Ultrasound imaging of the axilla

Abstract

Axilla is a pyramidal-in-shape "virtual cavity" housing multiple anatomical structures and connecting the upper limb with the trunk. To the best of our knowledge, in the pertinent literature, a detailed sonographic protocol to comprehensively assess the axillary region in daily practice is lacking. In this sense, the authors have briefly described the anatomical architecture of the axilla-also using cadaveric specimens-to propose a layer-by-layer sonographic approach to this challenging district. The most common sonographic pathological findings-for each and every anatomical compartment of the axilla-have been accurately reported and compared with the corresponding histopathological features. This ultrasound approach could be considered a ready-to-use educational guidance for the assessment of the axillary region. CRITICAL RELEVANCE STATEMENT: Axilla is a pyramidal-in-shape "virtual cavity" housing multiple anatomical structures and connecting the upper limb with the trunk. The aim of this review article was to describe the anatomical architecture of the axilla, also using cadaveric specimens, in order to propose a layer-by-layer sonographic approach to this challenging district.

Keywords: Axilla; Histopathology; Ultrasound.

Fig. 1

From bony to muscular edges of the axillary region. The axilla is an irregular-in-shape anatomical region (light blue) located between the ribs, the scapula, the clavicle (Cla) and the proximal humerus (Hum) (A). The pectoralis major (P Maj) and minor (P Min) muscle in the anterior compartment, the coracobrachialis (Cor B) and short head of the biceps brachii muscle (shBB) in the lateral section, the serratus anterior muscle (black arrowheads) medially; and the subscapularis muscle (SSC), the teres major and latissimus dorsi muscle (LD) in the posterior compartment, can be considered the “soft” anatomical landmarks to better define this complex anatomical space (B). Cor: coracoid; yellow asterisk: cervicoaxillary canal; black dotted lines: quadrants

Fig. 2

Schematic drawings of the axillary anatomical elements. The sagittal view (A) shows how the clavipectoral fascia (white line) wraps the pectoralis major (PM) muscle, the pectoralis minor (Pm) muscle and the subclavius (S) muscle; and blends with the axillary fascia (yellow line) and the suspensory ligament of the axilla (green lines). Likewise, the posterior view (B) shows the quadrilateral space (QS), the triangular space (TS) and the triangular interval (TI) originating by the crossing of the teres minor (Tm) muscle, the teres major (TM) muscle, the long head of the triceps (lhT) muscle and the humerus (H). Cla: clavicle

Fig. 3

Cadaveric anatomy of the axillary region. Using an anterior approach (A) and lifting the skin and pectoralis major muscle (P Maj), the axillary fat tissue (yellow arrowheads) can be easily observed filling the anatomical space between the upper limb and the trunk. Using an inferior approach B and selectively removing the fat pad, a panoramic view of the axillary cavity can be acquired until its medial border—i.e., the serratus anterior (SA) muscle. Of note, the more superficial vein is the lateral thoracic vein (light blue arrowhead); instead, the axillary vein (AV) is located in the deep portion of the axilla (B). P Min: pectoralis minor muscle; LD: latissimus dorsi muscle

Fig. 4

Neurovascular bundle of the axilla. The ulnar nerve (UN), the radial nerve (RN) and the median nerve (MN) wrap the axillary artery (AA) in a clockwise fashion (curved dark arrow) at the axillary base (A). They present a typical honeycomb pattern (yellow dotted line) and can be easily differentiated from the vascular elements using the color/power Doppler (B). V: vein; Hum: humerus; black dotted line: axillary sheath

Fig. 5

Epidermal inclusion cyst of the axilla. A hypoechoic and rounded formation (white asterisks) can be easily observed originating from the dermis, crossing the dermo-hypodermal interface (green arrowhead) and protruding within the subcutaneous tissue (SUBC) with a typical artifact of posterior acoustic enhancement (white arrow) (A). Of note, using high-frequency probes, the canaliculus (yellow arrowhead) between the cyst (white asterisks) and epidermis (white arrowheads) can be clearly recognized (B). High-sensitivity color Doppler does not show intra-cystic vascular signals but only some small vessels in the surrounding soft tissues (C). The histological examination confirms the presence of the unilocular and spherical cyst (D) lined by a multi-layered epithelium (black arrowheads) and containing laminated keratin (Ker). H&E: hematoxylin & eosin

Fig. 6

Hidradenitis suppurativa of the axilla. Massive hypoechoic thickening (yellow arrowheads) of the dermis (A) with pathological hypervascularization on high-sensitivity color Doppler (B) can be promptly observed at the level of the axillary skin. Very high-frequency ultrasound probes (C, D) better define additional sonographic signs of hidradenitis suppurativa as the superficial, saclike fluid collections (white asterisks) and the ballooning of hair follicles (white dotted line). HH Humeral head

Fig. 7

Pseudofolliculitis cutis of the axilla. Avoiding an excessive squeeze of the superficial soft tissues, also a small hypoechoic “mass” (yellow arrowheads) can be easily identified within the dermal layer of the axilla (A). Of note, a gentle sono-palpation (B) can be performed to promptly confirm the solid texture of inflammatory tissues (white dotted line) wrapping the hyperechoic “ingrown” hairs (white arrowhead). C–D The histological examination (C) confirms the simultaneous presence of an acute inflammatory infiltrate (1) with neutrophilic and eosinophilic granulocytes reacting against the epithelium of the follicular structure (black arrowheads)—external and internal root sheath—and a chronic inflammatory infiltrate (2) with lymphocytes, plasma cells and macrophages reacting against broken hair shafts and generating foreign body giant cell reaction in the peri-follicular area. Of note, inflammatory cells (3) are also leading to a progressive disruption of the follicular papilla (FP) (D). H&E: hematoxylin & eosin

Fig. 8

Subcutaneous abscess vs. hematoma of the axilla. A small, irregular-in-shape, fluid collection (white asterisk) can be identified within the axillary subcutaneous tissue (SUBC) not extending to the overlying dermo-epidermal complex (yellow arrowheads) nor to the underlying muscle planes (A); and high-sensitivity color Doppler shows intense hypervascularization of the surrounding soft tissues confirming inflammatory/infectious local phenomena (B). Of note, axillary hematoma (yellow asterisks) can present a sonographic pattern mimicking an abscess (C), but vascular signals are usually absent within the surrounding fat pad (D). White arrowhead: posterior acoustic enhancement

Fig. 9

Sono-histological architecture of lymph nodes. Histological samples of reactive lymph nodes (A, C) clearly show how the feeding microvasculature (yellow arrowhead) enters through the hilum (H) to progressively branch smaller ramifications (white arrowheads) toward the cortex (C)—i.e., the unipolar vascularization. Of note, all the aforementioned histological elements can be easily identified using modern high-frequency probes and high-sensitivity color/power Doppler (B). Vb: vascular bundle; H&E: hematoxylin & eosin

Fig. 10

A glance-in-depth at the multiparametric sonographic assessment of lymph nodes. Benign lymphadenopathy (A) usually preserves the hyperechoic hilum (H), the hypoechoic cortex (C) and a unipolar vascularization with feeding vessels branching from the hilum to the cortex (B, C) Instead, a disappearance of the hilum (D), a rounded/globular shape (E) and a multipolar vascularization (F) with feeding vessels (red arrowheads) originating from the peripheral portion, can be considered the most common sonographic findings of malignant lymphadenopathy. Of note, considering the extreme variability of sonographic patterns of lymph nodes, several “atypical” findings can be identified in daily practice—e.g., serpiginous protrusions (white arrowhead) related to ectatic vessels (red arrowhead) (G, H) a normal lymph node (white dotted line) coupled with a “snowfall” lymph node (green arrowheads) due to particle depositions (I, L) eccentric thickening of the cortex (white asterisks) with lateral displacement of the hilum (H) (M). PD: power doppler; CD: color doppler

Fig. 11

Additional sonographic findings of the axilla. Longitudinal scans (A–C) show a massive thrombosis (yellow asterisks) of the axillary vein (AV) coupled with a globular lymph node (LN) in a patient with neoplastic disease. Accessory breast tissue (white dotted line) is usually located in between the subcutis (SUBC) and muscle planes (D) showing a peculiar sonographic pattern with hyperechoic fibroductal matrix, hypoechoic fat islands and poor vascular signals in normal conditions (E). Articular effusion (white asterisks) distending the axillary recess (yellow line) can be easily identified using the standing position in a patient with glenohumeral osteoarthritis (F). HH Himeral head, G Glena, L Labrum

Fig. 12

Axillary web syndrome and lymphocele. Axillary web syndrome (A) is clinically characterized by very superficial, painful and retractile cords (black arrowhead). Transverse (B) and longitudinal (C) sonogram—using a high-frequency probe—shows the hypoechoic band (white arrowheads) coursing within the dermal layer (Der) of the axillary region. Of note, a large amount of gel allows a prompt identification of the skin bulging (yellow dotted line) where the lymphatic cord (white arrowhead) should be sonographically assessed (B). A large lymphocele (D) presents as anechoic irregular-in-shape fluid collection and its peduncular connection (yellow arrowhead) with the overlying surgical incision is clearly visible as well. SUBC: subcutaneous tissue