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KEY FACTS

Terminology
- Idiopathic enlargement of subarachnoid spaces (SAS) during infancy
Imaging
- Primary imaging modality: US - CT/MR used if fontanelle closing or to further investigate atypical clinical/US findings
- Best clue: Enlarged SAS and ↑ orbitofrontal circumference (OFC) (> 95th percentile) - Ventricles may be mildly enlarged
- Symmetric bifrontal and bitemporal SAS
- All modalities show veins coursing through SAS
- SAS follow CSF appearance on all modalities
- No compression of veins or gyri
- No inward displacement of arachnoid membrane by subdural fluid; small, nonhemorrhagic, subdural collections seen in ~ 4% of patients with enlarged SAS
Top Differential Diagnoses
- Atrophy
- Acquired progressive communicating hydrocephalus
- Nonaccidental trauma (NAT)
Pathology
- Etiology uncertain: Immature CSF drainage pathways likely
- Family history of macrocephaly > 80%
Clinical Issues
- Mild developmental delay alone should not prompt further imaging or subspecialty evaluation - Further evaluation required only in setting of focal neurologic signs &/or developmental regression
- Consider NAT if enlarged extraaxial spaces atypical - Moderate/large/complex subdural collection → NAT work-up
- SAS enlargement and developmental delay typically resolve without therapy by 2 years of age
- No treatment necessary
Diagnostic Checklist
- Further evaluation with brain MR or CT if US atypical
- Even small/simple subdural collections should be discussed with referring clinician to identify any concerns for NAT that merit further work-up

TERMINOLOGY

Synonyms
- Benign enlargement of subarachnoid spaces of infancy (BESSI)
- Benign external hydrocephalus
- Benign extracerebral fluid collections of infancy
- Benign communicating hydrocephalus
- Physiologic extraventricular obstructive hydrocephalus
- Benign macrocephaly of infancy
Definitions
- Enlarged subarachnoid spaces (SAS) in patient < 1 year of age with macrocrania [head circumference (HC) > 95%]

IMAGING

General Features

Best diagnostic clue

- Symmetric bilateral SAS enlargement ± mild ventriculomegaly
        - Patient with normal or mildly delayed development

Location

- Enlarged SAS in infant with macrocrania
        - Symmetric at bifrontal and bitemporal SAS

Size

- Normal SAS values differ significantly between studies
        - Normal maximum width peaks at 28 postnatal weeks (7 months of age)
        - Interhemispheric width: 95th percentile: ~ 8 mm
                  - Widest distance between hemispheres
        - Craniocortical width: 95th percentile: ~ 10 mm
                  - Widest vertical distance between brain and inner table of calvarium
        - Sinocortical width: 95th percentile: ~ 7 mm
                  - Widest distance between cortex and superior sagittal sinus

Morphology

- CSF space follows (not flattens) gyral contour

Radiographic Findings
- Radiography - ↑ craniofacial ratio

CT Findings

NECT

- Enlarged SAS with normal sulci; no hemorrhage
- Enlarged cisterns (especially suprasellar/chiasmatic)

CECT

- Demonstrates veins traversing SAS
- No abnormal meningeal enhancement

MR Findings

T1WI

- Normal brain parenchyma without edema
- Small subdural collections sometimes visible

T2WI

- Arachnoid membranes
- Small nonhemorrhagic subdural collections in ~ 4%

FLAIR

- SAS fluid follows CSF signal on all sequences
- Incomplete signal suppression in subdural collections

DWI

- ↑ fractional anisotropy and mean diffusivity in brain of patients with enlarged SAS compared to controls
        - Normalizes over time with resolution of SAS enlargement

T1WI C+
```
- Demonstrates veins traversing SAS
```
SSFSE - May be used for follow-up to avoid sedation in children

Ultrasonographic Findings

Grayscale ultrasound

- Primary modality used whenever possible
- Symmetric enlargement of bifrontal SAS
- ± mild ventricular enlargement

Color Doppler

- Cortical veins seen within subarachnoid fluid space 
        - No mass effect displacing veins against pia
        - No inward displacement of arachnoid membrane by subdural fluid
- Subdural collections lack traversing veins

Imaging Recommendations

Best imaging tool

- US if acoustic window available
- CT/MR if no acoustic window available

Protocol advice

- Doppler sonography: Documents veins traversing SAS
- Linear high-resolution US most sensitive for detection of associated subdural fluid
- After diagnosis, best follow-up: Clinical monitoring of HC and development of any neurologic findings
- Follow-up with MR/CT typically not necessary, unless
        - Focal neurologic signs/symptoms
        - Suspicion for subdural collection on US

DIFFERENTIAL DIAGNOSIS

Atrophy
- Small HC; sulcal prominence out of proportion - Forehead "pointed" due to metopic fusion
Incidental Bilateral Subdural Fluid Collections
- Subdural fluid not normally visualized - Small, nonhemorrhagic subdural collections seen in 4% of benign macrocrania patients - Characterized by crescentic fluid collection separating dura from arachnoid - No cortical veins traversing subdural space - Discrete arachnoid membrane displaced toward cortex; may be compressing SAS veins - May have different signal intensity on PD and other MR sequences compared to CSF - Discuss need for further work-up with referring clinician - Close clinical follow-up at minimum; work-up for nonaccidental trauma (NAT) at discretion of clinician
Nonaccidental Trauma
- Moderate/large or hemorrhagic subdurals or unusual clinical findings should raise concern
Glutaric Aciduria Type 1
- Enlarged sylvian fissures with delayed myelination
- Subdural collections may be present
- T2-hyperintense basal ganglia
Elevated Venous Pressures
- Causes: Cardiac disease, internal jugular vein sacrifice for ECMO, arteriovenous fistula, or sinus venous thrombosis
Communicating Hydrocephalus
- Often post hemorrhagic/post inflammatory/neoplastic - Density of extraaxial collection does not = CSF
- Achondroplasia and other skull base anomalies - Coarctation of foramen magnum (narrow)

PATHOLOGY

General Features

Etiology

- Remains incompletely understood
- Immature CSF drainage pathways: Most accepted theory
        - CSF primarily drained via extracellular space → capillaries
        - Pacchionian granulations (PGs) do not mature until 18 months
        - PGs then displaced into veins (as Starling-type resistors)
        - PGs regulate venous drainage of CSF when fontanels close
        - Benign SAS enlargement usually resolves at that time
- Disproportionate growth of skull and brain
        - Faster growth of skull results in ↑ SAS ± ↑ ventricles
        - This theory helps to explain frequent identification of subdural fluid collections
- Family history of macrocephaly > 80%

Associated abnormalities

- Subdural collections (typically small and incidental) in ~ 4%
- Predisposition to bleed with minor trauma: Controversial
        - Possibility of ↑ risk for bridging vein injury and subdural collection/hematoma in absence of major trauma
        - Venous "stretching" implicated
- May ↑ risk of arachnoid cyst development compared to normal population

Gross Pathologic & Surgical Features
- Deep/prominent but otherwise normal-appearing SAS
- No pathologic membranes
Microscopic Features
- Ependymal damage not seen in benign SAS enlargement

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Macrocrania: HC > 95th percentile
- Frontal bossing
- No signs of elevated intracranial pressure (ICP); normal pressure on lumbar puncture
- Danger signs
        - Elevated ICP
        - Persistent or rapid deviation of HC from normal curve
        - Developmental regression, focal neurologic signs, vomiting, bruising

Other signs/symptoms

- Mild developmental delay common (20-50%) and usually resolves over time
        - Should not necessarily prompt further evaluation

Clinical profile

- Family history of benign macrocephaly common
- Male infants, ± late to walk

Demographics
- Most common imaging diagnosis for macrocrania in patients < 1 year of age - Usually presents at 3-9 months
- Sex: M:F = 2:1
Natural History & Prognosis
- Enlarged SAS → ↑ suture/calvarial malleability/compliance → predisposes to posterior plagiocephaly
- Self-limited; resolves without therapy by 12-24 months - Spontaneous resolution of spaces and symptoms
- Macrocephaly may persist
Treatment
- No treatment necessary
- Normal outcome (developmental delay usually resolves as prominent SAS resolves)

DIAGNOSTIC CHECKLIST

Image Interpretation Pearls
- Crucial to know HC
- Further evaluation with brain MR or CT if US atypical - Moderate/large/complex subdural collection → NAT work-up - Even small/simple subdural collections should be discussed with referring clinician to identify any concerns for NAT that merit further work-up

17f5e314-01d9-44b0-81da-6468019ad492

References

Selected References

Differential diagnosis

Cistern, Subarachnoid Space Normal Variant

DDX:167a514e-0b18-4a16-9474-41a1d760607b

Subarachnoid Space Normal Variants

DDX:558a9979-3a38-473f-a5f8-bf6b6d6538e2

Cases

{'cases': [{'authors': [{'key': 'e8af6d26-3aad-47c9-9083-5128aab09af2', 'value': 'Susan I. Blaser, MD, FRCPC'}], 'caseVersionId': 'bd82b696-6c64-439d-a4e9-417829b23517', 'description': 'T2W axial images (#1-4) demonstrate "squaring" of the forehead and prominent pericerebral spaces. Small linear flow voids (arrows #2-4) represent veins traversing the subarachnoid space. The CSF spaces are at their widest at approximately 7 months of life. This process has also been called benign macrocephaly of infancy, physiologic extraventricular obstructive hydrocephalus, and external hydrocephalus. It is self-limited, usually resolving without therapy by 1 to 2 years of age.', 'history': 'Presented with macrocrania and frontal bossing.', 'imagePoolId': 'fd5ba87e-b9d9-48f7-aa78-96eac3fc925e', 'name': 'Crossing vessels', 'teachingPoint': None, 'demographics': '7 Months old male'}, {'authors': [{'key': 'e8af6d26-3aad-47c9-9083-5128aab09af2', 'value': 'Susan I. Blaser, MD, FRCPC'}], 'caseVersionId': '9e524b89-ebd0-4d81-9776-61b08302b2ec', 'description': 'NECT (#1, 2) demonstrate marked enlargement of the frontal pericerebral CSF spaces (arrow). Without the use of IV contrast material, traversing venous structures cannot be assessed. Similar findings are seen on sagittal and axial T1WIs (#3, 4) and axial FLAIR (#5). Axial (#6, 7) and coronal (#8) T2W images, however, are extremely useful. The T2W images reveal fine linear flow-voids due to traversing veins (#6-8, curved arrows), confirming that these enlarged spaces are in fact the subarachnoid space. \n\nComment: Enlarged pericerebral spaces in infancy are often a transient and benign condition. They resolve between 8 and 12 months of age, usually when the infant is able to be in the upright position for longer periods of time. They are slower to resolve in late walkers. The traversing veins may bleed with trauma, simulating non-accidental injury. Sequential follow-up of head-circumference (tape-measure, not imaging) is suggested. Repeat imaging is suggested when the macrocrania rapidly progresses, or when there are neurological symptoms.', 'history': 'Typically present with macrocrania between the ages of 3 and 8 months of age.', 'imagePoolId': 'ad479e42-534c-4d0e-9c71-416160f544a5', 'name': 'Marked', 'teachingPoint': None, 'demographics': '8 Months old male'}], 'caseType': 'typical', 'name': 'TYPICAL'}
{'cases': [{'authors': [{'key': 'e8af6d26-3aad-47c9-9083-5128aab09af2', 'value': 'Susan I. Blaser, MD, FRCPC'}], 'caseVersionId': '94dfc51a-38fa-4d6c-98ea-9215b294dad3', 'description': 'Axial NECT reveals an asymmetric prominence of pericerebral spaces (arrow, #1). T2W images obtained 5 months later demonstrate persistence of the asymmetry (arrows, #2, 3). No membranes or unequal signal intensity are seen on FLAIR (#4). Enhancing veins traverse the dilated subarachnoid space (curved arrows, #5). Enlargement of the subarachnoid space is common during infancy; danger signs requiring imaging evaluation would include rapid enlargement of head circumference, marked prominence of subarachnoid space, increased intracranial pressure and persistence or onset after 1 year of age. Asymmetry is also concerning, raising the suspicion for underlying subdural collection in non-accidental trauma.', 'history': 'Presented with macrocrania and possible seizures.', 'imagePoolId': 'd78c443b-8aae-4312-ab47-bfae3322a312', 'name': 'Asymmetric', 'teachingPoint': None, 'demographics': '3 Months old male'}, {'authors': [{'key': 'e8af6d26-3aad-47c9-9083-5128aab09af2', 'value': 'Susan I. Blaser, MD, FRCPC'}, {'key': '5cff4116-3654-4b3a-bb75-5ebe0b8c9850', 'value': 'Anne G. Osborn, MD, FACR'}], 'caseVersionId': 'c76edba4-99cf-4471-99a3-e58bc8676bab', 'description': 'There is marked asymmetry of subarachnoid space enlargement (#1-8). After contrast administration, however, veins are seen to traverse the subarachnoid space (arrow, #5-8). Compression of the right posterior subarachnoid space by positional lambdoid flattening (open arrow, #1, 2, 4) and calvarial deformation accounts, in part, for the prominence of the contralateral spaces. Asymmetry is of concern and should prompt a search for an underlying subdural component or of underlying brain dysgenesis. In this child, however, traversing veins confirm involvement only of the subarachnoid space.', 'history': None, 'imagePoolId': '91d9adcc-19f5-46a3-b1c1-2ceff2c37812', 'name': 'Asymmetric', 'teachingPoint': None, 'demographics': '3 Months old male'}, {'authors': [{'key': 'e8af6d26-3aad-47c9-9083-5128aab09af2', 'value': 'Susan I. Blaser, MD, FRCPC'}], 'caseVersionId': 'ddc8c96d-0330-4a2a-8065-24b477e33f11', 'description': 'Enlarged pericerebral spaces are identified on coronal sonography. Distance between the surface of the brain and the dura is 1.5 cm, or 3 times the maximum allowable measurement of 5 mm. High-resolution view reveals multiple linear veins (arrows) traversing the subarachnoid space, confirming the diagnosis. \n\nNECT demonstrates huge pericerebral spaces over the frontal convexities and widening the interhemispheric and Sylvian fissures. CECT confirms a plethora of veins (arrows) traversing the subarachnoid space. No subdural membrane is identified.\n\nEnlargement of the subarachnoid spaces, also known as physiologic extraventricular obstructive hydrocephalus (EVOH), external hydrocephalus or benign macrocephaly of infancy is usually a transient phenomenon peaking between 3 and 8 months of age. Surgical intervention is needed only when spaces reach massive size, as in this case. \n\nBe careful not to mistake enlarged arachnoid spaces (in the presence of macrocephaly) for atrophy. Always determine and document the head circumference.', 'history': 'Patient was noted to have macrocephaly at 2 months of age. Subarachnoid spaces enlarged dramatically over time. Patient required shunt at 7 months.', 'imagePoolId': 'e88a10c0-feca-4740-8c77-7996345f28ae', 'name': 'Massive', 'teachingPoint': None, 'demographics': '7 Months old male'}], 'caseType': 'variant', 'name': 'VARIANT'}

Images

Selected Images

Axial graphic shows classic enlarged subarachnoid spaces (SAS) in a macrocephalic infant (head circumference > 95%). Note the symmetric enlargement with idiopathic enlargement of SAS during the 1st year of life.

Axial T2 MR shows enlarged frontal & anterior interhemispheric pericerebral fluid spaces , mild ventriculomegaly, & right-sided posterior plagiocephaly in a 7-month-old boy with macrocephaly.

Coronal US in a 7-month-old boy with macrocrania shows enlarged SAS & normal ventricular size. Note the normal size of the sulci. This is a typical clinical history & imaging appearance for benign enlargement of the SAS.

Coronal color Doppler US in a 4-month-old girl shows vessels traversing the enlarged SAS . Doppler US can be helpful to exclude subdural collections by demonstrating normal veins in the SAS.

Coronal US in a 3 month old with macrocephaly shows prominent SAS as well as mild enlargement of the lateral ventricles . Mild lateral ventricular enlargement is common in benign enlargement of subarachnoid spaces (BESSI).

Coronal T2 MR in a 6 month old with macrocephaly shows symmetrically prominent bifrontal SAS with mild enlargement of the lateral ventricles . Mild enlargement of the lateral ventricles should not dissuade one from suggesting BESSI.

Coronal T2 MR at 13 months (left) & NECT at 5 years (right) of age show expected resolution of the enlarged SAS over a 4-year period. Enlarged SAS typically resolve by 24 months of age.

Coronal high-resolution US in a 4-month-old girl with macrocrania shows bilateral enlargement of the SAS . Also present are small, bilateral, subdural collections , which are anechoic compared to the SAS. Note the separation of the arachnoid membrane .

Axial PD MR in a 4-month-old girl with macrocrania shows enlarged SAS , which are isointense to the brain. Also note the small, bilateral, nonhemorrhagic, hyperintense subdural fluid collections .

Coronal T2 MR in the same patient shows symmetrically enlarged SAS as well as small, bilateral, nonhemorrhagic subdural fluid collections . Small, subdural fluid collections are seen in ~ 4% of patients with enlarged SAS.

Additional Images

Axial graphic shows classic enlargement of the subarachnoid spaces (SAS) in a macrocephalic infant. There is symmetric bifrontal enlargement of the SAS, which contain multiple bridging veins . Mild ventriculomegaly is present.

Axial T2 MR in a 6-month-old boy with enlarged SAS shows vessels coursing through the SAS. Note the lack of mass effect on the underlying brain parenchyma. There is mild enlargement of the lateral ventricles , a common finding in benign enlargement of the SAS.

Coronal T2 MR in the same 4-month-old girl with macrocrania shows symmetrically enlarged SAS as well as small to moderate, bilateral subdural fluid collections . The subdural collections are slightly hyperintense to the SAS. Small subdural fluid collections are seen in ~ 4% of patients with enlarged SAS.

Axial CECT shows enlarged SAS with enhancing traversing veins in a macrocephalic infant. This benign condition usually peaks at 7 months of age & resolves spontaneously by 12-24 months of age.

Axial T2 MR shows prominent frontal CSF spaces (craniocortical & interhemispheric) with mildly prominent ventricles in this macrocephalic infant. Note the squaring of the forehead, seen clinically as "frontal bossing." About 20-50% of cases have mild developmental delay (motor > > language), which nearly always resolves without therapy.

Axial NECT shows classic enlargement of SAS in this macrocephalic 5-month-old patient. Note the > 5-mm widening of the bifrontal craniocortical & anterior interhemispheric SAS.

Axial CECT shows veins traversing the enlarged SAS.

Axial T2 MR shows veins, represented by linear flow voids , traversing the enlarged SAS.

Coronal US shows dilated craniocortical SAS (note the space between the 2 markers) with veins traversing the SAS.

Coronal T2 MR shows markedly enlarged SAS with prominent ventricles & traversing bridging veins . Tiny, bilateral subdural collections are present .

36 KiB Raw Blame History

KEY FACTS

Terminology

Imaging

Top Differential Diagnoses

Pathology

Clinical Issues

Diagnostic Checklist

TERMINOLOGY

Synonyms

Definitions

IMAGING

General Features

Best diagnostic clue

Location

Size

Morphology

Radiographic Findings

CT Findings

NECT

CECT

MR Findings

T1WI

T2WI

FLAIR

DWI

T1WI C+

Ultrasonographic Findings

Grayscale ultrasound

Color Doppler

Imaging Recommendations

Best imaging tool

Protocol advice

DIFFERENTIAL DIAGNOSIS

Atrophy

Incidental Bilateral Subdural Fluid Collections

Nonaccidental Trauma

Elevated Venous Pressures

Communicating Hydrocephalus

PATHOLOGY

General Features

Etiology

Associated abnormalities

Gross Pathologic & Surgical Features

Microscopic Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Other signs/symptoms

Clinical profile

Demographics

Natural History & Prognosis

Treatment

DIAGNOSTIC CHECKLIST

Image Interpretation Pearls

References

Selected References

Differential diagnosis

Cistern, Subarachnoid Space Normal Variant

Subarachnoid Space Normal Variants

Cases

Images

Selected Images

Additional Images

36 KiB

Raw Blame History