Testing the Brain: What Neurological Exams Can Tell Us About Ourselves
Medicine today possesses tools capable of peering into the most hidden and seemingly inaccessible parts of the body. We can snake cameras through the chambers of the heart and the vessels of the lungs, see embryos in the womb, and even observe electric flashes of thought dance across the brain. In the midst of this profusion of ultra-high technology, it is occasionally important to take pause and evaluate the practices that we have adopted. As the costs of health care skyrocket, the medical community is under increasing pressure to find methods of diagnosis less expensive than MRI and PET scans; the result is that doctors are beginning to revive the lost art of the physical examination. In neurology particularly, a physical examination administered by a well-trained physician can sometimes pinpoint problems with an accuracy unmatched by even the most costly technological solutions.
In the nervous system, these problems usually take the form of small, localized injuries to nerve tissue. Stroke is one example of how the brain can be injured in a very localized way. A stroke can be the result of a cerebral embolism, a blood clot in the brain that cuts off the nutrient supply to nerve tissue. A stroke can also result from a cerebral aneurysm, a type of damage that occurs when a blood vessel in the brain bursts.
The brain can also be injured in a local way during edema, a swelling of inflamed tissue which damages nerve cells by compressing them. Edema is caused by a traumatic injury to the head, such as a concussion, or by disease.
What do Neurological Exams Examine?
The reason that these neurological problems are so amenable to diagnosis by physical examination has to do with the anatomy of the nervous system. Trauma to nerve tissue is often apparent as impairment of the functions that the tissue performed. Careful observation of which functions are impaired coupled with thorough knowledge of neuroanatomy can indicate with great precision where in the body nerve tissues were damaged.
The basic neurological examination systematically examines functions that are associated with identified anatomy in the brain and spinal cord. The examination focuses on twelve large nerves, called the cranial nerves, which arise from groups of cells called nuclei in the brainstem. These cranial nerves serve narrowly defined functions such as reflexive eye blinking and movement of the tongue during chewing. By noting abnormalities in cranial nerve functions, a neurologist can deduce the location of many types of trauma in the nervous system. With that diagnosis in hand, options for both surgical and non-surgical treatments are greatly simplified, both reducing the cost and increasing the efficacy of treatment.
Light Reflex Tests
A neurological exam tests the twelve cranial nerves by subtly dissociating their functions. Shining a small flashlight into one eye, for example, can distinguish between damage to CN II (the optic nerve) and damage to CN III (the oculomotor nerve). The optic nerve controls the direct pupillary light reflex, which means that shining light into one eye causes the pupil in that eye to constrict. The oculomotor nerve, on the other hand, controls the consensual pupillary light reflex, which means that shining light into one eye causes pupil in the other eye to constrict. In a healthy patient, shining light into either eye should cause both pupils to constrict; abnormalities in these reflexes are indicators of possible damage to the cranial nerves or to the brainstem itself.
Many other tests included in the neurological examination also probe reflexes mediated by cranial nerves. For example, by lightly brushing a wisp of cotton across the front of the eyeball, the examiner can induce the corneal blink reflex, a blinking of both eyelids controlled by CN V. A tongue depressor can be used to test for the gag reflex, controlled by CN X. Rapidly turning a patient’s head to one side while they attempt to stare at a fixed point can suggest lower brainstem damage if the eyes cannot maintain their fixation point. This reflex is also called caloric nystagmus (caloric meaning heat, and nystagmus meaning a lateral movement of the eyes), because it can be induced by squirting cold water into one ear. The cold water slows down the fluid in the semicircular canals of that ear, and the brain, which compares fluid movements in the canals of both ears to judge motion, is fooled into perceiving a head turn.
Estimating the Size of a Tumor
It is not difficult to imagine a clinical situation in which a thorough knowledge of neuroanatomy coupled with a good neurological examination can yield an insightful diagnosis. An example is a cancer called an acoustic neuroma, which is a tumorous growth of the Schwann cells that sheath CN VIII (the acoustic nerve). The growth of the tumor initially compresses CN VIII. That compression can initially present itself as tinnitus, a ringing in the ear, followed by loss of hearing. As the tumor grows, it will begin to press into the space between the brainstem and the cerebellum, eventually compressing a branch of CN V (the trigeminal nerve) which controls the corneal blink reflex. When CN V is compressed, a wisp of cotton brushed across the eyeball will no longer illicit blinking. The tumor will continue to grow over time until it compresses CN VII (the facial nerve), resulting in a paralysis of the muscles on one side of the face. Further growth of the tumor will result in compression of the cerebellum, affecting the patient’s limb movements. Eventually, by closing the cerebral aqueduct, tumorous growth can lead to hydrocephaly. By observing the constellation of symptoms, a physician can quite accurately predict the size and location of the neuroma. Following confirmation by MRI, the tumor can then be surgically excised with minimal damage to peripheral tissues.
Together with a patient’s general health and mental status, simple neurological examinations can reveal volumes of information about damage to the brain resulting from stroke, edema, disease, and injury. Observations of this sort can give us a profound appreciation for the delicate and complex systems regulating the function of our bodies; at the same time they suggest that insight into the workings of the mind might require no more than looking very carefully.
|I||Olfactory||Sense of smell||Sensory afferent||Anosmia|
|II||Optic||Vision and direct pupillary light reflex||Sensory afferent||Blindness, loss of direct pupillary light reflex|
|III||Oculomotor||Innervates 4 extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique), and the levator palpebrae, which raises the eyelid; also controls the iris and the lens||Motor efferent||Dilated and fixed pupil, ptosis, ipsilateral gaze fixed “down and out”|
|IV||Trochlear||Innervates the superior oblique muscle, which intorts the eye and rotates it downward and outward||Motor efferent||Weakness of Downward Gaze|
|V||Trigeminal||Divided into opthlamic, maxillary, and mandibular branches. Mediates general sensation of the face, eyes, nose, and mouth. Innervates the muscles of mastication||Sensory afferent and motor efferent||Loss of general sensation in the face, loss of corneal reflex, deviation of the jaw to the ipsilateral side|
|VI||Abducent||Innervates the lateral rectus muscle, which abducts the eye (rotates it outward)||Motor efferent||Convergent strabismus
|VII||Facial||Facial movements, taste, salivation, and lacrimation||Sensory afferentand glandular efferent||Flaccid muscles of facial expression, loss of blink, loss of taste from the anterior two-thirds of the tongue, Bell’s Palsy|
|Balance and hearing||Sensory afferent||Disequilibrium, vertigo tinnitus and deafness,|
|Taste, salivation, swallowing, monitoring of blood oxygen and pressure||Sensory afferent and motor efferent||Loss of pharyngeal gag reflex, loss of taste from posterior third of tongue|
|X||Vagus||Innervates the larynx (voicebox) and controls swallowing||Sensory afferent and motor efferent|
|Cranial division innervates the layrnx, spinal division innervates muscles in the neck||Motor efferent||Difficulty in turning the neck colaterally, ipsilateral drooping shoulder|
|Tongue movement||Motor efferent||Ipsilateral Paralysis of the tongue|