Additional information

Sodium channel disorders: Abnormalities in the structure and/or function of sodium channels can cause channelopathies. These can lead to diseases such as epilepsy, hyperkalemic periodic paralysis, Brugada syndrome, cerebellar ataxia, anosmia, atrial fibrillation, sudden infant death syndrome, long QT syndrome, erythromelalgia, and other pain syndromes.

Pheochromocytoma: This is a tumor arising from the chromaffin cells of the adrenal medulla. Normally, epinephrine is the major output of chromaffin cells. In pheochromocytoma, the tumor produces mainly norepinephrine (NE), with or without epinephrine. However, in extra-adrenal pheochromocytomas, only NE is produced.

Signs of cerebellar lesions: Cerebellar lesions produce ipsilateral signs, including cerebellar ataxia, dysdiadochokinesia, dysmetria, intention tremor, and a negative Romberg sign.

• Cerebellar ataxia: Due to lack of coordination of movements.
• Dysdiadochokinesia: The patient can’t perform rapidly alternating movements (e.g., pronation and supination).
• Intention tremor: Begins with initiation of voluntary movement (e.g., moving a hand to pick up an object) and increases in amplitude as the target is approached or at the end of the movement.
• Rebound phenomenon: The patient can’t stop a motion because antagonistic muscles are weakened. It’s tested by the Holmes-Stewart manoeuvre by flexing the arm against resistance.
• Dysmetria: Improper measuring of distance in muscular acts.
  • Hypermetria: Overreaching (overstepping).
  • Hypometria: Underreaching (understepping). It’s tested by the finger-nose test.

Two noteworthy points - I) Nigrostriatal pathway of substantia nigra is pro-movement. II) Internal segment of globus pallidus is key - inhibit it to initiate movement and activate it to reduce movement (internal segment is anti-movement).

The following areas receive direct visual inputs (apart from the visual pathway)

1. The suprachiasmatic nucleus: It regulates circadian rhythms depending on environmental light.
2. The pretectal nuclei: Role in pupillary light reflex.
3. Superior colliculus: Role in conjugate gaze movements.

Amacrine cells and horizontal cells:

• Amacrine cells: These are interneurons in the retina. They receive input from bipolar cells and relay signals to ganglion cells. They have short, branching dendrites and secrete a variety of neurotransmitters, including glycine and GABA. Amacrine cells are involved in detecting motion, speed, and changes in light intensity.
• Horizontal cells: These are inhibitory neurons that connect to rods and cones. They help enhance contrast.

Caloric testing and physiological nystagmus: Caloric testing is also known as Barany’s test or doll’s eye movements.

• Physiological nystagmus (vestibulo-ocular reflex): When the head is turned (e.g., to the right), the firing rate increases in the right vestibular nerve and decreases in the left vestibular nerve. If the brainstem and vestibular system are functioning efficiently, this produces physiological nystagmus.
  • Slow phase: The eyes initially move away from the direction of rotation.
  • Fast phase: A rapid corrective movement follows, so the eyes move toward the direction the head was turned.

The caloric test is used to assess brainstem function in comatose patients. The patient’s head is raised by about 30 degrees to orient the horizontal semicircular canal in a vertical plane. Cold or warm water is inserted into the ear, producing convection currents in the endolymph and ultimately stimulating the hair cells.

• Cold water irrigation: Produces nystagmus with a slow phase toward the irrigated ear and a fast phase directed away from it.
• Warm water irrigation: Produces nystagmus with a fast phase directed toward the irrigated ear.

Use the mnemonic COWS (cold opposite, warm same) to remember the direction of the fast phase. The vestibular nerve firing rate increases in response to warm water and decreases in response to cold water.

Weaker reflex in one eye suggests tumors of the VIII cranial nerve, vestibular neuronitis, Meniere’s diseases, migraine, and stroke. Cerebellar lesions, especially in the flocculus, cause exaggerated nystagmus in caloric testing. This can be seen in multiple sclerosis, ingestion of psychoactive drugs, and anxiety. Bilaterally decreased caloric response is seen in neurosyphilis, neurodegenerative diseases, intracranial hypertension, Wernicke-Korsakoff syndrome, etc. Absent caloric response is seen in hair cell damage (e.g., aminoglycoside toxicity), vestibular nerve damage, and brainstem damage.