A lesion of the trochlear nerve can cause trochlear palsy. To diagnose such paralysis of the trochlear nerve and superior oblique muscle, the physician uses the Bielschowsky head nerve test. Unlike many other diagnostic procedures, the test has no risks, nor side effects.
What is the Bielschowsky head-negative test?
Trochlear nerve palsies can affect one or both sides of the head. To diagnose them, the doctor makes use of what is called the Bielschowsky head nerve test. In so-called trochlear nerve palsy, the patient suffers from a lesion of the trochlear nerve. This is the fourth cranial nerve, the lesion of which can result in complete paralysis or paresis of the superior oblique muscle. This muscle is the oblique superior eye muscle, which contains only motor fibers of the trochlear nerve. Trochlear nerve palsies can affect one or both sides of the head. To diagnose them, the physician makes use of the so-called Bielschowsky head tilt test. Accordingly, the head pinch test is a diagnostic procedure specifically designed to diagnose or rule out lesions of the trochlear nerve. No instruments are needed for the head pinch test. The physician simply instructs the patient on the position of the head. In certain head positions, abnormal eye movement occurs during the test, which is indicative of trochlear nerve paresis. These abnormal eye movements include, for example, vertical gaze deviation. If this phenomenon is observed during the course of the test, a diagnosis of unilateral or bilateral trochlear nerve palsy is made. The namesake of the head tilt test is Bielschowsky, who first described the diagnostic test procedure.
Function, effect, and goals
Trochlear nerve palsy occurs as a result of lesions on the IV cranial nerve, also known as the trochlear nerve. Causes of such lesions may be congenital. However, paralysis may also be acquired and thus occur after trauma, vascular changes, or in aneurysms. Equally common is paralysis in the setting of diabetic microangiopathy and apoplexy. The setting may also be sinus cavernosus syndrome or sinus cavernosus thrombosis. In most cases, however, it is an idiopathic trochlear nerve palsy, the cause of which cannot be found. The paresis causes the superior obliquus muscle to fail, so that eye movements are dominated by the obliquus inferior muscle, which is antagonistic to it. This symptom manifests itself, for example, in gaze deviation of the affected eye when an appropriate eye movement is to be made. The eye deviates upwards as soon as an adduction of the gaze is required. If the gaze is to be lowered, the eye deviates upwards. The gaze misalignment causes double images. This so-called diplopia is usually compensated by tilting the head to the opposite side. The phenomenon is also known as ocular torticollis. Vertical gaze deviations with characteristic diplopia can undoubtedly be diagnosed with the help of the Bielschowsky head tilt test. The test is equivalent to a provocation test in which the physician asks the patient to tilt the head to the ipsilateral side. As soon as the patient tilts his head to the side with the supposed damage, his gaze deviates upward on the affected eye. This phenomenon confirms vertical gaze deviation. In most cases, the patient complains of more or less severe double vision at this moment. Thus, the vertical deviation of the gaze causes vertical diplopia, which characteristically occurs during the test. The physician then asks the patient to tilt his head to the opposite side. As soon as the patient tilts his head on the side without lesion of the nerve, the height difference of the eyes equalizes. As a result, the diploidy decreases. The Bielschowsky head tilt test can thus determine, in the context of trochlear nerve paresis, whether the paresis is unilateral or bilateral. If the paresis is unilateral, the test also allows the physician to determine which side is affected and to better assess the location of the nerve lesion in this manner.
Risks, side effects, and hazards
There are no risks or side effects associated with the Bielschowsky head nerve test.As this is a provocation test for paresis-related double vision, patients may find the test unpleasant. However, the provoked double images last only as long as the head is tilted to the side of the lesion. As a rule, the Bielschowsky head tilt test takes a few minutes at most. This explains its clinical relevance in the context of trochlear nerve palsy. A faster diagnostic procedure is hardly imaginable for the paretic phenomenon. Furthermore, since there are no risks and side effects associated with the test procedure, the diagnostic procedure is automatically preferred to those with possible risks and side effects. For example, before MRI or other imaging takes place to image the fourth cranial nerve, the Bielschowsky head pinch test is first used to determine whether a lesion of the nerve is even likely. If the head-negative test does not yield pathologic results, contrast-enhanced imaging may not be necessary. Contrast agent, for example, can cause side effects such as headache and nausea, which the patient may be spared after a negative head-need test. However, a negative test can only be said to occur if gaze deviation does not occur. Vertical deviation of the affected eye is an objective phenomenon that the physician can observe with his own eyes and thus has generalized reliability. Double vision, in contrast, is a subjective phenomenon. Thus, if the patient complains of double vision during head tilt, but the physician cannot observe any deviation of the gaze, an objective diagnosis cannot be made on the basis of testing alone.