Why is an abdominal ultrasound recommended in a female patient with NMDAR (N-methyl-D-aspartate receptor) encephalitis?

Screening for Ovarian Teratoma in NMDAR Encephalitis

Abdominal (pelvic) ultrasound is essential in girls and women with NMDAR encephalitis because 20-50% harbor an ovarian teratoma that triggers the autoimmune response, and surgical removal of this tumor is critical for treatment success and preventing relapse. 1

Why Tumor Screening is Mandatory

The relationship between NMDAR encephalitis and ovarian teratoma is direct and causal:

• The teratoma itself triggers the autoimmune response by expressing NMDA receptors, leading to antibody production that attacks the brain 1, 2
• Tumor removal is not optional—it is a critical component of treatment that improves prognosis and decreases recurrence risk 1, 3
• In female patients with NMDAR encephalitis, 20-50% will have an associated tumor, which is almost always an ovarian teratoma 1

Specific Imaging Approach for Female Patients

Young and middle-aged women with typical NMDAR encephalitis should be specifically screened with transvaginal or transabdominal pelvic ultrasound 4, 1:

• Ultrasound is the targeted screening tool specifically recommended for detecting ovarian teratomas in this population 1
• If ultrasound findings are equivocal, pelvic MRI should be obtained 4, 1
• CT of chest/abdomen/pelvis with contrast can serve as initial broad screening but CT is not preferred for pelvic imaging in children, pregnant women, or women of childbearing age 4

Critical Pitfalls: Teratomas Can Be Extremely Small

The most important caveat is that ovarian teratomas in NMDAR encephalitis can be very small (as small as 7-12mm) and easily missed on initial imaging, particularly in children 3:

• One case report documented bilateral teratomas (12mm and 7mm) that were not appreciated on initial abdominal CT or pelvic MRI but were identified on ultrasound follow-up at 5 months 3
• Intraoperative sonography may be needed to localize very small teratomas for excision 3
• Periodic sonography and MRI follow-up is warranted if the initial study is negative, as teratomas can develop in a delayed manner even months after initial presentation 3, 5

Long-Term Surveillance Strategy

Tumor screening should be performed annually for several years, particularly if treatment response is poor or relapses occur 1:

• One case demonstrated an ovarian teratoma appearing 8 months after initial presentation when imaging was initially negative 5
• The patient may remain asymptomatic even when the teratoma develops 5
• Awareness of this association improves diagnostic accuracy and prevents missed diagnoses 3

Additional Screening Considerations

If pelvic ultrasound is negative but suspicion remains high 4, 2:

• Whole body FDG-PET can detect occult teratomas when initial screening is negative 4, 1
• Teratomas can rarely occur at multiple sites (mediastinal and bilateral ovarian) 6
• Extraovarian germ cell tumors may be detected on CT or MRI-based general cancer screening 4