Paraneoplastic Cerebellar Syndrome Revealing Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Breast Cancer - A Case Report

Sep 29, 2026

Paraneoplastic cerebellar degeneration (PCD) is a rare neurologic disorder that may appear months to years before detection of an underlying malignancy. A 46-year-old woman developed progressive gait instability, dysarthria, and nystagmus. Brain MRI showed cerebellar atrophy without mass lesion, and serology revealed strong anti-Yo antibody positivity. Two years later, she was diagnosed with HER2-positive, hormone receptor-negative invasive ductal breast cancer. Neurologic symptoms persisted after mastectomy and adjuvant chemotherapy plus trastuzumab. The case underscores diagnostic challenges and the need for systematic malignancy screening.

LanguageEnglish

CategoryHealthcare

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Pages5

Reading Time13 min

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Review began 10/17/2025 Review ended 12/01/2025 Published 12/05/2025 © Copyright 2025 Bendadi et al. This is an open access article distributed under the terms of the Creative Commons Attribution License CCBY 4.0. , which permits unrestricted use , distribution , and reproduction in any medium , provided the original author and source are credited. DOI: 10.7759/cureus.98519 Open Access Case Report Paraneoplastic Cerebellar Syndrome Revealing Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Breast Cancer: A Case Report Sara Bendadi 1 , Leila Afani 2, Othmane Zouiten 1, Mohamed El Fadli 1, Rhizlane Belbaraka 1 1. Medical Oncology, Mohammed VI University Hospital, Marrakesh, MAR 2. Oncology, Mohammed VI University Hospital, Marrakesh, MAR Corresponding author: Sara Bendadi, [bendadisara@gmail.com](bendadisara@gmail.com) Abstract Paraneoplastic cerebellar degeneration (PCD) is a rare neurological disorder that can precede the diagnosis of an underlying malignancy by several months or even years. We report the case of a 46-year-old woman who presented with progressive gait instability, dysarthria, and nystagmus. Brain magnetic resonance imaging (MRI) revealed cerebellar atrophy without any mass lesion, and serologic testing showed strong anti-Yo antibody positivity. Two years later, she was diagnosed with a human epidermal growth factor receptor 2 (HER2)-positive, hormone receptor-negative invasive ductal carcinoma of the breast. The patient underwent a modified radical mastectomy followed by adjuvant chemotherapy and trastuzumab. Despite appropriate oncologic treatment, her neurological symptoms persisted and gradually worsened. This case highlights the diagnostic challenge of PCD, particularly when neurological manifestations precede tumor detection, and suggests that HER2-positive breast cancer may also be associated with anti-Yo antibodies. Early recognition of this syndrome and systematic malignancy screening are essential to improve patient outcomes. Categories: Neurology, Oncology, Allergy/Immunology Keywords: anti-yo antibody, her2-positive breast cancer, immune checkpoint inhibitor, immuno-clinical profiles, paraneoplastic cerebellar degeneration Introduction Paraneoplastic neurological syndromes (PNS) are a heterogeneous group of rare disorders that, by definition, are associated with malignancies but are not caused by direct tumor invasion, metastasis, metabolic or nutritional deficits, infections, or drug toxicity [1] . PNS may occur concomitantly with or after a cancer diagnosis; however, in many cases, they represent the first manifestation of an underlying malignancy, sometimes appearing months or even years before tumor detection [1] . Several clinical forms have been described, including paraneoplastic cerebellar degeneration (PCD), limbic encephalitis, encephalomyelitis, subacute sensory neuropathy, Lambert-Eaton myasthenic syndrome, opsoclonus-myoclonus syndrome, dermato-or polymyositis, pandysautonomia, and cancer-associated retinopathy [2] . Among these, PCD is one of the most frequent, typically associated with gynecological and breast malignancies [3] . We report the first documented case of anti-Yo antibody-positive paraneoplastic cerebellar syndrome revealing breast cancer at the Onco-Hematology Department of the University Hospital of Marrakech. Case Presentation A 46-year-old woman, divorced and mother of two, presented with a three-month history of progressive gait instability and limb incoordination. She also reported mild dysarthria for four months and intermittent nystagmus for two months. Neurological examination revealed marked cerebellar ataxia requiring assistance for ambulation, bilateral abducens nerve involvement, and pyramidal signs, while muscle strength and sensation were preserved. Initial investigations, including brain magnetic resonance imaging (MRI), infectious and nutritional laboratory testing, and cerebrospinal fluid analysis, were un

FAQs

What neurological symptoms did the patient initially present with?

She presented with progressive gait instability, dysarthria, and intermittent nystagmus. Examination showed marked cerebellar ataxia along with other neurologic signs.

How was paraneoplastic cerebellar syndrome supported before cancer was found?

Brain MRI demonstrated cerebellar atrophy without a mass lesion. A paraneoplastic antibody panel later showed strong anti-Yo antibody positivity.

What was the breast cancer diagnosis and how was it treated?

She was diagnosed with HER2-positive, hormone receptor-negative invasive ductal carcinoma. Treatment included modified radical mastectomy with axillary lymph node dissection, followed by adjuvant chemotherapy with taxanes and trastuzumab.

Sep 29, 2026

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