As the filariasis is a primary disease of lymphatic system it is essential to explain the anatomy of lymphatic system before going to filariasis per se. The adult worm of filariasis produces its pathogenesis in lymphatic system only when it enters this system. The microfilaria can enter into skin, blood circulation as well as lymphatic circulation but it manifests the pathogenesis only when it enters the lymphatic circulation. Hence it is necessary to explain the anatomy of lymphatic system keeping the pathogenesis of filariasis in mind. For detailed study of lymphatic system, the reader may refer to a textbook of anatomy. The lymph nodes affected in filariasis in frequency are as follows:
- Lymph nodes draining lower limb
- Lymph nodes draining upper limbs
- Abdominal lymph nodes
- Thoracic lymph nodes and thoracic duct
- Lymph nodes of cervical region (very rare).
LYMPHATICS OF LOWER LIMB
The lymph nodes draining the lymph of lower limb are distributed in inguinal region and in popliteal region. By inguinal region it means the lymph nodes are distributed near about inguinal ligament, and that of popliteal region is in popliteal fossa.
Inguinal Lymph Nodes
The inguinal lymph nodes are classified as:
- Superficial inguinal lymph nodes:
- Upper (Transverse) group.
- Lower (Longitudinal) group.
- Deep inguinal lymph nodes.
Superficial Inguinal Lymph Nodes
They are usually present superficial to the deep fascia (Fascia lata) of thigh in inguinal region. This group of lymph nodes are arranged in two sets like upper or transverse group and lower or longitudinal group. The upper group lymph nodes are distributed in parallel with inguinal ligament and they are usually 5 to 6 in number. They are sub-divided in 3 sets like (from lateral to medial) superficial circumflex iliac, superficial epigastric and superficial external pudendal. These sub-groups are named as they are present with corresponding veins in inguinal region. The lower group are 2 to 4 in number and are distributed in parallel with terminal part of long saphenous vein.
Deep Inguinal Lymph Nodes
They are present deep to fascia lata (deep fascia of thigh) and they are usually 1 to 3 in number. The lowest one is present in the lower angle formed at the union of long saphenous vein with femoral vein. The middle one is present in the femoral canal and this lymph node is otherwise named as gland of cloquet. The highest one is present lateral to the femoral ring and may be frequently absent.
The inguinal lymph nodes drain, the lower limb, penis and scrotum (except testis) (external genitalia below hymen in female), gluteal region, anal region, perianal region and adjoining part of abdominal wall below umbilicus.
Popliteal Lymph Nodes
The popliteal lymph nodes are embedded inside the popliteal pad of fat. They are usually 6 to 7 in number. One lymph node is constantly present in the angle formed by the union of short saphenous vein with popliteal vein and another present between posterior surface of knee joint and popliteal artery. Other nodes are present besides the popliteal vessels.
LYMPHATICS OF UPPER LIMB
The lymphatics of upper limb mainly drain into the lymph nodes situated in axilla. The lymphatics are arranged in two sets in upper limb, like superficial and deep. The deep set usually accompanies the neurovascular bundle of upper limb, whereas the superficial set follows the superficial veins of upper limb. However, they are terminated at axillary lymph nodes. The axillary lymph nodes are arranged in 5 groups such as:
Anterior (Pectoral) Group
These are usually 4 to 5 in number and present on the lower boarder of pectoralis minor in relation to lateral thoracic vessels.
Posterior (Subcapular) Group
These are about 6 to 7 in number and situated on the lower boarder of posterior axillary fold in relation to subcapular vessels.
Lateral (Humeral) Group
These are 4 to 6 in number and situated along the course of axillary vein, nearer to the neck of humerus.
This group of lymph nodes numbering 4 to 5, embedded in the fat present at the base of axilla. The above sets of lymph nodes drain to this group of lymph nodes. In addition to the above afferent lymphatics, this group also receives afferent lymphatic from breast as well as from back of the body as low as iliac crest.
This group is highest in number (6 to 12) as well as in position. These are present in a triangle, formed by 1st rib medially, axillary vein posteriorly and clavicle anteriorly. The central group of lymph nodes drain to this group of lymph nodes. The efferent lymphatics from these lymph nodes form the subclavian trunk which drains to either at the junction of internal jugular vein with subclavian vein or thoracic duct on left side (or jugular trunk on right side).
In addition to these lymph nodes there are other groups of lymph nodes which are distributed in the course of lymphatics. They are:
- Supratrochlear lymph nodes—They are 2 to 3 in number and situated on the medial side of humerous above medial epicondyle along the course of basalic vein.
- Infraclavicular lymph nodes—These lymph nodes are 1 to 2 in number and placed in the infraclavicular fossa along the course of cephalic vein.
- In addition to the above lymph nodes there may be occasional isolated lymph nodes distributed along with different vessels like ulnar, radial, and brachial. The axillary group of lymph nodes drain the area like upper limb, anterior and posterior surfaces of chest wall including breast, anterior wall of abdomen as below as umbilicus and back of the body as below as iliac crest.
LYMPHATICS OF ABDOMEN AND PELVIS
The lymph node of abdominal cavity and pelvis are arranged in different orders from individual to individual (Flow Chart 1.1). The abdomen and pelvis contain a large number of lymph nodes and is second in position. Out of 800 lymph nodes present in the body, the abdomen and pelvis nearly contain 200 of such lymph nodes. All organs of abdomen and pelvis are drained by the lymph nodes of abdomen and pelvis and return to the veins in neck via the thoracic duct. For better appreciation of lymph nodes in the abdomen and pelvis, a classification of lymph nodes is given below. The lymph nodes are primarily described in relation with abdominal aorta and its branches, hence they are as follows:
Preaortic Lymph Nodes
The preaortic lymph nodes are arranged mainly along the branches of coeliac trunk, superior mesenteric and inferior mesenteric arteries. They are classified as per their position of the draining organ. The organs drained by these groups of lymph nodes are: diaphragm, lower part of oesophagus, stomach, duodenum, jejunum, ileum, appendix, colon, rectum, spleen, pancreas, liver, gall bladder and their peritoneal coverings.
Para-aortic Lymph Nodes
These lymph nodes are arranged on both sides of abdominal aorta till to its termination. The organs drained by the main para-aortic lymph nodes are: kidneys, suprarenal gland, testis (ovaries), ureters, partly bladder, and partly uterus. The terminal para-aortic lymph nodes are mainly concerned with drainage of pelvic organs like, bladder, uterus, rectum, vagina, deep structure of perineum as well as pelvic wall.
Retro-aortic Lymph Nodes
Topographically there is no separate group of lymph nodes which are placed behind the aorta. These are mainly included in para-aortic group. These groups of lymph nodes are mainly placed in retro-peritoneal space. The posterior abdominal wall is mainly drained by these groups of lymph nodes. However, all abdominal and pelvic lymph nodes are inter-connected by lymphatic channels, even they also bear interconnecting relations with axillary and inguinal nodes.
LYMPHATICS OF THORAX AND THORACIC DUCT
It has been already explained that the superficial tissues of thoracic wall are drained by the axillary group of lymph nodes but the deep tissues are drained by the following lymph nodes.
Parasternal Lymph Nodes (Internal Thoracic)
These are 4 to 5 in number and are placed on each side of sternum inside the intercostal space in relation to internal mammary vessels (internal thoracic). The efferent lymphatics from these lymph nodes unite with the lymphatics of the mediastinum to form the jugular trunk (bronchomediastinal trunk) which opens at the junction of right subclavian vein with right internal jugular vein.
Intercostal Lymph Nodes
These lymph nodes are present in each intercostal space in relation to the neck of each ribs. The efferent lymphatics of these nodes drain to the thoracic duct in lower 4 to 5 spaces as-well-as left upper spaces, whereas the jugular trunk receives the efferent lymphatics from the rest of left upper spaces.
Diaphragmatic Lymph Nodes
These lymph nodes are arranged on the circumference of diaphragm in four sets such as anterior, lateral (right and left) and posterior. They all drain into thoracic duct.
The lymphatic drainage of the thoracic contents is performed by the lymph nodes present in the mediastinum. They are as follows:
Superior Mediastinal Lymph Nodes (Brachiocephalic)
These lymph nodes are 20 to 35 in number. They are placed surrounding the brachiocephalic vein in superior mediastinum. They mainly drain to either thoracic duct or jugular trunk.
Anterior Mediastinal (Parasternal) Lymph Nodes
Middle Mediastinal (Tracheobronchial) Lymph Nodes
These lymph nodes are many in number and arranged surrounding trachea as well as bronchi. They are classified as per their position.
Paratracheal (left and right) type: These are present on the side of trachea in tracheoesophageal groove.
Tracheobronchial type (superior and inferior): These lymph nodes are present above and below the principal bronchus. There may be 2 or 3 large nodes placed below the Carina.
Bronchopulmonary type: These lymph nodes are present just in the hilus of each lung.
Pulmonary type: These are present inside the substance of the lung on the course of the bronchus. All of the above lymph nodes drain mainly lungs and heart. On the left side the efferent lymphatics.
Posterior, Mediastinal Lymph Nodes
These are present behind the pericardium in relation to oesophagus and thoracic aorta. It is very much difficult to differentiate between posterior mediastinal and intercostal lymph nodes as these are placed nearer to each other.
The thoracic duct is the main canal through which major lymph conveys into blood. This duct drains the lymph from below diaphragm, left of chest wall, left side of face and neck, left upper limb as well as left organs of thoracic cavity. It is one of the 18 inches (45 cm) structures. It begins from abdomen and ends in neck. It starts from upper end of cisterna chyli which is placed over 1st and 2nd lumbar vertebrae. Hence, the thoracic duct starts from lower boarder of 12th thoracic vertebra and enters the thoracic cavity through the aortic opening in diaphragm. It ascends through the posterior mediastinum between thoracic aorta on its left and azygous vein on its right. In posterior mediastinum it remains anterior to lower thoracic vertebral column, hemiazygous vein and right intercostal arteries and posterior to oesophagus. At the level of 5th thoracic vertebra it turns to left side and enters the superior mediastinum. Here, it lies behind arch of aorta, left subclavian artery and apex of left lung. In the left side of neck it lies behind left common carotid artery left internal jugular vein and left vagus nerve. Here also it relates anterior to left phrenic nerve, sympathetic trunk and thyrocervical trunk. Then it opens into the junction of left subclavian vein with left internal jugular vein. This corresponds at the level of seventh cervical vertebra. The thoracic duct is segmented by the provision of one way valves in it. Because of its multiple connections with the lymphatics of chest and abdomen a single block in thoracic duct does not cause chylothorax or chylous ascites.
LYMPHATICS OF HEAD AND NECK
The lymph nodes concerned in the drainage of head and neck remain mostly in neck. Out of 800 lymph nodes in body about 300 lymph nodes are present in neck, still more in children. Hence, neck is the organ that accommodates maximum lymph nodes. The lymph nodes are arranged both superficial and deep to the deep fascia (Fascia Colles) of neck.
Superficial Lymph Nodes
They are present superficial to the deep fascia of neck. They are arranged both transversely and longitudinally.
- Submental nodes
- Submandibular nodes
- Postauricular nodes
- Superficial parotid nodes
- Buccal (Facial) nodes
- Occipital nodes.
- Submental nodes
- Suprahyoid nodes
- Infrahyoid nodes
- Suprasternal nodes (in space of Burn)
- Superficial cervical nodes (These are present along the course of external jugular vein)
- Anterior cervical (present on the mid line of neck below hyoid bone).
Deep Lymph Nodes
The deep group of lymph nodes are arranged both anterior and posterior to sternocleidomastoid muscle, hence anterior and posterior group.
Anterior Deep Cervical Group
- Submandibular nodes
- Jugulodigastric nodes
- Sublingual nodes.
- Retropharyngeal nodes
- Retrotracheal nodes
- Paratracheal nodes.
Posterior Deep Cervical Group
- Jugulo-omohyoid nodes
- Supraclavicular nodes (on left known as Virchow's gland)
- Subclavian nodes.
The filarial manifestation of cervical nodes is very rare. Hence, only the groups of lymph nodes are described in short in this chapter. The brain and spinal cord are devoid of lymphatic drainage for which filariasis is not manifest in this system.
CIRCULATION OF LYMPH
The lymphatic system is not provided with a pump as in circulation of blood. But it is still wonder the lymph circulates with same speed of blood. The factors responsible for circulation of lymph are:
Due to constant generation of tissue fluid a pressure is created in the tissue space. This pressure forces the extra fluid with the end products of metabolism into the lymphatics opened into the tissue space.
Due to contraction of voluntary muscles the lymph in lymphatic channel is squeezed for forward movement thus a circulation is maintained.
Negative Intrathoracic Pressure
Due to negative intrathoracic pressure and positive intra-abdominal pressure the lymph is pushed from abdomen to thorax. This movement usually occurs in each movements of diaphragm.
Negative Neck Veins Pressure
The lymph always enters the blood circulation through the veins in the neck. The venous pressure in neck is always in negative side. Hence the lymph is sucked into veins in neck during diastole of right atrium.
Pulsation of Arteries
Because of arterial pulsation the lymphatics are pressed constantly leading to the movement of lymph.
The lymphatics are also provided with smooth muscles. This plays a very minor role in lymph flow.