A fourth-nerve palsy is a common cause of acquired vertical diplopia in the adult. These patients typically complain of vertical diplopia and tilting (torsion) of objects. The history and exam should focus on any other neurologic signs or symptoms, recent head trauma, vasculopathic risk factors, and prior symptoms suggestive of a congenital or long-standing palsy.
The three-step test is the classic maneuver for diagnosis, including an incomitant ipsilateral hypertropia that is worse in contralateral horizontal gaze and worse on ipsilateral head tilt.
Seven steps The following seven steps might be useful for clinicians dealing with a patient with a fourth-nerve palsy.
Step 3: Rule out fourth-nerve palsy mimics. The differential diagnosis for binocular vertical diplopia includes restrictive etiologies (e.g., orbital floor fracture, orbital tumor or pseudotumor, or thyroid eye disease), partial third-nerve palsy, myasthenia gravis, and skew deviation. These typically can usually be distinguished on clinical grounds alone. Forced ductions might be necessary for restrictive disease. The pupil size and reactivity should be documented. The presence of lid retraction, lid lag, proptosis, variability, or fatigue should be noted. Skew deviation would be a rare isolated presentation and typically is accompanied by other signs of posterior fossa disease.
Step 4: Rule out congenital palsy. If the fourth-nerve palsy is isolated then consider old congenital fourth-nerve palsy by (1) reviewing old photographs for a long-standing head tilt, (2) documenting objective fundus excyclotorsion without subjective image tilting, (3) demonstrating large vertical fusional amplitudes (> 6 to 7 prism diopters), and (4) long-standing facial asymmetry. Bilateral fourth-nerve palsies are commonly due to trauma. Patients with unexplained bilateral fourth-nerve palsies should probably undergo neuroimaging with attention to the dorsal midbrain.
Step 5: Rule out traumatic fourth-nerve palsy. A patient with a significant head trauma prior to the onset of the fourth-nerve palsy probably does not require additional evaluation.
Step 6: Consider vasculopathic fourth-nerve palsy. Patients with vasculopathic risk factors (e.g., diabetes, hypertension) do not generally require any initial neuroimaging studies if the palsy is isolated. Most cases show improvement over the following 6 to 8 weeks, but in cases of progressive or unresolved palsies, or new neurologic signs, a neuroimaging study (typically post-contrast magnetic resonance imaging [MRI]) should be performed.
Step 7: Neuroimaging is recommended for patients with non-traumatic, non-congenital, non-vasculopathic, or progressive fourth-nerve palsy. Although most patients with an isolated fourth-nerve palsy have a benign course, if there is progression or if no etiology can be documented, cranial post-contrast MRI should be considered, with attention to the course of the fourth cranial nerve.