Bilateral Bronchiectasis in a 73 Year Old Man

1. The HRCT thorax shows cystic lesions and nodular branching opacities prominently in the middle, lingular and lower lobes and a small cavity in the right lower lobe. The appearance of branching centrilobular nodules with bronchiectasis is highly suggestive of although not diagnostic of nodular bronchiectatic form of NTM [1]. It may also be seen in panbronchiolitis and bronchiectasis associated with non-specific bronchiolitis. If bronchiolitis with bronchiectasis is present in all the lobes or there is associated lobular consolidation or cavity, NTM pulmonary infection is the most likely diagnosis [1].

The laboratory results showed normal haemogram and serum biochemistry. Enzyme linked immunosorbent assay for human immunodeficiency virus (HIV) was negative. The chest roentgenogram did not show any significant abnormality. He was subjected to high resolution computed tomography (HRCT) of thorax shown in Figure 1. The sputum smear examination for acid fast bacilli was positive.

Answers
1. The HRCT thorax shows cystic lesions and nodular branching opacities prominently in the middle, lingular and lower lobes and a small cavity in the right lower lobe. The appearance of branching centrilobular nodules with bronchiectasis is highly suggestive of although not diagnostic of nodular bronchiectatic form of NTM [1]. It may also be seen in panbronchiolitis and bronchiectasis associated with non-specific bronchiolitis. If bronchiolitis with bronchiectasis is present in all the lobes or there is associated lobular consolidation or cavity, NTM pulmonary infection is the most likely diagnosis [1].
2. Runyon devised the first good scheme for grouping based on the growth rate and colony pigmentation [2]. Recently, a more appropriate grouping has been described based on the type of clinical disease they produce: disseminated disease, pulmonary disease, cutaneous disease, and others (eye, mastoid, sinus, breast, meninges, peritoneum, appendix, pericardium, kidney, prostate, bursa, joints, bones, lymph nodes) [3]. Disseminated disease usually occurs in immunocompromised host whereas localized disease i.e. pulmonary, cutaneous & others are reported in immunocompetent host [3].
3. The radiological manifestation of pulmonary NTM has three classic forms: nodular bronchiectasis, upper lobe cavity and hypersensitivity pneumonitis ("hot-tub lung") ( Table 1) [4]. The nodular bronchiectatic form of pulmonary infection usually occurs in nonsmoker, middle-aged and elderly individuals without underlying lung disease. The typical HRCT findings of nodular bronchiectatic form of NTM are multiple small centrilobular nodules (<5 mm) with "tree in bud" appearance combined with bronchiectasis in the same lobe of the lung which progress over time [5]. The lesions are observed predominantly in the right middle lobe and lingular segment of the left upper lobe [4]. In contrast to nodular bronchiectatic form, the cavitary form is typically seen in smokers and elderly men with underlying chronic lung disease, usually chronic obstructive pulmonary disease. Though, radiologically the upper lobe cavitary form is similar to post primary pulmonary tuberculosis, clinically it has more indolent course. The pulmonary form of hypersensitivity pneumonitis (HP) is caused by hot tub exposure and is clinically and radiologically similar to HP due to other organisms [6].

Discussion
NTM exist free in nature and are well recognized causes of disease in humans and animals. An assortment of names has been employed to describe these mycobacteria like: anonymous mycobacteria, tuberculoid mycobacteria, atypical mycobacteria, opportunistic mycobacteria and mycobacteria other than tuberculosis. NTM commonly causing pulmonary infection are Mycobacterium (M) avium-intercellulare complex (MAC); M. kansaii; M. abscessus; M. chelonae and M. fortuitum of which MAC is the commonest pathogen [5]. The nodular bronchiectatic form is usually caused by MAC, however M. abscessus, M. kansasii and M. fortuitum are also known to cause the disease [7]. The progression of disease in nodular form is slow and treatment is indicated only if the disease is progressive. The cavitary form of disease is generally caused by MAC, M. kansasii and to a lesser extent by M. xenopi, M. abscessus and M. malomense. It is progressive in nature, and if left untreated can lead to extensive lung destruction and death. HP is commonly associated with MAC. The prognosis is good and prompt resolution of symptoms is expected following treatment [8]. Medical therapy for NTM involves multiple agents to facilitate clearance of organisms and minimize the emergence of drug resistance. Though it is important to identify the species for treatment the cornerstone agents include macrolides, ethambutol, and rifamycin. Patients with nodular bronchiectatic lung disease and those with less severe or indolent infections are effectively managed with a 3-times-weekly regimen that includes clarithromycin 1000 mg or azithromycin 500 mg plus ethambutol 25 mg/kg plus rifampin 10 mg/kg/day (maximum 600 mg/day) [7,9,10]. Intermittent drug therapy is not recommended for patients who have cavitary disease, patients who have been previously treated, or patients who have moderate or severe disease. The treatment is continued until culture is negative on therapy for 1 year. If the disease is progressive in spite of treatment and sufficiently localized and where lung function permits, surgical resection under chemotherapeutic cover is considered. In countries with high prevalence of tuberculosis, this entity needs to be remembered as it can lead to treatment failure [11]. To conclude, high index of suspicion is the key to the diagnosis of NTM infection. Patients with bronchiectasis and nodules on HRCT thorax should be advised a microbiological and culture examination of their sputum for NTM organisms for early diagnosis and appropriate management [12].  Cessation of exposure to hot water tubs, corticosteroids, antimycobacterial therapy