128 Clinical Presentation and Diagnosis

Thus, atypical presentations of SARS are a threat to patients, staff, and visitors. The WHO case definition is a useful epidemiological device; however, it is no substitute for daily, thorough clinical, laboratory, and radiological assessment of patients with symptoms of SARS (Fisher).

Table 3: Characteristics of four patients with atypical presentations of SARS*

* modified from Fisher et al.

Chest Radiographic Abnormalities

Imaging plays an important role in the diagnosis of SARS and monitoring of response to therapy. A predominant peripheral location, a progression pattern from unilateral focal air-space opacity to unilateral multifocal or bilateral involvement during treatment, and lack of cavitation, lymphadenopathy, and pleural effusion are the more distinctive radiographic findings (Wong 2003b).

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Chest Radiographs

At the onset of fever, 70-80 % of the patients have abnormal chest radiographs (Booth, Wong 2003b, Peiris 2003b). It should be noted that, in a substantial proportion of cases, chest radiographs may be normal during the febrile prodrome, as well as throughout the course of illness. In other cases, radiological evidence of pneumonic changes may precede the fever (Rainer), particularly in individuals with comorbidities who may be impaired in their ability to mount a fever (Fisher 2003a).

Chest X-ray findings typically begin with a small, unilateral, patchy shadowing, and progress over 1-2 days to become bilateral and generalized, with interstitial or confluent infiltrates. Air-space opacities eventually develop during the course of the disease. In patients who deteriorate clinically, the air-space opacities may increase in size, extent, and severity (Tsang, Lee).

In the first large cohort from Hong Kong, 55 % of the patients had unilateral focal involvement and 45 % had either unilateral multi-focal or bilateral involvement at the onset of fever (Lee). Within a prospective cohort, initial involvement was confined to one lung zone in 49% and was multi-zonal in 21% of the patients (Peiris 2003b).

The initial radiographic changes may be indistinguishable from those associated with other causes of bronchopneumonia. The research group from Hong Kong suggested that chest radiographs might offer important diagnostic clues, in particular when, after approximately one week, unilateral, predominantly peripheral areas of consolidation progress to bilateral patchy consolidation, and when the extent of the lung opacities is correlated with the deterioration in respiratory function (Lee).

There seems to be a predominant involvement of the peripheral-zone. Pleural effusions, cavitation, and hilar lymphadenopathy are usually absent. Respiratory symptoms and positive auscultatory findings are disproportionally mild compared with the chest radiographic findings (Lee).

One large study focused on radiographic appearances and the pattern of progression (Wong 2003b). Within this cohort of 138 patients, four patterns of radiographic progression were recognized: type 1 (initial

Kamps and Hoffmann (eds.)

130 Clinical Presentation and Diagnosis

radiographic deterioration to a peak level, followed by radiographic improvement) in 70.3%, type 2 (fluctuating radiographic changes) in 17.4%, type 3 (static radiographic appearance) in 7.3%, and type 4 (progressive radiographic deterioration) in 5.1% of the patients. Findings during deterioration are compatible with the radiological features of acute respiratory distress syndrome.

CT Scans

The predominant abnormalities found on initial CT scans are areas of sub-pleural focal consolidation with air bronchograms and groundglass opacities (Tsang). The lower lobes are preferentially affected, especially in the early stages. Patients with more advanced cases show a more bilateral involvement (Wong 2003a). The lesions tend to be peripheral and smaller in the less severely affected lungs, also suggesting an earlier stage of the disease. In patients with more advanced cases, there is involvement of the central, perihilar regions by larger (>3 cm) lesions. The majority of the lesions contained an area of ground-glass opacification with or without consolidation. Other findings include intralobular thickening, interlobular septal thickening, a crazy-paving pattern, and bronchiectasis (Wong 2003a). Obvious bronchial dilatation is generally not found (Lee).

Radiographically, SARS may be indistinguishable from other severe forms of pneumonia. It also shares CT features with other conditions that result in subpleural air-space disease, such as the pneumonia of bronchiolitis obliterans and acute interstitial pneumonia (Tsang).

Radiologists from the Prince of Wales Hospital, Hong Kong, recommend the following protocol for diagnostic imaging of suspected SARS patients (Wong 2003a):

a)Patients with symptoms and signs consistent with SARS and with abnormalities on chest radiographs are followed up with serial radiography. CT scanning is not required for diagnosis.

b)Patients with symptoms and signs consistent with SARS and with a normal chest radiograph undergo thin-section CT to confirm the diagnosis. They subsequently undergo serial radiography for follow-up.

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