Radiological presentation of extramedullary hematopoiesis
On computed tomography (CT) scans, foci of extramedullary hematopoiesis are most commonly identified as masses with homogeneous soft-tissue attenuation (soft-tissue attenuation). A characteristic topographic feature is that these hematopoietic tissue proliferations are typically bilateral and appear fairly symmetrical.
Interestingly, the structure of these masses can change depending on the course of the underlying disease. For instance, if the patient's anemia resolves, a fat component begins to be visualized within the pathological tissue, which serves as an important diagnostic marker for the dynamics of the process.
Clinical case: hemolytic anemia
For a better understanding of radiological semiotics, let us consider a classic case: a 43-year-old male presenting with hemolytic anemia.
- Chest X-ray (CXR): Reveals a large mediastinal mass. Meanwhile, the cardiac contours remain sharp and are not obscured. This phenomenon is termed the negative silhouette sign. It reliably indicates that the mass is located in the posterior mediastinum, behind the heart, and does not abut it directly.
- Contrast-enhanced CT: Confirms the presence of a posterior mediastinal mass. Its structure clearly differentiates areas of both fat and soft-tissue attenuation. Coronal and sagittal reconstructions clearly demonstrate the close apposition of the identified mass directly to the vertebral column.
Mesenchymal tumors: lymphangiomas (cystic hygromas)
In addition to hematopoietic foci, the differential diagnosis of mediastinal masses includes rare benign congenital malformations—lymphangiomas (Lymphangiomas) or cystic hygromas (Cystic Hygromas).
Morphologically, they represent focal proliferation of highly differentiated lymphatic tissue. The masses consist of complex intertwining lymphatic vessels or extensive cystic spaces. Clear or straw-colored fluid accumulates within these cavities. The vast majority of such lesions are present at birth and are detected within the first two years of life.
Localization and histological classification
The most common sites of involvement are the neck and axillary region. Approximately 10% of cervical lymphangiomas are capable of extending into the mediastinal cavity. Mediastinal lymphangiomas can arise in any compartment, but the predilection site is the anterior or superior mediastinum.
Within the mediastinum, they may exist as entirely isolated masses, but more frequently represent direct extension of a growing tumor from the neck region.
Depending on the caliber of the lymphatic vessels, three histological types are distinguished:
- Simple / capillary (simple / capillary).
- Cavernous (cavernous).
- Cystic / hygroma (cystic / hygroma) — the most common type.
Clinical manifestations and potential complications
Symptoms directly depend on the patient's age and the exact localization of the malformation:
- Cervicomediastinal forms: typical for early childhood. They manifest visually and upon palpation as a neck mass.
- Isolated mediastinal forms: characteristic of older children and adults. In most cases, this is an incidental finding during examination (an entirely asymptomatic mediastinal mass).
Due to their soft, compressible consistency, lymphangiomas extremely rarely cause clinical symptoms. However, upon reaching large sizes and compressing surrounding mediastinal structures, the patient may develop chest pain, persistent cough, and dyspnea (dyspnoea).
The main complications include airway compromise (airway compromise), secondary infection, and accumulation of lymph in the pleural (chylothorax / chylothorax) or pericardial (chylopericardium / chylopericardium) cavities.
Briefly: answers to topical questions
What diseases are the most frequent cause of extramedullary hematopoiesis?
The most common cause of extramedullary hematopoiesis is primary myelofibrosis (primary myelofibrosis). The process also occurs as a compensatory reaction in the following pathologies:
- Myeloproliferative neoplasms (myeloproliferative neoplasms) — polycythemia vera and primary myelofibrosis.
- Hemolytic anemias — hereditary (thalassemias, microspherocytosis, sickle cell anemia, G6PD deficiency) and acquired.
- β-Thalassemia major (β-Thalassemia Major) — severe anemia leads to massive extramedullary hematopoiesis.
With which posterior mediastinal masses must foci of extramedullary hematopoiesis be differentiated?
Foci of extramedullary hematopoiesis must be differentiated from other mass lesions of the posterior (paravertebral) mediastinum. These include:
- Neurogenic tumors (neurogenic tumours) — neuroblastoma, ganglioneuroblastoma, ganglioneuroma, schwannoma.
- Cystic lesions — neuroenteric cyst (neuroenteric cyst), meningocele (meningocele), pancreatic mediastinal pseudocyst.
- Spinal tumors — primitive neuroectodermal tumor (PNET), Ewing sarcoma.
- Infectious and traumatic lesions — spinal abscess, vertebral hematoma.
- Hernia — Bochdalek hernia (Bochdalek hernia).
What are the MRI features of extramedullary hematopoiesis foci?
On MRI, foci of extramedullary hematopoiesis are visualized as paravertebral soft-tissue masses. Their main magnetic resonance features include:
- Signal intensity (SI) — the masses themselves have heterogeneous SI on T1- and T2-weighted images.
- Signs of inactivity — presence of fat tissue signal within the masses.
- Signs of activity — contrast enhancement (enhancement).
- Associated changes — signs of bone marrow hyperplasia or fibrosis, manifested by diffuse or heterogeneous signal intensity decrease on T1-weighted images.
The negative silhouette sign can be remembered as the "Rule of Different Planes": if the cardiac contour is sharp, it means the mass does not touch its border, but is hidden behind it (in the posterior mediastinum).
Class and exam questions
What does extramedullary hematopoiesis look like on CT?
It manifests as bilateral, symmetrical masses of soft-tissue attenuation, in which a fat component may appear upon the resolution of anemia.
Where are mediastinal lymphangiomas most frequently localized?
In the mediastinum, they can occur in any compartment, but the most frequent localization is the anterior or superior mediastinum.
At what age are cervicomediastinal lymphangiomas most commonly detected?
The majority of such lesions are present at birth and are detected as a palpable neck mass within the first 2 years of life.
What complications are characteristic of mediastinal cystic hygromas?
Complications include airway compromise, infection of cystic cavities, as well as the development of chylothorax and chylopericardium.
What else to study on this topic
- Pathophysiology of extramedullary hematopoiesis in chronic anemias
- Differential diagnostic radiology of posterior mediastinal masses
- Surgical and conservative management of cervical cystic hygromas
- Causes of chylothorax and chylopericardium in mediastinal masses
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