🔍 Everything You Need to Know About Inferior Accessory Fissure of the Lung: A Normal Variant in Chest Pain Patients

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In this post, I will delve into the fascinating finding—the Inferior Accessory Fissure of the Lung—a structure often discovered during a CT scan for a patient presenting with chest pain. This anatomical structure is a normal variant that can often be mistaken for atelectasis or scarring.

This column aims to go beyond simple information, providing a comprehensive overview based on world-class expert knowledge and current literature. We will cover the Pathophysiology, Epidemiology, Clinical presentation, Imaging features, Differential diagnosis, Treatment, and Prognosis of the Inferior Accessory Fissure. 

👨‍⚕️ Case Presentation: A 56-Year-Old Male Complaining of Chest Pain

Patient Information:

  • Age: 56 years
  • Gender: Male
  • Chief Complaint: Chest pain

The patient underwent a Chest CT scan due to chest pain, which revealed the Inferior Accessory Fissure of the Lung, a normal variant. The findings also included a Right middle lobe atelectatic band.

[Figure 1] Axial lung window

Image Interpretation: An inferior accessory fissure is observed in the right lower lobe of the lung. This is an inferior accessory fissure of the lung which divides the medial basal segment from the rest of the lower lobe. This image clearly demonstrates the anatomical structure of the lung, specifically the location of the Inferior Accessory Fissure.

[Figure 2] Coronal lung window

Image Interpretation: The inferior accessory fissure of the lung is also seen on the coronal image, suggesting it separates the medial basal segment from the rest of the lower lobe. The arrows point to the inferior accessory fissure, which is a normal variant.


[Figure 3] Sagittal lung window

Image Interpretation: The inferior accessory fissure is clearly visible on the sagittal image, forming the boundary that divides the medial basal segment from the rest of the lower lobe. This view aids in the three-dimensional understanding of the inferior accessory fissure.

🌬️ In-Depth Analysis of the Inferior Accessory Fissure of the Lung

The Inferior Accessory Fissure of the Lung , also known as Twining's line, is one of the normal anatomical variations of the lung.

1. Anatomical Definition and Location

The inferior accessory fissure is a small accessory fissure existing within the Lower Lobe of the lung.

  • Function: This fissure completely or partially separates the medial basal bronchopulmonary segment from the rest of the lower lobe.
  • Twining's line: Historically, when observed on radiographs, it was named Twining's line.

Bronchopulmonary segments are the anatomical and functional basic units of the lung. The inferior accessory fissure represents a case where the boundary between these segments is fully developed.

2. Pathophysiology and Embryological Understanding

The Inferior Accessory Fissure of the Lung is an inborn embryological variation, not a disease state.

  • Embryological Origin: During fetal development, the bronchial buds differentiate, and lung parenchyma forms, dividing into lobar and segmental structures. The fissures separating the lobes are formed by the invagination of the visceral pleura covering the lung surface. The inferior accessory fissure is formed when the segmental boundary between the medial basal segment and adjacent segments is completely or abnormally covered by the visceral pleura.
  • Clinical Significance: The inferior accessory fissure itself is usually asymptomatic and is not directly related to symptoms like the patient's chest pain. However, its presence can influence the pattern of disease spread in conditions such as pneumonia or atelectasis. For example, if the fissure is complete, infection may be more likely to be confined to that specific segment.

3. Epidemiology

Accessory Fissures of the Lung are relatively common findings, and the inferior accessory fissure is one of the more frequent accessory fissures.

  • Prevalence: Although the prevalence varies in the literature, the Inferior Accessory Fissure of the Lung may be found in approximately 1% to 8% of the general population on CT scans. Higher frequencies have been reported in cadaveric dissection studies.
  • Gender and Race: There is insufficient evidence to suggest a significantly higher prevalence in a specific gender or race. The patient in this case is a 56-year-old male.

4. Clinical Presentation

  • Asymptomatic: In most cases, the Inferior Accessory Fissure is asymptomatic and is an incidental finding on Chest CT or X-ray performed for other reasons.
  • Current Case: The patient presented with chest pain, but the Inferior Accessory Fissure is not the direct cause of the pain; it is a normal variant. The cause of the chest pain is likely another cardiac, pulmonary, or musculoskeletal condition, and the patient also had a Right middle lobe atelectatic band.

5. Imaging Features

The Chest CT scan is the most accurate diagnostic tool for the Inferior Accessory Fissure.

  • Chest CT Findings:
    • Location: It is typically observed in the medial aspect of the lower lobe, visible on Axial, Coronal, and Sagittal images.
    • Appearance: It appears as a thin, linear opacity and typically separates the medial basal segment from the rest of the lower lobe.
    • Distinction: Since the inferior accessory fissure is a structure formed by invaginated visceral pleura and not lung parenchyma, it must be distinguished from linear opacities caused by pulmonary diseases.
  • Plain Chest X-ray Findings: If the Inferior Accessory Fissure is complete and provides sufficient contrast with surrounding tissues, it may be observed as a thin linear opacity, the Twining's line, on a standard Chest X-ray. However, it is generally less clear than on a CT scan.

6. Differential Diagnosis

The clinical importance of the Inferior Accessory Fissure lies in the fact that it can be misinterpreted as a pathological structure.

Differential Diagnosis Item

Distinction from Inferior Accessory Fissure

Clinical Importance

Atelectasis

Atelectasis is accompanied by a volume loss and increased opacity of the lung parenchyma, which pulls on surrounding bronchi and vessels. The Inferior Accessory Fissure is observed within normal lung volume.

Requires treatment (airway obstruction, respiratory failure, etc.)

Pulmonary Scarring

Pulmonary scarring usually results from previous inflammation or injury and can be irregularly thick and accompanied by bronchial traction. The Inferior Accessory Fissure appears as a smooth, thin line.

Requires confirmation of disease history and follow-up.

Pleurisy or Pleural Thickening

Pleural thickening is generally thicker and more irregular, and pleurisy is accompanied by symptoms like chest pain. The Inferior Accessory Fissure is a thin visceral pleural structure.

Requires treatment and follow-up.

Pulmonary Fibrosis

Pulmonary fibrosis is accompanied by widespread changes such as honeycombing and traction bronchiectasis. The Inferior Accessory Fissure is a localized normal variant.

Severe lung disease requiring treatment.

Key Takeaway: It is crucial to recognize the Inferior Accessory Fissure as a normal variant and to clearly differentiate it from pathological findings such as atelectasis or scarring based on CT image anatomy.

7. Treatment and Prognosis

  • Treatment: Since the Inferior Accessory Fissure of the Lung is a normal variant, no treatment is required. It is sufficient to explain to the patient that it is a normal anatomical variation to provide reassurance.
  • Prognosis: As the Inferior Accessory Fissure is a normal structure that does not affect the patient's health or lifespan, the prognosis is excellent. However, its presence can be important for surgical planning and defining anatomical boundaries during procedures, especially pulmonary segmentectomy.

Quiz

1. Question (Anatomical Definition of the Inferior Accessory Fissure)

Which of the following statements best describes the Inferior Accessory Fissure of the Lung?

  1. It is an abnormal fissure that separates the right upper lobe from the middle lobe.
  2. It is a normal variant within the lower lobe that separates the medial basal segment from the rest of the lower lobe.
  3. It is a finding of fibrosis commonly seen in the CT of patients with interstitial pneumonia.
  4. It is the typical form of pulmonary scarring that occurs after the resolution of pneumonia.
  5. It is the most common imaging finding of acute atelectasis caused by airway obstruction.

Answer: 2. Explanation: The Inferior Accessory Fissure of the Lung, also called Twining's line, is a normal anatomical variation formed by the invagination of the visceral pleura in the lower lobe, separating the medial basal bronchopulmonary segment from the rest of the lower lobe.

2. Question (Differential Diagnosis and Clinical Misinterpretation)

A thin, smooth linear opacity was observed in the lower lobe on a chest CT performed for chest pain, and it was diagnosed as an Inferior Accessory Fissure. What pathological entity is most commonly misinterpreted as this structure, and what is the most crucial CT finding for distinguishing them?

  1. Pulmonary nodule; Irregularity of the border
  2. Pulmonary fibrosis; Associated honeycombing opacities
  3. Lung cancer; Pattern of contrast enhancement
  4. Atelectasis or Scarring; Presence or absence of lung parenchyma volume loss
  5. Bronchiectasis; Increased diameter of the bronchi

Answer: 4. Explanation: The Inferior Accessory Fissure can be mistaken for atelectasis or pulmonary scarring43. The most crucial finding for differentiation is the change in lung parenchyma volume. Atelectasis is accompanied by volume loss, whereas the Inferior Accessory Fissure is a normal variant 444444 that does not cause surrounding lung parenchyma volume loss.

3. Question (Clinical Significance of the Inferior Accessory Fissure)

Which statement is the most accurate regarding the clinical significance of the Inferior Accessory Fissure of the Lung?

  1. It is the main cause of severe chest pain and requires immediate analgesic treatment.
  2. It has a high probability of transforming into a malignant tumor if left untreated.
  3. It is a normal variant that requires no treatment45, but it can influence the localization of the lesion if pneumonia occurs.
  4. It causes severe obstructive ventilatory impairment on pulmonary function tests.
  5. It is a lesion that must be surgically removed during lung transplantation.

Answer: 3. Explanation: The Inferior Accessory Fissure is a normal variant 464646and requires no treatment47. However, if the fissure is complete, it can lead to the localization of lesions, such as pneumonia, within the medial basal segment, affecting the spread of the disease. This reflects the clinical importance of segmental lung anatomy.

References

  1. Standring, S. (Ed.). (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
  2. Naidich, D. P., Bankier, A. A., & MacMahon, H. (Eds.). (2019). Computed Tomography and Magnetic Resonance of the Thorax (6th ed.). Elsevier.
  3. Twining, E. W. (1937). The Inferior Accessory Fissure of the Lung. The British Journal of Radiology, 10(117), 606-613. (Classical paper from which Twining's line is derived)
  4. Arakawa, H., & Fujimoto, K. (2014). Accessory Fissures of the Lung: Identification and Clinical Importance. Seminars in Ultrasound, CT and MRI, 35(1), 1-13.
  5. Nishino, M., & Soares, M. R. (2018). Accessory Fissures of the Lung: A Practical Pictorial Review. Applied Radiology, 47(4), 16-20.
  6. McLoud, T. C., & Kalantari, S. (2015). Fissures and Accessory Fissures of the Lung. In: Fleischner Society: A Centenary of Pulmonary Radiology. Springer.
  7. Ross, J. S., & Reade, C. C. (2021). Thoracic Imaging: A Practical Approach (3rd ed.). Wolters Kluwer.
  8. Sakai, M., Tanaka, K., Muroya, Y., & Nakashima, Y. (2022). The clinical significance of accessory fissures in surgical planning for segmentectomy. Journal of Thoracic Disease, 14(3), 856-865.

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