By Marcello Cherchi, MD PhD
Practical summary
There are several general pathologic processes that can cause a discrete lesion in any part of the central nervous system (CNS). The main ones are:
- Infarction. Hemorrhagic and ischemic strokes can affect any part of the brain or spinal cord.
- Demyelination. Any part of the central nervous system is vulnerable to demyelination. Some demyelinating diseases have a predilection for specific area of the central nervous system (e.g., neuromyelitis optica spectrum disorders).
- Space occupying lesions. Any part of the brain and spinal cord can be affected by tumors (malignant and non-malignant), vascular anomalies and other processes that exert mass effect on neural structures.
Other disease processes that can, at least in principle, affect any part of the CNS include infection and sarcoidosis.
As with any subdiscipline of neurology, diagnostic thinking proceeds through the steps of:
- Anatomy — localizing the lesion.
- Pathophysiology — what physiologic dysfunction is occurring.
- Etiology — what is the underlying cause of the pathophysiology.
A good example of this thought process in general neurology might be a young healthy patient brought to the emergency room after having been found with abrupt-onset left-sided weakness (face, arm and leg) that is now rapidly improving, and on examination she has left-sided Hoffman’s, Troemner’s and Babinski signs.
- Anatomy — Long tract signs from the left upper motor neuron to right side of body.
- Pathophysiology — Transient upper motor neuron dysfunction, rapidly improving. Processes that can behave in this fashion include transient ischemia, and a post-seizure state.
- Etiology — Transient ischemic attack (such as from embolization) would be somewhat unlikely in a young patient with no vascular risk factors. If this weakness is post-ictal (i.e., following a seizure), then cortical irritation, such as from a tumor, is a consideration.
Diagnosis in otoneurology would follow a similar thought process. Take the example of a 70-year-old patient with atrial fibrillation who abruptly developed disequilibrium and total left-sided hearing loss that have remained constant since onset several days ago, and who on examination he has left-sided sensorineural hearing loss and spontaneous right beat nystagmus that increases on rightward gaze and decreases on leftward gaze.
- Anatomy — The left-sided sensorineural hearing loss and spontaneous right beat nystagmus both implicate a left-sided auditory and vestibular deficit.
- Pathophysiology — The labyrinth could be dysfunctional, or the vestibulocochlear nerve could be dysfunctional.
- Etiology — Left-sided labyrinthitis is a consideration as it is common. Left-sided labyrinthine infarction is uncommon, but still a consideration in an older patient with vascular risk factors. A vestibular schwannoma, while possible, would be unlikely to present so abruptly.
Thus from the perspective of arriving at a diagnosis in otoneurology, it is not usually productive to speak in general terms about infarction, demyelination or tumors. Those etiologies may be worth considering depending on the anatomy and pathophysiology suggested by the history, physical examination and perhaps ancillary test results.
