Horizontal Fissure

1. Overview

The horizontal fissure, also known as the transverse fissure of the right lung, is an anatomical division that separates the right upper lobe from the right middle lobe. It is one of two fissures present in the right lung—the other being the oblique fissure. The left lung does not have a horizontal fissure. This structure plays an important role in organizing lobar anatomy and serves as a surgical and radiological landmark.

2. Location

The horizontal fissure is located in the right lung, and its approximate surface projection is:

• From the oblique fissure posteriorly, at about the level of the 4th rib in the mid-axillary line

• Running anteriorly and slightly superiorly to reach the 4th costal cartilage at the sternal margin

Internally, it divides the right superior (upper) lobe from the right middle lobe, while the oblique fissure separates the middle lobe from the right lower (inferior) lobe.

3. Structure

The horizontal fissure is formed by a double layer of visceral pleura that invaginates into the lung parenchyma. It contains:

• A potential space allowing relative movement between lobes during respiration

• Associated interlobar vessels, lymphatics, and bronchial branches (at the hilum level)

On imaging, the fissure appears as a thin line on lateral or oblique radiographs and may be more clearly defined on CT scans.

4. Function

While the fissure itself is not a functional organ, it serves important organizational and clinical purposes:

• Structural separation: Divides lung lobes into distinct anatomical and functional units

• Lobar movement: Allows independent expansion and contraction of lobes during respiration

• Surgical guidance: Defines boundaries for segmentectomy or lobectomy in thoracic surgery

5. Physiological Role(s)

The horizontal fissure supports key respiratory system functions indirectly by:

• Compartmentalizing the lung: Localizes infections, tumors, or other pathology to specific lobes, aiding in disease containment

• Optimizing lung mechanics: Lobar segmentation allows differential lung compliance and air distribution

• Enabling segmental ventilation and perfusion: Facilitates more efficient gas exchange and regional lung function

6. Clinical Significance

Radiological Identification

The horizontal fissure is often seen on chest X-rays and CT scans. It can appear as a horizontal line in PA or lateral chest views. Blunting or obscuration may indicate:

• Pleural effusion

• Lobar pneumonia (especially middle lobe)

• Lung mass or collapse

Middle Lobe Syndrome

Recurrent or chronic atelectasis of the right middle lobe, often due to obstruction or poor ventilation, can be localized due to the anatomy defined by the horizontal fissure. Symptoms may include:

• Persistent cough

• Recurrent infections

• Volume loss seen in the middle lobe region on imaging

Surgical Landmark

The horizontal fissure is a critical reference point in lobectomies, segmental resections, and thoracoscopic procedures. Accurate identification reduces complications and preserves as much healthy lung tissue as possible.

Fissure Variants

Incomplete or absent horizontal fissures are common anatomical variations. In such cases:

• The lobes may be partially fused

• Surgical planning becomes more complex

• May lead to altered disease spread patterns (e.g., in pneumonia or carcinoma)

Pleural Effusion Tracking

Pleural fluid may track along the horizontal fissure, especially in supine patients. This can be visualized as a horizontal density on imaging studies, sometimes mimicking consolidation or mass effect.