P03.12: Posterior fossa arachnoid cyst: value of mid‐sagittal view of the vermis
Bibliographic record
Abstract
Primary arachnoid cysts are benign, non-communicating fluid collections within arachnoid membranes. If located in the posterior fossa, they can be confused with Dandy-Walker malformation (DWM), inferior vermian hypoplasia, mega-cisterna magna, and Blake's pouch cyst. We report a case misdiagnosed as DWM. A 30-year-old G3P2 woman was referred at 37.5 weeks gestation. The couple is healthy, non-consanguineous and of Caucasian descent. Ultrasound at 20 weeks was normal. Another ultrasound was performed at 37 weeks due to decreased fundal height, with a diagnosis of DWM. Repeat US at our institution showed a female AGA fetus. BPD was 9.8 cm and head circumference (HC) 35 cm (both at 95th centile). There was marked dilatation of both lateral ventricles (atrial width 20 mm) and third ventricle (13 mm). Cisterna magna measured 23 mm. It was continuous with a large 5.5 × 4.5 × 3.5 cm cyst in the posterior fossa, which contain several septa. Transverse cerebellar diameter was normal for gestation. Mid-sagittal view of the posterior fossa showed normal morphology of the vermis with small 4th ventricle. The cyst was posterior to the cerebellum, with mass effect on the cerebellum and elevation of the torcular. These findings of a retrocerebellar arachnoid cyst causing ventriculomegaly were confirmed with subsequent fetal MRI. Delivery was at 39 weeks gestation by Caesarean section. The baby's birth weight was 3.6 kg (50th–90th centile), length 51 cm (50th centile) and HC 38 cm (97th centile). Her APGAR scores were 9 and 9 at 1 and 5 minutes respectively. A shunt was inserted into the cyst at 16 days of age with decrease in size of ventricles and head circumference. She has been discharged home and is now 23 days old. This case shows the importance of evaluating the vermis by obtaining a mid-sagittal view, either by transvaginal ultrasound (if cephalic presentation) or by reconstruction from a 3D volume acquired in the axial plane. Both of these approaches were used in this case.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.004 | 0.002 |
| Insufficient payload (model declined to judge) | 0.006 | 0.002 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".