter of the splenic artery (P<.05), the size of the spleen (P<.01), and the ratio of the splenic to the transplant hepatic arteries (P<.05) was significant between the two groups. The diameter of the splenic artery was larger than 4 mm in all patients in the study group.

CONCLUSIONS

Conventional angiography was mandatory for the diagnosis of splenic artery steal syndrome. MDCTA is a noninvasive method. Some computed tomography criteria are important for early diagnosis and treatment.

Transjugular Intrahepatic Portosystemic Shunt Creation in Budd-Chiari Syndrome: Percutaneous Ultrasound-Guided Direct Simultaneous Puncture of the Portal Vein and Vena Cava

Budd-Chiari syndrome (BCS) is an uncommon disorder that can be life-threatening, depending on the degree of hepatic venous outflow obstruction. Transjugular intrahepatic portosystemic shunt (TIPS) provides decompression of the congested liver but the hepatic vein obstruction makes the procedure more difficult. We describe a modified method that involved a single percutaneous puncture of the portal vein and inferior vena cava simultaneously for TIPS creation in a patient with BCS.

Use of the rendezvous technique in the treatment of biliary anastomotic disruption in a liver transplant recipient

In liver transplant recipients, the treatment of a biliary leak resulting from anastomotic disruption usually requires surgical intervention. However, reoperation results in significant morbidity in such patients, whose clinical status may preclude a second surgery. Restoring the integrity of a disrupted biliary anastomosis can be difficult, and in some patients with that complication, neither the percutaneous technique nor the endoscopic approach effectively stents the biliary anastomosis. In those individuals, bile drainage can be achieved by means of a combined percutaneous transhepatic and endoscopic transpapillary approach (the rendezvous technique). In conclusion, we present the case report of a liver transplant recipient with an anastomotic bile leak that was successfully treated with the rendezvous technique.

Nonvascular complications in pediatric liver recipients: multidetector computed tomography evaluation

In pediatric liver transplantation postoperative diagnosis of complications is crucial for graft salvage. Multidetector computed tomography (MDCT) is a technique to evaluate complications. In this study we present nonvascular abdominal complications encountered in pediatric recipients after liver transplantation. We retrospectively examined 113 MDCT examinations in 43 pediatric patients who underwent liver transplantation between 1997 and 2005. Computed tomography (CT) examinations were made by a 16-detector multislice CT scanner. The pathological findings on CT images were: intraperitoneal free fluid, intrahepatic bile duct dilatation, graft liver infarction, perihepatic and intraperitoneal fluid collections (six biloma), colonic and/or intestinal dilatation, splenic infarction, perihepatic hematoma, right adrenal hemorrhage, perihepatic abscess, incisional hernia, intrahepatic biloma and periportal collar. In one patient intestinal hemorrhage was suspected. Intestinal perforation was suspected in three patients. Among these three patients, one patient died before any surgical intervention. In two patients the diagnosis was confirmed at surgery. In pediatric patients, the short examination time, brief sedation duration, and high-resolution images make MDCT an effective radiological method to evaluate nonvascular transplant complications.

Ultrasound-guided percutaneous cholecystostomy for acute cholecystitis in critically ill patients: one center's experience

BACKGROUND/AIMS

The standard treatment for acute cholecystitis is cholecystectomy; however, cholecystectomy is not an option in some patients who are too high-risk for emergency surgery. Ultrasound-guided percutaneous cholecystostomy is an alternative for such patients. This study presents one center's five-year clinical experience with ultrasound-guided percutaneous cholecystostomy for treatment of acute cholecystitis.

METHODS

In this study the records of all patients (18 total; mean age, 68.2+/-15.4 years; range, 42-91 years) who underwent ultrasound-guided percutaneous cholecystostomy for acute cholecystitis between June 1998 and October 2003 were reviewed. Duration of hospitalization, duration of tube placement, mortality and morbidity after tube placement, complication rates, culture results for aspirated bile, and clinical outcomes were analyzed.

RESULTS

Fourteen patients were diagnosed with acute calculous cholecystitis and four were diagnosed with acalculous cholecystitis. The average hospital stay was 19+/-12.6 days (range, 5-52 days), and the average duration of catheter drainage was 20.5+/-19.1 days (range, 1-75 days). Six patients underwent open cholecystectomy between days 16 and 26 of catheter drainage, and none had postoperative complications.

CONCLUSIONS

Ultrasound-guided percutaneous cholecystostomy is a relatively safe and easy method for treating acute cholecystitis in critically ill patients. The risk of complications is low and the likelihood of success is high.

The retinal ganglion cell layer and visual acuity of the camel

We examined the retinal ganglion cell layer of the dromedary camel, Camelus dromedarius. We have estimated that there are 8 million neurons in the ganglion cell layer of this large retina (mean area of 2,300 mm(-2)). However, only approximately 1 million are considered to be ganglion cells. The ganglion cells are arranged as two areas of high cell density, one in the temporal and one in the nasal retina. Densities of ganglion cells between these two high density regions is much lower, often less than 100 per mm(-2). In between these two high density regions, on the nasal side of the optic nerve head, is a unique and dense vertical streak of mostly non-ganglion cells; the function of this specialization is unknown. On the basis of ganglion cell density we estimate that the peak acuity in the dromedary camel is about 10 and 9.5 cycles per degree in the temporal and nasal high density regions respectively and falls to 2-3 cycles per degree in the central retina. Behavioral acuity was estimated for one bactrian camel and was found to be approximately 10 cyc deg(-1). The camel has a retina with a mean thickness of 104 microm, less than the 143 microm thickness that has previously been thought to be necessary for a retinal vasculature. Nevertheless, there is an extensive vitreal vasculature that does not appear to spare any retinal region.

Myctolaimellus robiniae n. sp. (Diplogasterida: Cylindrocorporidae) from Larval Cavities of the Locust Borer, Megacyllene robiniae Forster

A new nematode species of the family Cylindrocorporidae and the genus Myctolaimellus from subcortical cavities made by the locust borer (Megacyllene robiniae Forster) in black locust (Robinia pseudoacacia L.) is described. Males of the new species have a length of 700 to 1,050 microm; a bursate tail, peloderan with nine pairs of rays; and knobbed, curved spicules with tips bending gently into a hook. The distinctive gubernaculum is half the length of the spicules, deeply grooved longitudinally along both its dorsal and ventral surfaces, and has a spoon-shaped end. Females have a length of 830 to 1,340 microm, an amphidelphic reproductive tract with long ovaries crossing each other to extend beyond the equatorial vulva, and a gradually tapering tail.

Neuronal density in the human retinal ganglion cell layer from 16-77 years

Literature assessing whether or not neurons (retinal ganglion cells and displaced amacrine cells) are lost from the retinal ganglion cell layer in mammals with age is still controversial, some studies finding a decrease in cell density and others not. To date there have been no studies estimating the total number of neurons in the retinal ganglion cell layer of humans throughout life. Recent studies have concentrated on the macular region and examined cell densities, which are reported to decrease during aging. In a study of the human retinal pigment epithelium (RPE), we showed that, while RPE cell number does not change, cell density increases significantly in central temporal retina (macular region) as the retina ages. We speculated that the increase in density represents a "drawing together" of the retinal sheet to maintain high cell densities, in this region of the neural retina, in the face of presumed cell loss from the ganglion cell layer due to aging. Here, therefore, we have sampled the entire ganglion cell layer of the human retina and estimated total neuron numbers in 12 retinae aged from 16 to 77 years. Human retinae, fixed in formalin, were obtained from the Queensland Eye Bank and whole-mounted, ganglion cell layer uppermost. The total number of neurons was lower in the older than younger retinae and neuronal density was lower in most retinal regions in older retinae. Retinal area increased with age and neuronal density fell throughout the retina with a mean reduction of 0.53% per year. However, the percentage reduction in density was much lower for the macular region, with a value of 0.29% per year. It is possible that this lesser reduction in cell density in the macula is a result of the drawing together of the retinal sheet in this region as we speculated from RPE data.

First evidence of diversity in eutherian chiasmatic architecture: tree shrews, like marsupials, have spatially segregated crossed and uncrossed chiasmatic pathways

In the optic chiasm of mammals, axons either cross the midline to the opposite side of the brain or remain uncrossed. In the eutherian species studied to date, uncrossed axons in the caudal nerve are found in all regions. In the chiasm, they are dispersed through the hemichiasm, with many axons approaching the midline and then turning back to enter the same side of the brain as the originating eye. In marsupials, by contrast, uncrossed axons never approach the midline; instead, they remain grouped in the lateral nerve and chiasm. The impression gained from these data is that there is a major difference in chiasmatic architecture between eutherian and marsupial mammals. Therefore, the mechanisms by which axons choose their route through the chiasm was also thought to differ between the two major groups of mammals. However, the present study shows that the chiasm of a highly visual eutherian mammal, the tree shrew, is similar to that found in marsupials, with uncrossed axons confined to lateral regions and not approaching the midline. However, unlike marsupials, in the tree shrew, optic fascicles in the chiasm are often separated by thick collagen bundles. It is probable that the chiasmatic structure described to date for eutherian mammals is not ubiquitous, as was previously thought, and theories explaining the mechanisms by which axons chose their route through the chiasm during development will have to be expanded.

Spermiogenesis and spermiation in a marsupial, the tammar wallaby (Macropus eugenii)

Fourteen steps of spermatid development in the tammar wallaby (Macropus eugenii), from the newly formed spermatid to the release of the spermatozoon into the lumen of the seminiferous tubules, were recognised at the ultrastructural level using transmission and scanning electron microscopy. This study confirmed that although the main events are generally similar, the process of the differentiation of the spermatid in marsupials is notably different and relatively more complex than that in most studied eutherian mammals and birds. For example, the sperm head rotated twice in the late stage of spermiogenesis: the shape of the spermatid changed from a T-shape at step 10 into a streamlined shape in step 14, and then back to T-shape in the testicular spermatozoa. Some unique figures occurring during the spermiogenesis in other marsupial species, such as the presence of Sertoli cell spurs, the nuclear ring and the subacrosomal space, were also found in the tammar wallaby. However, an important new finding of this study was the development of the postacrosome complex (PAC), a special structure that was first evident as a line of electron dense material on the nuclear membrane of the step 7 spermatid. Subsequently it became a discontinuous line of electron particles, and migrated from the ventral side of the nucleus to the area just behind the posterior end of the acrosome, which was closely located to the sperm-egg fusion site proposed for Monodelphis domestica (Taggart et al. 1993). The PAC and its possible role in both American and Australian marsupials requires detailed examination. Distinct immature features were discovered in the wallaby testicular spermatozoa. A scoop shape of the acrosome was found on the testicular spermatozoa of the tammar wallaby, which was completely different to the compact button shape of acrosome in ejaculated spermatozoa. The fibre network found beneath the cytoplasm membrane of the midpiece of the ejaculated sperm also did not occur in the testicular spermatozoa, although the structure of the principle piece was fully formed and had no obvious morphological difference from that of the epididymal and ejaculated spermatozoa. The time frame of the formation of morphologically mature spermatozoa in the epididymis of the tammar wallaby needs to be determined by further studies.

Tropospheric ozone: respiratory effects and Australian air quality goals

OBJECTIVE

To review the health effects of tropospheric ozone and discuss the implications for public health policy.

DESIGN

Literature review and consultation with scientists in Australia and overseas. Papers in English or with English language abstracts were identified by Medline search from the international peer reviewed published reports. Those from the period 1980-93 were read systematically but selected earlier papers were also considered. Reports on ozone exposures were obtained from environmental agencies in the region.

RESULTS

Exposure to ozone at concentrations below the current Australian air quality goal (0.12 ppm averaged over one hour) may cause impaired respiratory function. Inflammatory changes in the small airways and respiratory symptoms result from moderate to heavy exercise in the presence of ozone at levels of 0.08-0.12 ppm. The changes in respiratory function due to ozone are short lived, vary with the duration of exposure, may be modified by levels of other pollutants (such as sulphur dioxide and particulates), and differ appreciably between individuals. Bronchial lavage studies indicate that inflammation and other pathological changes may occur in the airways before reductions in air flow are detectable, and persist after respiratory function has returned to normal. It is not known whether exposures to ozone at low levels (0.08-0.12 ppm) cause lasting damage to the lung or, if such damage does occur, whether it is functionally significant. At present, it is not possible to identify confidently population subgroups with heightened susceptibility to ozone. People with asthma may be more susceptible to the effects of ozone than the general population but the evidence is not consistent. Recent reports suggest that ozone increases airway reactivity on subsequent challenge with allergens and other irritants. Animal studies are consistent with the findings in human populations.

CONCLUSION

A new one hour air quality ozone goal of 0.08 ppm for Australia, and the introduction of a four hour goal of 0.06 ppm are recommended on health grounds.

Interactions of cimetidine and ranitidine with aluminum-containing antacids and a clay-containing gastric-protective drug in an "artificial stomach-duodenum" model

Interactions of cimetidine and ranitidine with aluminum-containing antacids and clay-containing gastric-protective drugs were analyzed in vitro by using an artificial stomach-duodenum model. The model reproduced near-physiologic conditions, taking into account gastric and duodenal flux variations and interactions between gastric mucosa and drugs added to the gastric content. Clay bound cimetidine in acid medium, but the drug was released when the pH increased, resulting in cimetidine amounts in the duodenal site close to those in controls. In contrast, clay bound ranitidine in acid medium and did not release it in the duodenal site. Aluminum-containing antacids did not significantly modify the amount of cimetidine or ranitidine available for absorption. Several factors play a role in the interactions of cimetidine and ranitidine with aluminum-containing antacids and clay-containing gastric-protective drugs: the structure of the antisecretory drugs, gastroduodenal pH, interactions of the antacid and clay with the gastric mucosa, and release of aluminum that could adsorb the drugs or prevent their adsorption by the mucosa. These phenomena are intricate and difficult to analyze without using a physicochemical approach.

Horizontal cells in the retina of the brush-tailed possum

Most species of eutherian (placental) mammals examined have two types of horizontal cell, one is axonless and the other has a short axon. We have recently shown that a marsupial, the quokka wallaby, also has two types of horizontal cell and that the axonless cell in this species has unusual stubby processes that pass through the inner nuclear layer to reach the inner plexiform layer. In order to discover whether these descending processes are a feature of marsupials in general, I examined the morphology of retinal horizontal cells in the brush-tailed possum, using horseradish peroxidase labelling. There are two types of horizontal cell in the possum. One type is axonless and has long, fine dendrites somewhat similar to that in the quokka; however, there are several marked differences between the axonless cells seen in the two species. The axonless cell in the possum has on average ten secondary dendrites, twice as many as seen in the quokka. These dendrites are arranged in a radial distribution, unlike those in the quokka, which are polarised in a direction often orthogonal to the overlying ganglion cell axons. Axonless horizontal cells in the possum do not have descending processes that reach the inner plexiform layer as has been seen in the quokka. The second horizontal cell type, the short-axon cell, has an axon and an axonal arbor and is similar to the short-axon cell seen in the retina of the quokka. Therefore, the morphology of the axonless horizontal cell appears to be variable, while that of the short-axon cell is conserved in marsupials as in eutherians.