Hair-on-End Sign on Skull CT: When the Skull Reveals Chronic Hemolytic Anemia

A Rare Radiologic Clue That Should Never Be Missed

Most radiologic signs are subtle. Some, however, are unforgettable.

One of the most striking examples is the Hair-on-End Sign, a skull imaging appearance in which vertical bony trabeculae project outward from the calvarium, creating the impression of hair standing upright. Although uncommon in modern clinical practice, this imaging finding remains a classic indicator of long-standing bone marrow hyperplasia, particularly in patients with chronic hemolytic anemia.

Recognizing this pattern is far more than identifying an interesting radiologic curiosity. It can provide a critical diagnostic clue that immediately directs clinicians toward underlying hematologic disorders such as sickle cell disease or thalassemia major, sometimes before laboratory confirmation is complete.

Clinical Case

A 35-year-old man with a known history of sickle cell disease presented to the emergency department during a vaso-occlusive crisis accompanied by severe headache. Initial evaluation included a non-contrast CT examination of the head to exclude acute intracranial pathology.

Instead of hemorrhage or mass effect, CT demonstrated a markedly thickened calvarium with vertically oriented trabecular proliferation extending through the diploic space, producing the characteristic Hair-on-End appearance.

The patient's headache improved rapidly with conservative treatment, and no neurological complication developed during hospitalization.

Imaging Findings

CT Features

The non-contrast skull CT demonstrated:

Expansion of the diploic space
Thickened calvarial bone
Prominent vertically oriented trabeculae
Coarse marrow architecture
Classic Hair-on-End appearance

These findings reflect chronic compensatory marrow expansion rather than acute bone pathology.

Unlike fractures or destructive lesions, cortical integrity is generally preserved while the internal architecture becomes dramatically remodeled.

Why Does the Hair-on-End Sign Occur?

The answer lies in bone marrow physiology.

Patients with chronic hemolytic anemia continuously destroy red blood cells faster than they can be replaced.

To compensate, the body stimulates:

Increased erythropoietin production
Massive erythroid hyperplasia
Progressive marrow expansion

Because the skull normally contains active marrow, prolonged stimulation causes enlargement of the diploic space and remodeling of the bony trabeculae.

Eventually, vertical trabeculae become thickened enough to produce the characteristic radiographic appearance.

This process represents adaptive remodeling, not primary bone disease.

Diseases Associated with the Hair-on-End Sign

Although classically linked to thalassemia, several disorders may produce this imaging appearance.

1. Sickle Cell Disease

The most recognized modern cause.

Repeated hemolysis results in lifelong marrow hyperplasia that remodels the skull.

2. Thalassemia Major

Historically the most common cause.

Before regular transfusion programs became available, Hair-on-End appearance was frequently observed on plain skull radiographs.

3. Hereditary Spherocytosis

Chronic hemolysis occasionally produces similar marrow expansion.

4. Severe Iron Deficiency (Rare)

Extremely prolonged anemia may induce mild marrow hypertrophy, although the classic appearance is unusual.

5. Cyanotic Congenital Heart Disease

Long-standing hypoxia stimulates erythropoiesis and may produce comparable skeletal remodeling in rare cases.

Differential Diagnosis

Radiologists should distinguish Hair-on-End Sign from other calvarial abnormalities.

Disease	Imaging Appearance
Osteomyelitis	Bone destruction with irregular cortical erosion
Langerhans Cell Histiocytosis	Well-defined punched-out lytic lesions
Fibrous Dysplasia	Ground-glass matrix with expansion
Paget Disease	Mixed lytic and sclerotic thickening
Hair-on-End Sign	Vertical trabecular proliferation with diploic expansion

Recognition of the trabecular orientation is the key diagnostic feature.

Clinical Importance

Although modern treatment has reduced its frequency, Hair-on-End Sign remains clinically valuable because it suggests:

Chronic hemolytic anemia
Long-standing marrow hyperactivity
Increased erythropoietic demand
Potential systemic complications

Its presence should prompt careful review of the patient's hematologic history and laboratory findings.

In many developing regions where sickle cell disease remains prevalent, this imaging finding continues to have important diagnostic relevance.

The Pathophysiology Behind the Imaging

The Hair-on-End Sign is a vivid demonstration of how chronic hematologic disease can reshape the skeleton.

In sickle cell disease (SCD), a single point mutation in the β-globin gene (HBB) produces abnormal hemoglobin S (HbS). Under conditions such as hypoxia, dehydration, or acidosis, HbS polymerizes, causing erythrocytes to assume the characteristic sickle shape.

These rigid cells have two major consequences:

Chronic hemolysis, resulting in persistent anemia.
Microvascular occlusion, leading to ischemia and repeated vaso-occlusive crises.

To compensate for ongoing red blood cell destruction, the kidneys increase erythropoietin production, stimulating the bone marrow to generate more erythrocytes. Over many years, this continuous demand expands the marrow cavity, particularly within the skull, producing widening of the diploic space and vertically oriented trabeculae.

Thus, the Hair-on-End Sign is not a disease itself—it is a radiologic manifestation of prolonged compensatory erythropoiesis.

The Evolution of Imaging Findings

Historically, the Hair-on-End Sign was identified on plain skull radiographs, where vertical bony spicules projected perpendicular to the calvarium.

Today, multidetector CT and MRI provide much more detailed visualization.

Skull Radiography

Classic "crew-cut" or "hair-on-end" appearance
Widened diploic space
Thickened calvarium

Computed Tomography (CT)

Superior depiction of trabecular architecture
Thickened vertical trabeculae
Expansion of marrow space
Excellent evaluation of acute complications such as hemorrhage or infarction

Magnetic Resonance Imaging (MRI)

MRI demonstrates:

Marrow hyperplasia
Fatty marrow replacement
Bone infarction
Cerebral ischemia
Silent cerebral infarcts frequently associated with sickle cell disease

Modern imaging therefore evaluates both the chronic skeletal manifestations and the acute neurological complications of SCD.

An Additional Pediatric Case

The educational material accompanying this case also presents a remarkable pediatric example.

A 9-year-old girl from Uganda presented with acute stroke, vomiting, seizures, and altered consciousness. CT imaging revealed:

Intraparenchymal hemorrhage
Multiple cerebral infarctions
Classic Hair-on-End Sign
Expanded diploic marrow space

Subsequent hemoglobin electrophoresis confirmed sickle cell disease. The patient improved following treatment with hydroxyurea, intravenous fluids, antibiotics, and analgesics before being discharged in stable condition.

This case illustrates an important clinical lesson:

The Hair-on-End Sign is not merely an imaging curiosity—it can serve as a radiologic clue to an undiagnosed systemic disease presenting with life-threatening neurological complications.

AI in Radiology: Can Artificial Intelligence Detect the Hair-on-End Sign?

The increasing integration of artificial intelligence into radiology raises an intriguing question:

Could AI automatically recognize rare skeletal signs such as the Hair-on-End appearance?

The answer is increasingly yes, although several challenges remain.

Modern deep learning algorithms excel at identifying reproducible imaging patterns. With sufficient annotated training data, AI systems could potentially detect:

Calvarial thickening
Expansion of the diploic space
Vertical trabecular orientation
Abnormal skull morphology
Associated cerebral infarction or hemorrhage

Beyond simple detection, AI could integrate imaging findings with laboratory values and electronic health records to suggest underlying chronic hemolytic disorders.

For example, an AI-assisted workflow might:

Detect abnormal skull trabeculation on CT.
Flag possible marrow hyperplasia.
Review clinical history for anemia or sickle cell disease.
Recommend hematologic correlation.
Alert the radiologist to potential systemic complications.

Such decision-support tools could be particularly valuable in emergency settings, where uncommon radiologic signs may otherwise be overlooked.

Radiology Pearls

Pearl 1

Hair-on-End Sign reflects marrow expansion, not primary skull disease.

Pearl 2

The most common causes are chronic hemolytic anemias, particularly:

Sickle cell disease
Thalassemia major

Pearl 3

CT demonstrates the trabecular changes more clearly than conventional radiography.

Pearl 4

MRI is the preferred modality for evaluating associated neurological complications, including silent cerebral infarction.

Pearl 5

Whenever this sign is identified, radiologists should recommend evaluation for chronic hemolytic disorders if no diagnosis is already known.

Figure Placement Guide

Figure 1. Non-contrast Skull CT (Coronal and Axial): Demonstration of the classic Hair-on-End Sign with vertically oriented trabeculae and expansion of the diploic space.

Figure 2. Pediatric Case: Skull CT demonstrating Hair-on-End Sign associated with acute stroke in a child with sickle cell disease.

Figure 3. Pathophysiologic Mechanism

Figure 4. AI-Assisted Diagnostic Workflow

Key Clinical Takeaways

The Hair-on-End Sign is far more than a memorable radiologic pattern—it is a visual marker of chronic systemic disease. Although encountered less frequently today because of earlier diagnosis and improved treatment of hemolytic disorders, recognizing this sign remains essential for radiologists, emergency physicians, neurologists, hematologists, and trainees.

Several practical lessons emerge from this case:

Always consider chronic hemolytic anemia when vertical calvarial trabeculation is identified.
Review the patient's hematologic history before attributing the finding to a primary skull abnormality.
Evaluate for neurological complications, particularly in patients presenting with headache, altered mental status, or focal neurological deficits.
Differentiate the Hair-on-End Sign from destructive skull lesions, as the management pathways differ completely.
Recognize that modern CT and MRI provide earlier and more detailed detection than conventional radiography.

For radiologists, this classic imaging appearance serves as a reminder that bones often reflect systemic disease long before symptoms become severe.

Future Perspectives

Advances in medicine continue to reshape the outlook for patients with sickle cell disease.

Current therapeutic developments include:

Hydroxyurea therapy to reduce vaso-occlusive crises
Chronic transfusion protocols for stroke prevention
Stem cell transplantation in selected patients
Gene-editing technologies targeting the underlying HBB mutation
AI-assisted imaging tools for early detection of skeletal and neurological complications

As these therapies become more widely available, classic manifestations such as the Hair-on-End Sign may become increasingly uncommon. Nevertheless, radiologists must remain familiar with these findings because patients from underserved regions or areas with limited healthcare access may still present with advanced disease.

Conclusion

Some radiologic signs become iconic because they tell an entire clinical story in a single image.

The Hair-on-End Sign is one of those signs.

It reflects years of chronic marrow stimulation, compensatory erythropoiesis, and the body's remarkable attempt to overcome persistent hemolytic anemia. While visually striking, its true importance lies in the systemic disease it represents.

For patients with sickle cell disease, timely recognition of this imaging finding can prompt comprehensive evaluation, appropriate hematologic management, and surveillance for serious complications such as stroke and organ damage.

As artificial intelligence becomes increasingly integrated into radiology, classic imaging signs like the Hair-on-End appearance may also become valuable targets for automated pattern recognition. Rather than replacing radiologists, AI has the potential to enhance diagnostic confidence by identifying uncommon but clinically significant findings and ensuring that no critical clue is overlooked.

Ultimately, the Hair-on-End Sign reminds us that radiology is not simply about interpreting images—it is about understanding the biology, physiology, and human story that every image reveals.

References

Rees DC, Williams TN, Gladwin MT. Sickle-cell disease. The Lancet. 2010;376(9757):2018–2031.
Kato GJ, Piel FB, Reid CD, et al. Sickle Cell Disease. Nature Reviews Disease Primers. 2018;4:18010.
Ware RE, de Montalembert M, Tshilolo L, Abboud MR. Sickle cell disease. The Lancet. 2017;390:311–323.
Vichinsky EP. Clinical manifestations of sickle cell disease. Hematology Am Soc Hematol Educ Program. 2016.
Osunkwo I, et al. American Society of Hematology Guidelines for Sickle Cell Disease. 2020.
ACR Appropriateness Criteria® – Head Trauma and Cranial Imaging.
Original educational case material describing the Hair-on-End Sign in sickle cell disease and associated imaging findings.

Hair-on-End Sign on Skull CT: A Classic Imaging Manifestation of Sickle Cell Disease Every Radiologist Should Recognize