Electroclinical, MRI, and Neuropathological Study of 10 Patients with Nodular Heterotopia, with Surgical Outcomes

Tassi L, Colombo N, Cossu M, Mai R, Francione S, Lo Russo G, Galli C, Bramerio M, Battaglia G, Garbelli R, Meroni A, Spreafico R

Brain 2005;128:321–337

We present the results of a retrospective study on 10 patients operated on for intractable epilepsy associated with nodular heterotopia, as identified by high-resolution MRI. Seven patients had unilateral heterotopia, one patient had symmetrical bilateral heterotopia, and two patients had asymmetrical bilateral heterotopia. By stereoelectroencephalogram (SEEG; nine patients), interictal activity within nodules was similar in all cases, and ictal activity never started from nodules alone but from the overlying cortex or simultaneously in nodules and cortex. Excellent outcomes (Engel class Ia, 1987) were achieved in the seven patients with unilateral heterotopia, showing that surgery can be highly beneficial in such cases when the epileptogenic zone is carefully located before surgery by MRI and particularly SEEG. For the bilateral cases, surgical outcomes were Engel IIa (one patient) or Engel IIIa (two patients).

Histologic/immunohistochemical studies of resected specimens showed that all nodules had similar microscopic organization, even though their extent and location varied markedly. The overlying cortex was dysplastic in nine patients, but of normal thickness. We suggest that nodule formation may be the result of a dual mechanism: (1) failure of a stop signal in the germinal periventricular region leading to cell overproduction; and (2) early transformation of radial glial cells into astrocytes, resulting in defective neuronal migration. The intrinsic interictal epileptiform activity of nodules may be due to an impaired intranodular GABAergic system.

The Role of Periventricular Nodular Heterotopia in Epileptogenesis

Aghakhani Y, Kinay D, Gotman J, Soualmi L, Andermann F, Olivier A, Dubeau F

Brain 2005;128:641–651

A temporal resection in patients with periventricular nodular heterotopia (PNH) and intractable focal seizures yields poor results. To define the role of heterotopic grey matter tissue in epileptogenesis and to improve outcome, we performed stereoencephalography (SEEG) recordings in eight patients with uni- or bilateral PNH and intractable focal epilepsy. The SEEG studies aimed to evaluate the most epileptogenic areas and included the allo- and neocortex and at least one nodule of grey matter. Interictal spiking activity was found in ectopic grey matter in three patients, in the cortex overlying the nodules in five, and in the mesial temporal structures in all. At least one heterotopion was involved at seizure onset in six patients, synchronous with the overlying neocortex or ipsilateral hippocampus. Two patients had their seizures originating in the mesial temporal structures only. Six patients had surgery, and the resected areas included the seizure onset, with follow-up from 1 to 8 years. An amygdalohippocampectomy was performed in two (Engel classes Id and III), an amygdalohippocampectomy plus removal of an adjacent heterotopion in two (class Ia), and a resection of two contiguous nodules plus a small rim of overlying occipital cortex in one patient (class Id). One patient with bilateral PNH had three adjacent nodules resected and an ipsilateral amygdalohippocampectomy, resulting in a reduction of the number of seizures by 25 to 50%. The best predictor of surgical outcome is the presence of a focal epileptic generator; this generator may or may not include the PNH. Invasive recording is required in patients with PNH; it improves localization and is the key to better outcome.

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Intracranial EEG in the 21st Century

Surgery for Heterotopia: A Second Look