Remember, the neurological examination evaluates mentation, cranial nerves, postural reactions, gait/posture and segmental spinal reflexes.
MENTATION: Levels of consciousness can be described (in descending order) as alert, quiet, disoriented, obtunded, stuporous (semi-coma) and coma. I would describe Charlie as obtunded. He does not react to normal stimuli, but does react to vigorous stimuli.
- Menace response: absent in the right eye, present in the left eye. The menace response is elicited by directing a ‘menacing gesture’ toward the eye. The normal response is to blink the eyelids. The menace response tests vision (retina, cranial nerve II/Optic nerve, optic chiasm, optic tracts, lateral geniculate nucleus, optic radiations, occipital/visual cortex), the connections to the motor cortex and the conscious closure of the eyelids (motor nucleus of cranial nerve VII/Facial nerve, neuromuscular junction, orbicularis oculi muscle). Abnormal response indicates a problem somewhere in this pathway. Note that this is a response that requires cortical input.
- Palpebral reflex: absent in the right eye, present in the left eye. The palpebral reflex is tested by gently touching the skin of the medial and lateral canthi. The palpebral reflex tests sensation of the skin (cranial nerve V/maxillary branch of the Trigeminal nerve), the sensory nucleus of the trigeminal nerve and it’s connections to the facial nerve (motor nucleus of VII/Facial nerve, neuromuscular junction, orbicularis oculi muscle). Note that this is a reflex that does not require cortical input.
- Nasal sensation: absent on the right, present in the left. Nasal sensation is tested by applying a noxious stimulus to the medial aspect of the naris. This tests facial sensation (cranial nerve V/maxillary branch of the Trigeminal nerve, sensory nucleus of the trigeminal nerve and it’s connections in the thalamus, to be consciously perceived in the opposite sensory/parietal cortex.
Commonly referred to as “CP’s” or conscious proprioception, this refers to knowledge of position of where the body (for example, the pelvic limb) is. We test for this by flipping over the paw. A normal patient will (usually) quickly replace the paw to it’s normal position. It is important to support the patient during this so that they do not fall over. A second way of testing postural reactions is to ‘hop’ the patient on one leg. As the clinician moves the patient to either side, the normal patient will ‘hop’ on one leg to keep the leg directly below them. Postural reactions test long stretches of the neurological system and are relatively sensitive for neurological disease, but by themselves are poor at localizing where in the nervous system the problem is.
Charlie is clearly non-ambulatory. There is minimal movement in all four legs. He has a head turn to the left. A head turn is different from a head tilt. Head turn usually indicates a problem in the forebrain (prosencephalon) while a head tilt usually points towards a balance (vestibular) problem.
SPINAL REFLEXES (normal, not shown in video):
- These are local reflex arcs that start in the area being tested, travel up to the spinal cord, synapse within the spinal cord, then descend to the effector organ. The most common example is the patellar reflex. Tapping gently on the patellar tendon causes a stretch of the tendon. The muscle spindle fires and the sensory impulse travels up the femoral nerve, enters the spinal cord via the dorsal nerve root and synapses on a motor neuron in the L3-5 spinal cord. This second neuron exits the spinal cord via the ventral nerve root, travels in the femoral nerve and synapses on the quadriceps muscle to cause extension of the stifle. Another example of a spinal reflex is the flexor withdrawal reflex. Gentle but firm pressure to the digits should cause the limb to be pulled toward the body.
- Reflexes do not require cerebrocortical input. This cannot be overemphasized and is one of the most common misconceptions that veterinarians have about neurology. Withdrawing the limb when stimulated is a reflex. It does not indicate that the information has been consciously perceived in the cerebral cortex.
Based on Charlie’s examination (seizures, absent menace response OD, absent palpebral response OD, absent nasal sensation on the right, head turn to the left, absent postural reactions and normal spinal reflexes) what is your neuroanatomical localization?
Keep it simple–is it the brain, spinal cord or peripheral nervous system?
So we’ve localized the problem to the brain– can we localize it further? Forebrain, midbrain or medulla?
Seizures indicate a problem in the forebrain.
Can we pick a side, left or right?
The head turns TOWARDS the side of the lesion. Remember, that vision is perceived in the opposite occipital (visual) cortex. Sensation is perceived in the opposite parietal (somatosensory) cortex. Since Charlie’s head turn is toward the left, and he is not perceiving stimuli (visual, tactile) sensation on the right, his lesion is localized on the LEFT cerebral cortex.
What are our differential diagnoses? Take into account the signalment, history (acute vs. chronic, progressive vs. non-progressive). Use the DAMNIT-V scheme.
Degenerative conditions typically do not cause asymmetrical signs.
Anomalies (cysts, etc) usually don’t start at 5 years of age, but are still possible.
Metabolic conditions typically are not asymmetrical.
Neoplasia can be acute, progressive and asymmetrical and is certainly possible.
Inflammatory and infectious disease can be acute, progressive and asymmetrical and are certainly possible.
Trauma can cause acute, asymmetrical signs, but is usually not progressive.
Vascular (stroke) is typically acute and asymmetric, but usually does not progress over a 4 day period.
Referral CBC, chemistry panel and thoracic and cervical spinal radiographs were unremarkable.
Below are two images of Charlie’s brain. The first is a sagittal slice, as if he is in profile. Rostral is to the left of the picture, dorsal is to the top of the picture. The second image is a ‘slice’ in the transverse (90 degree) plane. By convention, Charlie’s right side is on the left of the screen. Dorsal is toward the top. The “bright” areas are regions of inflammation. These are characteristic of encephalitis. Note that there are multiple areas of inflammation on both the right and the left, as well as in the medulla and the cerebellum. These are abnormalities that a CT or low-field MRI would be likely to miss. High-field MRI offers several advantages over low-field MRI or CT scans. While CTs use X-rays to make a ‘slice’ image, they cannot acquire images in multiple planes. Furthermore, MRI has greater soft tissue detail for imaging muscle, joints and especially the nervous system. Lastly, MRI allows excellent visualization of the medulla which is typically not able to be adequately imaged by CT due to an artifact called ‘beam hardening’.
Enephalitis in dogs is typically autoimmune. Infectious causes of encephalitis are uncommon in dogs. Treatment involves immune suppression with prednisone and adjunctive immunosuppressants. Adjunctive immunosuppressants that have been used to treat encephalitis in dogs include cytarabine arabinoside (Cytosar), azathioprine (Immuran), cyclosporine (modified), procarbazine, leflunomide and others. Prognosis is guarded in that many dogs respond to treatment and stay normal, many dogs improve but suffer relapses, and some dogs don’t respond at all despite aggressive therapy.
Charlie was treated with prednisone and cytarabine. Phenobarbital was also prescribed.
Here are the important “Take Home” points from Charlie:
1. Neck pain can be caused by things other than a slipped disk, most notably meningitis.
2. Seizures can be caused by problems outside of the brain, problem inside the brain or idiopathic epilepsy. Seizures combined with an abnormal neurological examination strongly suggest a problem within the brain.
3. Encephalitis in dogs is typically autoimmune and is one of the most common causes of brain disease in adult, smaller-breed dogs.
4. Many dogs that are dramatically affected–even those that might appear ‘brain dead’ can have a good outcome if diagnosed appropriately and treated aggressively.
5. MRI is the diagnostic test of choice for diagnosing all cause of brain disease including encephalitis, stroke, tumors and others.