Right Middle Lobe Collapse

Introduction

The right middle lobe (RML) is the odd one out- there is no 'left middle lobe' as such. The lingula segments of the left upper lobe are the equivalent of the RML. This is worth bearing in mind as the plain film pathological appearances of these lobes can demonstrate similar patterns..

The Right Middle Lobe Anatomy

adapted from <a class="external" href="http://books.google.com.au/books?id=Bif0zpmEWtAC" rel="nofollow" target="_blank">By Fred W. Wright Radiology of the Chest and Related Conditions: Together with an Extensive Illustrative Collection of Radiographs CRC Press, 2002</a>

The right middle lobe has two pulmonary segments which are situated side by side; the more lateral segment, approximates the size of its adjacent neighbour (medial segment). The medial segment abuts the right heart border medially , while the lateral segment extends to and comprises a portion of the lateral border of the right lung. <a class="external" href="http://lib.cpums.edu.cn/jiepou/tupu/atlas/www.vh.org/adult/provider/radiology/LungAnatomy/RightLung/RtLungSegAnat.html" rel="nofollow" target="_blank">http://lib.cpums.edu.cn/jiepou/tupu/atlas/www.vh.org/adult/provider/radiology/LungAnatomy/RightLung/RtLungSegAnat.html</a>. When viewing chest radiographs with pathology involving the right middle lobe, it is important to think about the shape and position of the RML in three dimensions. This may not be easy at first. Note the description of the lobes is very approximate.

adapted from <a class="external" href="http://books.google.com.au/books?id=Bif0zpmEWtAC" rel="nofollow" target="_blank">By Fred W. Wright Radiology of the Chest and Related Conditions: Together with an Extensive Illustrative Collection of Radiographs CRC Press, 2002</a>

The lateral view image gives a better appreciation of its middleness. Further information on lung anatomy here

Important Characteristics of all Lobar Collapse

1. Collapse and consolation can occur independently or together

2. Collapse can be partial or complete

3. It is often not clear to what extent the appearance is due to collapse or consolidation or both. The degrees of each are often unclear.

4. If a lobe is only partially collapsed and there is no accompanying consolidation, there may be no increase in opacity

5. In cases of pure collapse, only when the collapse is virtually complete will there be a significant increase in density of the affected lung

Plain Film Appearance

• The plain film appearance of collapse of the right middle lobe is often characterised by a loss of volume, an increase in density, and a loss of clarity of the right heart border.

• A partial RML collapse can be difficult to appreciate because the increase in opacity of the lobe can be superimposed over the heart shadow

A collapse of the lingula segment of the left upper lobe (LUL)

If there is sufficient collapse of the RML, the horizontal fissure may be pulled down with the collapsing lobe. This will not always be evident given that the horizontal fissure is not always demonstrated. Importantly, the horizontal fissure will tend to disappear from the AP/PA chest image as the RML collapses because it is no longer horizontal.	The child has abnormal opacity in the region of the RML. There are air bronchogram lines and these bronchii appear have a sudden change in calibre and to be displaced medially.
The lateral view is often the confirming image when there is RML collapse. The RML may appear dense with collapse of the lobe and movement of the fissures.	Not unlike RUL collapse, the fissures are attached at the hilum and the RML collapse appears like a fan being closed.

	an appreciation of the collapse of the RML can be gained by consideration of the normal position of the horizontal and oblique fissure

False RML Collapse/Consolidation in Patients with Pectus Excavatum

This patient presented for chest radiography with a history of recent facial nerve palsy. There is loss of visualisation of the right cardiac border.

The lateral chest image demonstrates no abnormal RML opacity. There is evidence of pectus excavatum (depression of the sternum). Pectus excavatum is a known cause of false RML disease and is likely to be a contributing cause of the pseudo-silhouette sign on the PA chest image. In patients, with pectus excavatum the heart tends to be displaced towards the left as a result of the limited space between the depressed sternum and the spine.Other contributing factors could be pericardial fat (although none is seen in this patient) and overlying pulmonary vessel. Pectus Excavatum <a class="external" href="http://en.wikipedia.org/wiki/File:Pectus1.jpg" rel="nofollow" target="_blank">http://en.wikipedia.org/wiki/File:Pectus1.jpg</a>

False RML Collapse/Consolidation in Patients with Pericardial fat

Pericardial Fat

<a class="external" href="http://jcmr-online.com/content/11/1/15" rel="nofollow" target="_blank">Validation of cardiovascular magnetic resonance assessment of pericardial adipose tissue volume</a><a class="external" href="http://jcmr-online.com/content/11/1/15" rel="nofollow" target="_blank">Adam J Nelson , Matthew I Worthley , Peter J Psaltis , Angelo Carbone , Benjamin K Dundon , Rae F Duncan , Cynthia Piantadosi , Dennis H Lau , Prashanthan Sanders , Gary A Wittert and Stephen G Worthley</a><a class="external" href="http://jcmr-online.com/content/11/1/15" rel="nofollow" target="_blank">Cardiovascular Research Centre, Royal Adelaide Hospital & Disciplines of Medicine and Physiology, University of Adelaide, Adelaide, SA, Australia</a>

This is a sheep's heart with the pericardium partly dissected off. Note the pericardial fat. Felson (<a class="external" href="http://www.amazon.com/Chest-Roentgenology-Benjamin-Felson/dp/0721635911/ref=sr_1_2?ie=UTF8&s=books&qid=1252240078&sr=1-2" rel="nofollow" target="_blank">Chest Roentgenology, W.B. Saunders, 1973, p55</a>) notes that "For reasons that escape me, fat in the thorax is often impossible to differentiate from water density. Perhaps the adjacent pulmonary gas density has something to do with this illusion." This is an interesting point given the clear differentiaton between fat and water density structures seenin the abdominal plain film.

Case 1

This 49 year old female patient presented for chest radiography with "recent onset of chest pain". There is loss of clarity of the right heart border. In addition, there is loss of clarity of the left heart border. There are good reasons to consider that these silhouette signs are not caused by lung consolidation. The patients history and clinical signs do not suggest a diagnosis of infection/consolidation there is no evidence of alveolar opacity the lateral view does not demonstrate alveolar opacity that you would expect with a lung consolidation The likely cause of the loss of clarity of the cardiac silhouette is the presence of pericardial fat.

There is no abnormal RML or left upper lobe (lingula segment) opacity. Where there is a significant amount of pericardial fat visible on the PA/AP chest image, you would expect to see evidence of this on the lateral image (white arrow)

patient with minimal pericardial fat for comparison (ignore arrow)

Hidden RML Collapse/Consolidation

There is suggestion of some loss of visualisation of the right heart border <a class="external" href="http://bjr.birjournals.org/cgi/content/abstract/74/877/89" rel="nofollow" target="_blank">K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97, 2001</a>	The arrowed structure looks like a fissure but it is not in the correct position for either the horizontal fissure nor the oblique fissure. <a class="external" href="http://bjr.birjournals.org/cgi/content/abstract/74/877/89" rel="nofollow" target="_blank">K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97, 2001</a>	The lordotic view demonstrates a completely collapsed RML (arrowed) <a class="external" href="http://bjr.birjournals.org/cgi/content/abstract/74/877/89" rel="nofollow" target="_blank">K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97, 2001</a> note- this is unlikely to represent an acute collapse

The centray ray is aligned to the orientation of the RML in the lordotic position. The lordotic position can reveal RML pathology which is hidden on the conventional PA and lateral views

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