CASE 18726 Published on 09.10.2024

MRI imaging features of BHAG into the sigmoid sinus

Section

Neuroradiology

Case Type

Clinical Case

Authors

Aparna Juneja, Syed Mehdi

Department of Radiology, Colchester Hospital, East Suffolk and North Essex NHS Foundation Trust, Colchester, United Kingdom

Patient

36 years, male

Categories
Area of Interest Anatomy, Neuroradiology brain ; Imaging Technique CT, MR
Clinical History

We present a case of a 36-year-old male who presented with a history of 6–7 days of generalised headache, two episodes of hallucinations, a few episodes of running into doors and objects, dizziness, and double vision. There were no focal neurological deficits, neck stiffness or photophobia, no weakness, or sensory changes.

Imaging Findings

Non-contrast CT head (Figures 1a and 1b) showed a prominent 1 cm arachnoid granulation related to the right sigmoid sinus. MRI brain (Figures 2a and 2b) was subsequently performed, which showed a small filling defect in the right sigmoid sinus with signal intensity similar to the brain parenchyma and showing continuity with the right inferior temporal lobe gyrus, suggesting a part of brain parenchyma herniating into the arachnoid granulation. Very faint signal intensity changes were seen on sagittal T2 weighted images (Figures 3a and 3b) representing early focal gliosis of the herniated tissue.

The patient was placed on symptomatic treatment with paracetamol, ibuprofen, and codeine, after which the patient's symptoms improved, which makes us to assume this was an incidental finding. However, due to limited literature regarding this entity, it is unclear whether the patient should be kept on follow-up given the focal element of gliosis within the herniated brain parenchyma.

Discussion

Arachnoid granulations are normal cerebrospinal fluid (CSF) signal intensity outpouchings of the subarachnoid membrane into the dural venous sinuses, which helps in the absorption of CSF [1,2]. Sometimes, these arachnoid granulations can be large enough to cause blockage to the passage of blood flow within the dural sinus, when they are termed as giant arachnoid granulation [3]. Very rarely, brain parenchyma herniates into them, when it is termed as BHAG (Brain herniation into arachnoid granulation), with a reported prevalence of 0.32% and with a mean age between 37 to 63 years [4]. It is assumed that progressive dural thinning secondary to an increase in intracranial pressure could be responsible for the development of such herniations [5]. A few case reports have shown BHAG to be associated with headache [6], seizures [7], tinnitus [8], and raised intracranial pressure [9]; however, no causal relationship has been confirmed by any of the studies. To the best of our knowledge, this is the first case of BHAG reported from the United Kingdom.

The most important diagnostic clue to identify BHAG is to look for the continuity of the herniated contents with the brain parenchyma and any associated vessel accompanying the herniation. Abnormal signal intensity changes of the herniated brain parenchyma should be clearly identified, as most reported symptomatic BHAGs have shown gliotic changes [10]. It is also important to visualise this entity in all three imaging planes, as the continuity of the brain parenchyma may not be seen on the axial slices alone. In our opinion, thin-section post-contrast T1 weighted images are the best for its characterisation (where herniated brain parenchyma appears as a discrete filling defect), and T2 weighted images are important for looking for any associated gliotic changes. Other common causes of filling defects within the transverse sinuses, such as blood clots, tumours, and arachnoid granulations, should be excluded. Blood clots show variable signal intensity depending on the age of the clot and show blooming on T2* sequences. Tumours like meningioma show enhancement on post-contrast imaging, while arachnoid granulations parallel the signal intensity of CSF on all the sequences.

In conclusion, very few case reports and case series have been published in the literature, mostly associating BHAG with symptoms of headache and increased intracranial hypertension. There has been no case report of BHAG associated with hallucinations. Whether these entities have any causal relationship with patient symptoms needs further research. Therefore, more case reports and studies need to be encouraged to determine clinical or imaging follow-up of such patients and establish BHAG as normal anatomical variants or pathological entities requiring long-term follow-up.

Written informed patient consent for publication has been obtained.

Differential Diagnosis List
Giant arachnoid granulations
Brain herniation into arachnoid granulation (BHAG)
Sinus thrombosis
Dural based tumours
Final Diagnosis
Brain herniation into arachnoid granulation (BHAG)
Case information
URL: https://eurorad.org/case/18726
DOI: 10.35100/eurorad/case.18726
ISSN: 1563-4086
License