Letter to the Editor Response

We thank Dr. Finsterer for his interest in this case and elegant discussion of Erdheim-Chester disease (ECD).

The lesson from our case is the single neurological symptom of seizures associated with bilateral temporal lobe lesions at the time of presentation. The term “manifestation” as used by Dr. Finsterer aligns with what we said and agree with: diabetes and retroperitoneal fibrosis are features of ECD that in retrospect were associated with the histopathological findings confirming ECD. The importance of a wide differential and the identification of symptom associations is emphasized in our text.

Regarding the first “limitation” mentioned by Dr. Finsterer, the patient’s seizure history, including a family history, and alternative causes were assessed at the time of the first seizure in 2016 and were negative. Our wording indicates that the 2016 seizure was the first-ever seizure event. It is notable, and was previously mentioned, that after this first seizure, magnetic resonance imaging was performed, and a temporal lesion was identified.

Regarding the second limitation, electroencephalography (EEG) was conducted when the patient presented to us. Electrographic seizures were not identified, but a focus of cortical irritability was, concordant with the location of one of the brain lesions. To focus on our article’s main point—progression of diagnosis to ECD—we believe noting this concordance is sufficient.

We present 1 month of follow-up with continued medications and the patient’s seizure-free status. Dr. Finsterer’s third limitation suggests long-term follow-up. However, the article was written to discuss the diagnosis of ECD from a neurosurgical perspective, not to educate on multisystemic ECD progression after diagnosis. The patient continues to do well at this time.

Regarding the final limitation, we do not describe the status of the cerebral arteries and other vessels because symptoms or findings related to these vessels were not identified prior to the ECD diagnosis, nor were they present on retrospective analysis. Mentioning every possible ECD association was not relevant to a succinct neurosurgical case focused on diagnosis.

Addressing the remaining comments, we did mention the patient’s history of type 2 diabetes and that the diagnosis of immune encephalitis had been made prior to patient presentation. Details on that diagnosis were beyond the scope of a succinct case presentation.

We completely agree with Dr. Finsterer’s closing summary, as it reinforces a major point we make in our paper. The patient’s only neurological symptom at presentation was seizures. After the ECD diagnosis by histopathological analysis, other nonneurological associations of ECD were identified, such as diabetes and retroperitoneal fibrosis.

On this last point, it is important to note that retroperitoneal fibrosis is rare, largely idiopathic, and typically seen, as in our case, in males 40–60 years old.¹ Systemic inflammatory disorders such as IgG4-related disease can present with retroperitoneal fibrosis and autoimmune pancreatitis-related diabetes, as well as intracranial manifestations of pachymeningeal disease and hypophysitis—also the more common manifestations of central nervous system ECD.^1,2 Therefore, the described systemic imaging findings are not ECD pathognomonic.

In our patient, ECD was not diagnosed until multisystemic involvement had already occurred; thus, associations were made in retrospect. This is the main case lesson, namely to consider the diagnosis of ECD in addition to neoplasm when seizures occur and bilateral brain lesions are found.