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Special interest group

Nosology & Scoring

Lead: Darcy Raches, PhD

The Nosology & Scoring group works on how the syndrome is defined, named, and measured, and is integral to the Society-wide effort to reach a shared definition of posterior fossa syndrome.

The consensus definition: postoperative pediatric cerebellar mutism syndrome (ppCMS)

Core features
Delayed-onset mutism or reduced speech, and emotional lability, after cerebellar or fourth ventricle tumor surgery in children.
Common additional features
Hypotonia; oropharyngeal dysfunction or dysphagia.
Frequently accompanied by
The cerebellar motor syndrome; the cerebellar cognitive affective syndrome; brainstem dysfunction, including long tract signs and cranial neuropathies.

Source: Iceland Delphi consensus of the Posterior Fossa Society (Gudrunardottir et al., 2016).1 Read the consensus paper

We encourage researchers to apply this definition, and to state it in their methods, so that findings can be compared across studies.

A note about severity

Research groups have converged on separating complete mutism from reduced speech. St. Jude uses PFS1 for absent speech and PFS2 for reduced speech, defined as utterances of fewer than three words,2, 3 while the European CMS study records mutism and severely reduced speech (speech limited to single words or short sentences) as two severity levels of postoperative speech impairment (POSI).4 All of these designations fall within the ppCMS definition.

The distinction matters: complete mutism and reduced speech are associated with different patterns of surgical injury5, 6 and different neurological recovery.2

Alongside the consensus definition, we encourage researchers to report children with complete mutism separately from those with reduced speech.

Toward a shared definition of posterior fossa syndrome

The Society's 2016 Iceland Delphi consensus defined postoperative pediatric cerebellar mutism syndrome.1 The literature, however, uses many names for the condition, including cerebellar mutism, posterior fossa syndrome, cerebellar mutism syndrome, cerebellar cognitive affective syndrome, transient cerebellar mutism, mutism with subsequent dysarthria, and akinetic mutism.7, 8 These differences contribute to reported incidence ranging from 8% to 32% after posterior fossa tumor surgery, and from 24% to 39% after medulloblastoma resection.7

At its first open global meeting in 2020, the Society adopted “CMS” while agreeing that “PFS” was acceptable.3 Posterior fossa syndrome remains the term most often used in clinics and by families, but its definition varies by institution. At the 2026 Global Meeting in Copenhagen, members agreed that “PFS” may be the preferable term, and the Society has renewed its consensus process to agree on a shared definition of posterior fossa syndrome.

The Nosology & Scoring group is integral to that Society-wide effort. Updates will be posted here as the work progresses.

Scoring

Consistent measurement of severity and duration is the other half of the group's remit. Several groups have published tools for diagnosing and grading the syndrome. They overlap considerably but differ in who completes them, which symptoms they record, and how they grade severity, and none is yet widely adopted in clinical practice.9

ToolFirst publishedCompleted byWhat it recordsSeverity levels
Children's Oncology Group questionnaire102006A clinical research associate, after consulting the neurosurgeon or neurologist, within two treatment trialsTime of onset, and severity of mutism, ataxia, hypotonia, and irritabilityEach feature mild, moderate, or severe by duration (under 1 week, 1 to 4 weeks, over 4 weeks), combined into an overall rating
NOPHO–European CMS study forms4, 112017Pediatricians or neurosurgeons, before surgery and at set times afterward, with standardized speech samplesSpeech status, with neurological examinationHabitual speech, reduced speech, or mutism; reduced speech and mutism together form postoperative speech impairment (POSI)
St. Jude PFS1/PFS2 classification22021Neurologists, through standardized serial examinationsSpeech output, with ataxia scored on SARAPFS1 (complete mutism), PFS2 (reduced speech, utterances < 3 words), PFS3 (grade-3 ataxia, significant irritability, reduced utterance length ≥3 words), and PFS4 (grade-3 ataxia with normal behavior)
Posterior Fossa Syndrome Questionnaire (PFSQ)122022The clinical team; short and long formsSpeech and language, motor, and emotional lability symptoms, each as current, prior, or neverDimensional symptom profile
Post-operative pediatric CMS scale132021Clinicians, from onset through remissionDuration and severity of mutism, emotional lability, hypotonia, and dysphagiaMild, moderate, or severe overall

General clinical measures are also used alongside these tools, such as the Scale for the Assessment and Rating of Ataxia (SARA)14 and, for cognition and language during acute recovery, the Cognitive and Linguistic Scale (CALS).15

Toward a harmonized approach

These tools are listed as public resources for researchers and clinicians, not as endorsements. The Society aims to work toward a harmonized approach to diagnosis and severity scoring that builds on the consensus definition1 and the distinction between complete mutism and reduced speech. Refining the definition and symptom scoring has been a working-group priority since the Society's 2018 consensus meeting.16

Selected references

  1. Gudrunardottir T, Morgan AT, Lux AL, et al. Consensus paper on post-operative pediatric cerebellar mutism syndrome: the Iceland Delphi results. Childs Nerv Syst. 2016;32(7):1195–1203. Link
  2. Khan RB, Patay Z, Klimo P, et al. Clinical features, neurologic recovery, and risk factors of postoperative posterior fossa syndrome and delayed recovery: a prospective study. Neuro Oncol. 2021;23(9):1586–1596. Link
  3. Toescu SM, Pizer B, Gump W, et al. Toward reducing the risk of cerebellar mutism syndrome: consensus statement from the Posterior Fossa Society. J Neurosurg Pediatr. 2025;36(6):789–797. Link
  4. Grønbæk JK, Wibroe M, Toescu S, et al. Postoperative speech impairment and surgical approach to posterior fossa tumours in children: a prospective European multicentre cohort study. Lancet Child Adolesc Health. 2021;5(11):814–824. Link
  5. McAfee SS, Zhang S, Zou P, et al. Fastigial nuclei surgical damage and focal midbrain disruption implicate PAG survival circuits in cerebellar mutism syndrome. Neuro Oncol. 2023;25(2):375–385. Link
  6. Ji Q, McAfee SS, Scoggins M, et al. Cerebellar mutism syndrome and dentato-thalamo-cortical tract disruption in diffusion tractography following surgery for medulloblastoma. Radiology. 2024;311(2):e232521. Link
  7. Som T, Ravichander A, Khurana E, et al. The current state of research on postoperative cerebellar mutism syndrome: a bibliometric analysis. J Child Neurol. 2025;40(8):620–633. Link
  8. Thomale UW, Driever PH. Inconsistent terminology for cerebellar mutism. Childs Nerv Syst. 2013;29(5):717–718. Link
  9. Obdeijn IV, van Baarsen KM, Avula S, et al. Neuroimaging of postoperative pediatric cerebellar mutism syndrome: a systematic review. Neurooncol Adv. 2025;7(1):vdae212. Link
  10. Robertson PL, Muraszko KM, Holmes EJ, et al. Incidence and severity of postoperative cerebellar mutism syndrome in children with medulloblastoma: a prospective study by the Children's Oncology Group. J Neurosurg. 2006;105(6 Suppl):444–451. Link
  11. Wibroe M, Cappelen J, Castor C, et al. Cerebellar mutism syndrome in children with brain tumours of the posterior fossa. BMC Cancer. 2017;17:439. Link
  12. Wickenhauser ME, Khan RB, Raches D, et al. The posterior fossa syndrome questionnaire: using science to inform practice. J Neurooncol. 2022;157(3):465–473. Link
  13. Ricci FS, D'Alessandro R, Somà A, et al. Development and application of a diagnostic and severity scale to grade post-operative pediatric cerebellar mutism syndrome. Eur J Pediatr. 2022;181. Link
  14. Schmitz-Hübsch T, du Montcel ST, Baliko L, et al. Scale for the assessment and rating of ataxia: development of a new clinical scale. Neurology. 2006;66(11):1717–1720. Link
  15. Raches D, Mule T, Conklin HM. Investigating severity of symptoms associated with posterior fossa syndrome as predictors of long-term cognitive and functional outcomes: a case series. Brain Disorders. 2023;11:100080. Link
  16. Posterior Fossa Society Consensus Meeting 2018: a synopsis. Childs Nerv Syst. 2019. Link

Get involved

Society members can join this group by contacting its lead or writing to info@posteriorfossasociety.org. Not yet a member? Membership is open to clinicians and researchers working on posterior fossa syndrome.

Membership