The lymphatic system Introduction
The lymphatic system functions is the main immunologic surveillance mechanism of the body. The lymphatic system consists of a rich network of interconnected lymphatic vessels which exist in virtually every tissue of the body. lnterstitial fluids, normal and abnormal cells, microorganisms and antigens are carried through lymphatic channels to loco-regional lymph nodes which function as peripheral lymphoid organs. Lymphatic fluid eventually returns to the venous circulation mainly via the thoracic duct which drains in the right subclavian vein. The spleen, a lympho-reticular organ serves as a major site of clearance for circulating microorganisms, antigens, damaged and abnormal circulating cells. Enlargement of lymph nodes, spleen, thymus or liver is one of the most common clinical findings and can be secondary to an increase in the number of normal cells such as lymphocytes and macrophages during antigenic stimulation, infiltration by inflammatory cells, proliferation of malignant lymphocytes or macrophages, infiltration by metastatic malignant cells and infiltration by lipid-Iaden macrophages in lipid storage diseases. It is therefore not surprising that lymph node involvement can be observed in a wide spectrum of infectious, autoimmune and neo-plastic disorders (Table 1). Under 30 years of age 80% of lymphadenopathies are benign whereas in patients older than 50 years, only 40% are benign.
Table 1.
Pathologic conditions in which lymph node involvement can be observed.
INFECTIOUS DISEASES
a - Viral infections: infectious hepatitis, infectious mononucleosis, AIDS, rubella, varicella, herpes
b - Bacterial infections: Streptococci, staphylococci, salmonella, brucella, listeria monocytogenes, pasteurella pestis, cat-scratch diseases, yersinia
c - Fungal infections: coccidiodomycosis, histoplasmosis
d - Chlamydial infections: lymphogranuloma venereum, trachoma
e - Mycobacterial infections: tuberculosis, leprosy
f - Parasitic infections: trypanosomiasis, microfilarissis, toxoplasmosis
g - Spirochetal infections: syphilis, yaws, leptospirosis
MALIGNANT DISEASES
a - Hematologic: lymphomas, leukemias and malignant histiocytosis
b - Metastatic: (common) melanoma, tumors of lung, breast, prostate, gastrointestinal tract, kidney and head and neck. Seminona, Kaposi's sarcoma
ENDOCRINE DISEASES: hyperthyroidism
LIPID STORAGE DISEASES: Gaucher, Niemann-Pick diseases
DISEASES OF UNKNOWN ETIOLOGY: sarcoidosis, amyloidosis, giant follicular
lymph node hyperplasia (Castleman's disease), lymphomatoid granulomatosis, familian mediterranean fever,
sinus histiocytosis, muco-cutaneous
lymph node syndrome
ROLE OF DIAGNOSTIC IMAGING
The radiologist is often called upon to help evaluate diseases involving the lymphatic system. Imaging is primarily used to
- detect the presence of lymphatic system involvement in regions in accessible to physical examination
- stage the extent and degree of lymphatic involvement
- guide and perform image guide d tissue sampling whenever necessary to obtain a specific diagnosis in regions inaccessible to excisional biopsy or for staging purposes in malignancies
- to assess the level of response to therapy by monitoring size changes and other features such as signal intensity, contrast uptake and metabolic activity.
Elias Zerhouni