Pulmonary Tuberculosis

Pulmonary Tuberculosis

Introduction

Chapter One presented the basic principles of chest radiography, how to distinguish the images of the normal thoracic anatomy from abnormal findings, the common radiographic patterns of thoracic disease, and standard terminology used to describe abnormal findings.

Chapter Two reviews the radiographic manifestations of pulmonary tuberculosis and how to apply the terminology covered in Chapter One to describe the radiographic findings. By the end of this chapter, readers will be familiar with the various radiographic manifestations of tuberculosis.

Because tuberculosis has a two-phase pathogenetic sequence by which the disease develops, and because each phase in the sequence is associated with different radiographic features, this chapter begins with an overview of the pathogenesis of tuberculosis.

Overview of the pathogenesis of tuberculosis

When tubercle bacilli (Mycobacterium tuberculosis) are inhaled into the lungs they are deposited in the airways and alveoli in more ventilated areas of the lung - typically in the middle- to lower-lung zones. In previously uninfected persons the bacilli cause an inflammatory reaction which may or may not be seen radiographically. If there is a radiographically visible abnormality, it is referred to as a primary or Ghon focus. This initial infection generally does not produce symptoms.

During this early stage of infection, organisms can spread via lymphatics to the draining lymph nodes in the chest and result in enlargement of hilar and mediastinal lymph nodes. Bacilli can also enter the bloodstream where they spread hematogenously throughout the body. Disease presenting at this stage is referred to as primary tuberculosis and is associated with particular radiographic findings.

After several weeks, the host develops a cell-mediated immune response as indicated by a positive tuberculin skin test or interferon gamma release assay. It is estimated that approximately four point zero percent of people infected with tuberculosis will develop tuberculosis disease by the end of one year following infection. A cumulative total of approximately eight point zero percent will have developed tuberculosis by the end of the twenty-fifth year after infection. Healed lesions may contain viable bacilli that can progress to active tuberculosis, although the risk decreases as time after infection increases. Such progression is termed reactivation or post-primary tuberculosis. Reactivation disease is also associated with characteristic radiographic findings with typical abnormalities occurring in the upper lobes. This entire pathogenetic sequence is a continuum and many of the radiographic manifestations of primary and reactivation tuberculosis overlap. Moreover, in immunodeficiency states such as HIV/AIDS, or treatment with immunosuppressing drugs, atypical radiographic abnormalities are common.

This chapter reviews the radiographic manifestations of tuberculosis in line with this pathogenetic sequence. It is important to note that the distinction between primary and reactivation tuberculosis has little clinical relevance. Active tuberculosis should be treated regardless of whether it is thought to be primary or reactivation. As this chapter will describe, abnormalities thought to be "characteristic" are not specific for tuberculosis disease. However, such findings should initiate a clinical and microbiological evaluation that includes tests for tuberculosis.