Answer

We present a case of Thanatophoric Dysplasia with abnormal sulcation in the temporal lobes.

Discussion

Thanatophoric dysplasia is a skeletal dysplasia characterized by severe micromelia, short ribs and pulmonary hypoplasia. It is nearly always lethal in the neonatal period due to respiratory failure [1]. It has a prevalence of 0.21 to 0.80 per 10,000 births [2]. Thanatophoric dysplasia is usually caused by a de novo mutation of fibroblast growth factor receptor 3 gene (FGFR3) on chromosome 4, which results in constitutive activation of the FGFR3 tyrosine kinase [1,3]. Thanatophoric dysplasia is divided into two subtypes: type 1 features bowed/bent femurs and metaphyseal flaring, resulting in a characteristic ‘telephone receiver’ appearance. Type 2 is characterized by straight femurs and a ‘cloverleaf’ skull, though this finding may also appear in type 1 [1,3].

In addition to the skeletal defects, fetuses with thanatophoric dysplasia demonstrate characteristic central nervous system findings. Temporal lobe abnormalities were first dscribed in thanatophoric dysplasia in 1971, and include deep transverse sulci across the infero-medial temporal surface, as well as temporal lobe enlargement, especially along the medial-lateral axis resulting in a globular appearance of the brain [4]. This is in contrast to the surface of a normal 18 to 22 week fetal brain which is essentially smooth. Additional central nervous system findings include megalencephaly, hippocampal dysplasia, rudimentary dentate gyrus, and polymicrogyria [4].

Temporal lobe dysplasia in cases of short-limb skeletal dysplasia is considered virtually pathognomonic for thanatophoric dysplasia, and can be identified on prenatal ultrasound and MRI between 19 to 21 weeks of gestation [1,3]. The routine axial biparietal diameter plane cuts through the cavum septi pellucidi and the cerebral peduncles, above the cerebellum and the inferior portion of the temporal lobe where the abnormal sulcation is most apparent. Therefore, abnormal gyration of the temporal lobes may be overlooked in this plane. Obtaining an axial image just inferior to the biparietal diameter plane, which roughly includes the orbits and brainstem/midbrain, or a coronal image that includes the cerebellum or midbrain better visualizes the temporal lobe dysplasia [2,3]. Using these modified planes, the abnormal deep medial temporal sulcation can be visualized as early as 16 weeks gestation [2].

In conclusion, the combination of short limbs, narrow chest, macrocephaly, and temporal lobe dysplasia may be a pathognomonic sonographic sign for thanatophoric dysplasia in mid-trimester fetuses. Sequencing of the FGFR3 gene, which is more affordable than whole genome sequencing, should be considered when these findings are identified on ultrasound or MRI. Awareness of these brain malformations in thanatophoric dysplasia can improve prenatal diagnostic accuracy and can aid in patient counseling.

References

[1] Fink AM, Hingston T, Sampson A, Jessica Ng, Palma-Dias R. Malformation of the fetal brain in thanatophoric dysplasia: US and MRI findings. Pediatr Radiol 2010; 40: S134-S137.
[2] Wang DC, Shannon P, Toi A, Chitayat D, Mohan U, Barkova E, et al. Temporal lobe dysplasia: a characteristic sonographic finding in thanatophoric dysplasia. Ultrasound obstet Gynecol 2014; 44: 588-594.
[3] Blaas H.-G.K, Vogt C, Eik-Nes S.H. Abnormal gyration of the temporal lobe and megalencephaly are typical features of thanatophoric dysplasia and can be visualized prenatally by ultrasound. Ultrasound Obstet Gynecol 2012; 40: 230-234.
[4] Hevner RF. The cerebral cortex malformation in thanatophoric dysplasia: neuropathology and pathogenesis. Acta Neuropathol 2005; 110: 208-221.

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