Cholesterol crystal embolism--new aspects of an "old" disease

Cholesterol crystal embolism is increasing in frequency due, on the one hand to the aging of population and, on the other hand, to the great extent of invasive cardiovascular procedures, anticoagulant and thrombolytic therapy. Very often it has a protean symptomatology being considered as a "great masquerader". Due to its particular clinical picture it is sometimes misdiagnosed. This article presents the major clinical manifestations, the diagnostic strategy, the methods of prevention and treatment of this disease.

Chronic cholesterol crystal embolism with a spontaneous onset

A 74-year-old man was referred to our hospital because of hypertension, blue toe syndrome and an elevation of serum creatinine from 0.8 to 1.4 mg/dl for eleven months. He had no history of invasive vascular procedures. Atherosclerosis was initially suspected, but renal impairment was accelerated following anticoagulant therapy. A renal biopsy established the diagnosis of cholesterol crystal embolism. Withdrawal of anticoagulants and the combination therapy with LDL apheresis and corticosteroids led to stabilization of the renal function. In patients with risk factors for atherosclerosis, cholesterol crystal embolism should be included in the differential diagnosis of chronic kidney disease.

Cholesterol Crystal Embolism Syndrome in Dialysis Patients: An Emerging Clinical Diagnosis?

BACKGROUND

Cholesterol crystal embolism syndrome (CCE) is an increasing end-stage renal disease cause. Few cases have been described on dialysis, despite the high prevalence of the predisposing factors.

METHODS

The diagnostic criteria of the present study were: skin lesions, myalgia, fatigue, fever and acute inflammatory serologic signs, in the presence of severe vasculopathy. The precipitating factors were: anticoagulation, endovascular intervention and ulcerated atherosclerotic plaque.

RESULTS

Between October 2003 and September 2005, CCE was diagnosed in 6 dialysis patients (of 200-210 on chronic treatment): 5 males, 1 female, median age 59.5 years (47-70) and end-stage renal disease follow-up 11.5 years (3-25). All had severe vasculopathy, 5 cardiopathy, and 4 were failed graft recipients. The treatment included: peritoneal dialysis, daily dialysis, 'conventional' hemodialysis (2 cases) and hemodiafiltration. The diagnosis was based on the clinical-laboratory picture in 1 patient. In the 5 others clues were present (dicumarol therapy, angioplasty, femoral artery thrombosis, CCE predialysis and ulcerated aortic plaque). The therapeutic approach consisted of corticosteroids (5 cases), statins (4 cases) and prostaglandin analogues (4 cases).

CONCLUSION

The differential diagnosis of CCE should also be considered in dialysis patients (necrotic lesions, limb pain and vasculitis-like signs).

Cholesterol embolism: Still an unrecognized entity with a high mortality rate

BACKGROUND

Cholesterol embolism (CE) is an increasingly common but often underdiagnosed medical problem. The recognition of clinical manifestations of CE is the first step toward a correct diagnosis.

OBJECTIVE

Our aim was to characterize the features of CE and the risk factors for fatal outcome.

METHODS

Clinical records of patients with clinical and histopathologic diagnoses of CE seen from January 1993 through March 2003 were reviewed.

RESULTS

Twenty-six male patients were identified. Mean age was 64 years (range, 48-88 years). All patients had two or more risk factors for atherosclerosis. All but one patient had preexisting symptomatic atherosclerotic disease. At least one precipitating factor was identified in 23 patients (88%). Diagnosis of CE at admission was made in 9 patients only (35%). Cutaneous lesions (88%) and renal failure (73%) were the most common clinical findings. Complications (dialysis, acute pulmonary edema, amputation, or gastrointestinal surgery) were present in 21 patients (81%), and 15 patients died (58%). Previous chronic renal failure was the only variable associated with mortality (relative risk: 4.54, 95% confidence interval 1.26-16.6; P = .02).

LIMITATIONS

The results were obtained from patients admitted to a university hospital. This fact may have selected a higher proportion of severe cases.

CONCLUSIONS

CE was frequently misdiagnosed. Skin lesions were the most common clinical findings and skin biopsy provided histologic confirmation in most of the patients. Chronic renal failure was the only factor related to death.

Epidemiology and prognostic implications of contrast-induced nephropathy

Contrast-induced nephropathy (CIN), usually defined as an increase in serum creatinine of 0.5 mg/dL (44.2 mumol/L), or a 25% increase from the baseline value 48 hours after the procedure, is a common and potentially serious complication of the use of iodinated contrast media in patients at risk of acute renal injury. It is an important cause of hospital-acquired renal failure, responsible for approximately 11% of cases. CIN may be difficult to distinguish from cholesterol embolization, another cause of postprocedure renal impairment. The reported incidence of CIN varies depending on the patient population studied. The impact of postprocedural renal impairment on clinical outcomes has been evaluated most extensively in patients undergoing percutaneous coronary intervention. CIN is associated with increased mortality both in hospital and at 1 year. A higher incidence of in-hospital and late cardiovascular events, as well as longer hospital stays, has been reported in patients developing CIN. In a small proportion of patients, CIN is severe enough to require dialysis, and these patients have a particularly poor prognosis. Many of the risk markers for CIN are also predictive of a worse prognosis.

Cholesterol crystal embolism: diagnosis and treatment

Cholesterol crystal embolization (CCE) is a dreaded complication of radiology, vascular surgery, and/or anticoagulation in patients with atherosclerosis and ulcerated aortic plaques. It also represents a cause of early graft failure and of poor results of renal artery surgery. Crystals lodge in small caliber renal arteries, where they induce early, transitory thrombosis followed by delayed, definitive obstruction by endarteritis, accompanied by evidence of inflammation and eosinophilia. Massive CCE leads to early oligoanuria. In subacute forms, renal insufficiency is often delayed by weeks or months following the triggering event. A third, chronic subset of CCE is easily mistaken for atherosclerotic renal ischemia and/or nephrosclerosis. The kidney is rarely the sole organ involved in acute/subacute forms, in which the central nervous system, the coronary arteries, the spinal cord, and the mesenteric and pancreatic blood supply compromise represent the main causes of death. Cutaneous, retinal, and muscle involvement allow diagnosis by inspection or scarcely invasive biopsies in about 80% of cases, whereas renal biopsy as the only diagnostic procedure is required in 20% of cases. Prevention is based on avoidance of endovascular radiology maneuvers, vascular surgery, and excess anticoagulation in atherosclerotic patients. Treatment of acute/subacute forms of renal insufficiency consisting of stopping anticoagulation and forbidding any new radiologic and/or vascular surgery procedure; treating hypertension with angiotensin 2 antagonists and vasodilators, strict volemic control by loop diuretics and ultrafiltration, along with parenteral nutrition and prednisone, has been credited with improved outcome. Iloprost may obtain favorable results. Statins definitely ameliorate the renal and patient's prognosis.

A Case of Cholesterol Embolism Confirmed by Skin Biopsy and Successfully Treated with Statins and Steroids

Although cholesterol embolism syndrome was recognized as a clinicopathologic entity more than 50 years ago, it is attracting growing attention recently. It is a multisystemic disorder in which cholesterol crystals released from atherosclerotic plaques obstruct small arterioles, resulting in local ischemia and end-organ damage. There are no established treatments, and with the limited treatment options available, it is important to make the diagnosis as early as possible. We present the case of a 68-year-old man with cholesterol embolism who had a few fluttering atheromas in the aorta, as demonstrated by transesophageal ultrasonography. The diagnosis was confirmed by skin biopsy, and treatment with statins and steroids proved effective, as renal failure progressively improved. This case emphasizes the importance of early diagnosis and shows the possible therapeutic effects of statins and steroids for patients with this syndrome.

[Cholesterol embolism of the upper urinary tract as a clinical feature of atherosclerotic embolic vascular disease. Report of three cases, one of them simulating neoplasia]

OBJECTIVES

To report three cases of atherosclerotic embolic vascular disease with clinical presentation in the lower urinary tract. This disease is not frequent; it mainly affects the skin, kidneys and skeletal muscle. Other organs of the urinary tract are rarely affected and they are exceptionally the clinical site of debut without previous known involvement of other areas.

METHODS/RESULTS

The first patient presented with hematuria associated with an ultrasound/cystoscopical image suspicion for neoplasia. Pathologic report of the transurethral resection chips showed polypoid cystitis with some small size arteries occupied by cholesterol needles, associated with inflammation, ulcers and hematic extravasation. The second patient underwent cystoprostatectomy for a transitional cell carcinoma and, incidentally, numerous cholesterol emboli were found, mainly in the lamina propria, but also in other layers of the bladder wall, prostate, urethra, veru montanum, and one seminal vesicle; this two latter sites have not being reported in previous publications. The third patient showed the embolus within a prostate biopsy core.

CONCLUSIONS

This unfrequent disease may present as a bladder or prostatic process and be diagnosed by biopsy of these organs. It may simulate a bladder neoplasia on ultrasound or cystoscopy and should be included among related or etiologic entities of polypoid cystitis, a well-known simulator of neoplasia.

Occult popliteal artery stenosis masquerading as atheroemboli in a patient with previous knee replacement

The purpose of this article is to report the unusual presentation of a 63-year-old patient with Rutherford grade 2, category 5 tissue ischemic changes involving the right foot secondary to an occult popliteal stenosis that was obscured behind a prosthetic knee on diagnostic angiograms. Conventional abdominal angiography with bilateral lower extremity runoff showed no evidence of significant disease and the patient was misdiagnosed with atheroemboli syndrome secondary to ipsilateral common femoral access following recent catheterization. Ultimately, a meticulous physical examination disclosed a bruit in the right popliteal fossa and selective right leg angiography with oblique views confirmed eccentric complex luminal encroachment in the right popliteal artery that was eclipsed by a prosthetic knee on antecedent nonselective angiography. The lesion responded favorably to endovascular treatment with durable clinical improvement. This case illustrates the importance of a meticulous physical examination and noninvasive studies prior to angiography in patients with ischemic tissue changes and emphasizes the importance of oblique views to image any vessel that may be obscured by a metal prosthesis.

Cholesterol embolism after cardiac catheterization mimicking infective endocarditis

We present a 65-year-old man with rheumatic combined valvular heart disease showing persistent fever 3 weeks after diagnostic cardiac catheterization. Infective endocarditis was strongly suspected from the clinical course, however, serial blood cultures were negative. Transesophageal echocardiography, done to investigate vegetation, revealed multiple mobile plaques in the descending aorta. Administration of both steroid and simvastatin improved both symptoms and renal function. Cholesterol embolism should be considered to be one of the possible causes of low-grade fever after cardiac catheterization especially in patients with anticoagulation.

Cholesterinemboliesyndrom nach Koronarangiographie*

Cholesterol crystal embolization (cholesterol embolism, cholesterol embolic disease) is a multiorgan disease, which is a severe iatrogenic complication from an invasive vascular procedure, such as manipulation of the aorta during angiography or vascular surgery, and after anticoagulant and fibrinolytic therapy. The diagnosis is made postmortem in two-thirds of cases. Cholesterol crystal embolism is an increasing and still underdiagnosed disease. Pathognomonic is the constellation of acral pain, nonhealing ulcerations and necrosis, livedo racemosa with intact peripheral arterial pulses and sudden onset of renal failure and arterial hypertension. Biopsy of the affected organs is essential for diagnosis. We report the case of a 66-year-old man who following coronary arteriography with PTCA and implantation of stents developed acral necrosis and cyanosis, livedo racemosa and acute irreversible renal failure.

Cholesterol crystal embolization diagnosed on bladder transurethral resection

Cholesterol crystal embolization (CCE) is a severe systemic disorder caused by vascular migration of cholesterol crystals originating from ulcerative atherosclerotic plaques located in large arteries. We report 2 cases of CCE diagnosed on bladder transurethral resection in 2 men aged 94 and 72 years. Both patients had atherosclerosis disease. One patient had been treated by heparin 1 month before for pulmonary embolism and the other had had a coronary angiography and bypass graft surgery 5 months before for silent myocardial infarction. One patient presented with hematuria and the other with acute renal failure. Cystoscopy showed multiple papillary tumors of the bladder wall. Bladder transurethral resections showed transitional cell carcinoma with cholesterol crystals occluding the lumen of small arterioles in the submucosa. Eight cases of CCE in the bladder wall have been reported in the literature in 3 women and 5 men aged 56 to 79 years. Cholesterol crystal embolization is often discovered in the bladder wall on necropsy specimens. Only 2 cases have been fortuitously discovered on bladder transurethral resection performed for transitional cell carcinoma. Cholesterol crystal embolization in the bladder wall is often a marker of severe disease although the evolution is quite favorable in our patients, still alive 1 and 2 years after diagnosis.

Enfermedad por émbolos de cristales de colesterol

Cholesterol crystal embolism (CCE) is an infrequent entity that primarily appears in males over the age of 60 with generalized arteriosclerosis after angiographic procedures, vascular surgery or, more rarely, with oral anticoagulant treatment with heparin or with fibrinolytics. We present the case of a patient with several risk factors for CCE, who presented with the pathognomonic triad of leg and foot pain, livedo reticularis and palpable pedal pulses. The diagnosis was based on the fact that cholesterol crystals were seen in the arterioles in the skin biopsy. Due to the frequency with which the skin manifestations appear and the importance of early diagnosis and treatment, an awareness of these crystals is fundamental in diagnosing these processes.

Technology Insight: targeting of biological molecules for evaluation of high-risk atherosclerotic plaques with magnetic resonance imaging

Identification of high-risk atherosclerotic lesions prone to rupture and thrombosis may greatly decrease the morbidity and mortality associated with atherosclerosis. The development of magnetic resonance imaging contrast agents that specifically target components of the atherosclerotic plaque might enable non-invasive detection of high-risk lesions. This review discusses a variety of molecules present in atherosclerotic plaque that could serve as targets for specific contrast agents. Ultimately, such agents may allow the identification of high-risk atherosclerotic lesions in patients and enable treatment of these patients before lesion progression and complications.

[Pseudovasculitis--confusingly alike systemic vasculitis...but completely different treatment is required]

"Vasculitis mimicry" or "pseudovasculitis" are terms used for the description of a variety of disorders that simulate systemic vasculitis and often represent a challenge in daily rheumatological practice. These conditions may present with clinical, radiological, and laboratory features resembling those of systemic vasculitis. Nevertheless, the pathogenesis, therapeutic approach, and prognosis are different. The pseudovasculitides constitute an extremely heterogenous group of disorders with conditions caused by infections, malignancies, drugs and thromboembolism. A detailed history is mandatory and this should include dietary habits, concurrent medication, recent infections or invasive investigative procedures such as PTCA or conventional angiography. Incorrect diagnosis may lead to serious consequences due to delay of appropriate management and exposure of the patients to the risks of potentially toxic drugs. Thus, pseudovasculitis must be part of the differential diagnosis of patients presenting with manifestations of systemic vasculitis.

Deterioration of vascular dementia caused by recurrent multiple small emboli from thoracic aortic atheroma

We report a case of a 77-year-old man with deteriorating dementia caused by repeated multiple small cerebral embolisms from a thoracic aortic atheroma. Multiple small embolisms were confirmed by diffusion-weighted magnetic resonance imaging (DWI). The patient ultimately died due to aortic dissection. Pathological examinations revealed that no causative embolic source for multiple embolisms could be detected other than severe atheromatous ulcer in thoracic aorta. This case demonstrates that severe aortic atheroma has the potential to precipitate deterioration of vascular dementia.

Primary sarcoma of the distal abdominal aorta: CT angiography findings

Primary aortic angiosarcomas are extremely rare. Clinically and radiographically, they mimic atherosclerosis and atheroembolic disease. For a definitive diagnosis, histologic evaluation of the tumor or of peripheral emboli is required. The imaging findings are frequently nonspecific and in most published cases did not allow a definitive preoperative diagnosis. This is the first report of the computed tomographic angiographic findings of a primary intimal abdominal aortic sarcoma and a review of previously described imaging findings in these tumors.

[Atheroembolic disease and pulmonary haemorrhage]

The cholesterol embolism syndrome is a multisystemic disease resulting from cholesterol crystal emboli deriving from ulcerous atherosclerotic plaques of the aorta or large arteries. Cholesterol crystal embolization can affect multiple organ, including the skin, kidney, brain, eyes, gastrointestinal tract and extremities, and mimic other systemic diseases like vasculitis. Cholesterol crystal embolization of lungs has been described and should be included in the differential diagnostic of pulmonary-renal syndromes. The diagnosis of cholesterol embolism should be considered in elderly patients with pre-existing atherosclerotic disease who develop renal failure and clinical features of peripheral cholesterol crystal embolization in association with precipitating event, without the need for histological demonstration of cholesterol clefts. This syndrome is associated with high morbidity and mortality but recent reports suggest that an aggressive therapeutic management with patient-tailored supportive measures, avoids precipitating factors, and the use of corticosteroids may be associated with a favorable clinical outcome.

Cholesterol crystal embolization (CCE) after cardiac catheterization: a case report and a review of 36 cases in the Japanese literature

Cholesterol crystal embolization (CCE) is a complication of atherosclerosis. A 67-year-old Japanese man underwent coronary artery bypass grafting. After the surgery, he underwent coronary angiography via the right femoral artery. Twelve days later, he suddenly developed acalculous cholecystitis and was treated with antibiotics. Gradual deterioration in renal function, purplish discoloration of the distal portion of his toes, and eosinophilia were noted. We performed a skin biopsy and made a diagnosis of CCE. Cilostazol and intravenous heparin improved the symptoms and decreased the creatinine level. We retrospectively studied the clinical features of 36 cases registered with a diagnosis of CCE in the Japanese literature.

Cholesterol crystal embolization: an atherosclerotic disease with frequent and varied cutaneous manifestations

In this paper the basic pathogenesis of cholesterol crystal embolization (CCE) is described, its clinical characteristics are presented and diagnosis and therapy are discussed. The main focus will be on the cutaneous manifestations; however, considering that CCE is a systemic illness, findings in other organs will also be highlighted, particularly the commonly involved renal and gastrointestinal systems.

An unusual case of cholecystitis

A 56-year-old woman with diabetes who had undergone a coronary artery bypass surgery for triple-vessel coronary artery disease presented 2 weeks after discharge with classic features of cholesterol embolization, blue toes, renal insufficiency, and intractable abdominal pain. Despite a multitude of investigations, the cause of her abdominal pain was elusive. Laparoscopic cholecystectomy revealed the cause: acute cholecystitis secondary to cholesterol crystal embolization. Although rare, cholecystitis as a manifestation of cholesterol embolization can occur, and prompt recognition will prevent unnecessary investigations and ensure immediate treatment.

The incidence and risk factors of cholesterol embolization syndrome, a complication of cardiac catheterization: a prospective study

BACKGROUND

Cholesterol embolization syndrome is a systemic disease caused by distal showering of cholesterol crystals after angiography, major vessel surgery, or thrombolysis.

METHODS

We prospectively evaluated a total of 1,786 consecutive patients 40 years of age and older, who underwent left-heart catheterization at 11 participating hospitals. The diagnosis of CES was made when patients had peripheral cutaneous involvement (livedo reticularis, blue toe syndrome, and digital gangrene) or renal dysfunction.

RESULTS

Twenty-five patients (1.4%) were diagnosed as having CES. Twelve patients (48%) had cutaneous signs, and 16 patients (64%) had renal insufficiency. Eosinophil counts were significantly higher in CES patients than in non-CES patients before and after cardiac catheterization. The in-hospital mortality rate was 16.0% (4 patients), which was significantly higher than that without CES (0.5%, p < 0.01). All four patients with CES who died after cardiac catheterization had progressive renal dysfunction. The incidence of CES increased in patients with atherosclerotic disease, hypertension, a history of smoking, and the elevation of baseline plasma C-reactive protein (CRP) by univariate analysis. The femoral approach did not increase the incidence, suggesting a possibility that the ascending aorta may be a potential embolic source. As an independent predictor of CES, multivariate regression analysis identified only the elevation of pre-procedural CRP levels (odds ratio 4.6, P = 0.01).

CONCLUSIONS

Cholesterol embolization syndrome is a relatively rare but serious complication after cardiac catheterization. Elevated plasma levels of pre-procedural CRP are associated with subsequent CES in patients who undergo vascular procedures.

[Immunoallergic interstitial nephritis vs. cholesterol atheroembolism. Differentiating characteristics]

The commonest clinical presentation of both immunoalergic interstitial nephritis (IIN) and atheroembolic renal disease (ATD) is an acute renal failure accompanied by skin lesions and eosinophilia. As a consequence, differential diagnosis between both entities is often very difficult. We have performed a comparative retrospective study of those patients diagnosed as having IIN or ATD in our Hospital in the period 1980-2000. A total of 42 patients have been diagnosed of IIN and 16 of ATD. Demographic data, as well as clinical and laboratory parameters and outcomes of every studied patient were analysed. We found a significantly higher prevalence of male sex (100% vs 57%, p < 0.01), previous history of hypertension (100% vs 55%, p < 0.01), chronic renal insufficiency (56% vs 17%, p < 0.01), ischemic heart disease (56% vs 14%, p < 0.001), peripheral ischemic disease, endovascular procedures (87% vs 7%, p < 0.001) and anticoagulant treatments (25% vs 5%, p < 0.001) among patients with ATD as compared with IIN, respectively. On the contrary, previous infections (45% vs 12%, p < 0.01) and exposure to new drugs (100% vs 40%, p < 0.001) were significantly more frequent among IIN patients in compare with ATD. ATD patients showed skin lesions consisting of livedo reticularis and digital infarcts (63% vs 31%, p < 0.05) accompanied by blood pressure increase (100% vs 24%, p < 0.001), whereas IIN patients showed fever (41% vs 19%, p < 0.05) and cutaneous rash as significant clinical manifestations, respectively. The number of ATD patients with proteinuria > 1 g/24 h was significantly higher, but no differences between both groups in the prevalence of urinary sediment abnormalities were observed. The prevalence of absolute eosinophilia was high in both groups (88% among ATD patients, 64% among IIN patients; pNS). Prognosis of both entities was clearly different: Almost all patients with ATD died (69%) or evolved to end-stage renal failure, whereas most patients with IIN showed a recovery of renal function after withdrawal of responsible drugs and steroid treatment. In summary, the analysis of clinical and laboratory data allows an initial differential diagnosis in patients suspected as having IIN or ATD.

[Cholesterol-crystal embolization after PTCA. Case report and review of the literature]

HISTORY

A 62-year-old male patient with depression after coronary intervention (PTCA) 4 weeks earlier.

THERAPY AND CLINICAL COURSE

The patient developed insufficiency of liver and kidney as well as painful toe lesions. Some more symptoms were observed such as recurrent pulmonary edema, arterial hypertension, hardly responding to medical therapy, and severe gastrointestinal symptoms. The variety of symptoms and the progress of the disease were similar to a generalized vasculitis. Renal biopsy yielded the diagnosis of cholesterol-crystal embolization (CCE). After short-term remission, the patient died following prolonged and severe progression of the disease.

CONCLUSION

CCE is a rare disease with devastating outcome. Clinicians should be aware of the disease, especially with regard to an increasing number of coronary interventions in older patients.

Cholesterol crystal embolization to the digestive system: characterization of a common, yet overlooked presentation of atheroembolism

In the 1359 published patients with multiorgan cholesterol crystal embolism (CCE), the digestive system seems to be the third most frequently affected system. Yet, this system received hitherto only little attention in the medical literature. Therefore, the aim of the present study was to clinically characterize the subset of patients with CCE involving the digestive system, based on our institutional experience and a review of the literature. Cases with CCE in a 7-yr period (1995-2001) were sought in the computerized records of our medical center. Of the CCE patients, those with digestive system involvement that could be related to CCE were included in this study. The clinical features of CCE were determined and compared with those found in published series. Fourteen cases with CCE were identified, giving an annual incidence of 0.8 per 10(5). Digestive system involvement was found in five (36%) of the 14 patients. All five patients had established atherosclerosis. Precipitating factors were vascular manipulations or anticoagulation treatment in four of these five patients. Two patterns of disease appeared: acute catastrophic multiorgan disorder with poor prognosis and chronic and more indolent GI disease. Abdominal pain, GI bleeding, fever, and diarrhea were the most common manifestations, resulting from bowel infarction, mucosal ulcerations, hepatocellular liver disorder, and/or pancreatitis. CCE is a systemic disorder with a frequent involvement of the digestive system and protean clinical manifestations. It should, therefore, be considered in any gastroenterological patient with atherosclerosis and recent vascular manipulations or systemic anticoagulation.

[Cholesterol emboli during coumarin therapy]

A 71-year-old patient suddenly developed a painful, bizarre livid erythema on the right foot. Based upon the clinical and histological presentation, cholesterol emboli were diagnosed. These cholesterol emboli were induced by therapy with phenprocoumon (a coumarin derivative), which had been initiated 5 months previously. Arterial emboli may rarely occur during anticoagulation and have to be included in the differential diagnosis.