Neuroradiology Case of the Week

Case 13

Ramon de Guzman, MD, Yuji Numaguchi, MD, PhD,
P-L Westesson, MD, PhD, DDS, Larry Buadu, MD, PhD
Ahmed Abdelhalim, MD, Manoj Ketkar, MD, and Jose Echeverri, MD

Clinical Presentation: The patient is a 20-year-old with history of left facial spasm and seizures. Previous head MRI examinations showed a mass at the quadrigeminal plate cistern and posterior fossa findings suggestive of Dandy-Walker variant.

Radiographic Findings: MRI of the head showed interval increase in size of the previously noted mass at the quadrigeminal plate cistern which is hyperintense in T1 weighted images, T2 weighted images and FLAIR images. No definite enhancement is seen in post-contrast images.
   Posterior fossa findings include absence of the vermis and apposition of the cerebellar hemispheres at the midline (Fig. 1). The superior cerebellar peduncle is enlarged and oriented horizontally (Fig. 2). Axial images (Fig. 3) shows “molar tooth” configuration of the superior cerebellar peduncle and fourth ventricle. The fourth ventricle opens into the cisterna magna.

Figure 1: Apposition of cerebellar hemispheres due to absence of normal vermian folia (arrow).	Figure 2: Enlarged, horizontal, superior cerebellar peduncle (arrow). Hyperintense mass at the quadrigeminal plate cistern most likely a pineal region germinomas/teratoma.
Figure 3: Axial image at the midbrain level shows the enlarged superior cerebellar peduncles (arrow).

Diagnosis: Joubert’s Syndrome

Discussion:
Clinical Discussion: Patients usually presents with episodic hyperpnea, abnormal eye movement, ataxia and mental retardation. There is hypogenesis and midline clefting of the cerebellar vermis, dysplasias and heterotopia of cerebellar nuclei, near total absence of the pyramidal decussations, and anomalies in the structure of the inferior olivary nuclei, descending trigeminal tract, solitary fascicle, and dorsal column of nuclei [1]. Recent evidence suggests an absence of decussation of the superior cerebellar peduncles and central pontine tracts. These suggest that the underlying developmental abnormality may be an inability of posterior fossa axons to cross the midline.

Neuroimaging Discussion: Imaging studies are characteristic [2]. Sagittal images show an agenesis or nearly complete lack of normal vermian folia. The cerebellar hemispheres appose one another in the midline. The superior cerebellar peduncles do not cross in the dorsal midbrain. They are large, nearly horizontal and can be clearly seen as they extend between the midbrain and cerebellum, surrounded by CSF. The midbrain is small in its anteroposterior diameter, probably because of the absence of the decussation of the superior cerebellar peduncles. The characteristic appearance of the midbrain, with the enlarged superior cerebellar peduncles and the absence of their decussation has been called the “molar tooth sign”. Associated supratentorial anomalies are uncommon, but cerebral cortical dysplasia and gray matter heterotopia have been reported.
   Not all patients with the neuroimaging findings just described have Joubert’s syndrome. At least they do not have the syndrome as classically defined (the precise clinical characteristics that define the syndrome are currently being debated). Some patients present with nystagmus, occulomotor apraxia, or lack of smooth ocular pursuit during the first 2 years of life [3,4], with no respiratory symptoms in infancy and, therefore do not have the “classical” Joubert’s syndrome [4]. The classification is disputed by other authors [5,6] who use the finding of vermian hypogenesis as a key component in defining Joubert’s syndrome and do not require any respiratory symptoms to make the diagnosis. From the radiologist’s perspective, it is important to remember two facts: first, there is some dispute as to how the syndrome is defined; and second, the two groups of patients cannot be differentiated by neuroimaging alone. Therefore, the determination as to whether the patient has Joubert’s syndrome is often dependent on how the clinician defines the syndrome [6].

References:

1. Friede RL. Developmental neuropathology, 2nd ed. Berlin: Springer-Verlag, 1989.
2. Kendall B, Kingsley D, Lambert SR, Taylor D, Finn P. Joubert syndrome: a clinical radiological study. Neuroradiology 1990;31:502-506.
3. Sargent MA, Poskitt KJ, Jan JE. Congenital ocular motor apraxia: imaging findings. American Journal of Neuroradiology 1997;18:1915-1922.
4. Adamsbaum C, et al. Vermian agenesis without posterior fossa cyst, Pediatric Radiology 1994;24:543-546.
5. Maria B, et al. “Joubert” syndrome revisited: key ocular motor signs with magnetic resonance imaging correlation. Journal Child Neurology 1997;12:423-430.
6. Steinlin M, et al. Follow-up in children with Joubert syndrome. Neuropediatrics 1997;28:204-211.
7. Barkovich, AJ. Pediatric Neuroimaging 5;345-348. Lippincott Williams & Wilkins 2000.