Lesion localizer

The leg and foot are represented on the medial surface of the motor cortex, supplied by the ACA; frontal bladder control is also affected.

Mixed upper and lower motor neuron signs in the same region with no sensory loss.

Infarction of the anterior two-thirds of the cord (e.g. after aortic surgery) spares the dorsal columns.

Surgical neck fracture or anterior dislocation — weak deltoid with the "regimental badge" patch of numbness.

Corticospinal and corticobulbar tracts are destroyed but the reticular activating system above is spared — the patient is awake and communicates with vertical eye movements.

Ipsilateral weakness and vibration/proprioception loss below the lesion; pain and temperature are lost on the opposite side because the spinothalamic fibers have already crossed.

Weak elbow flexion and wrist extension, reduced biceps/brachioradialis reflex, numbness of the thumb and lateral forearm.

The most common cervical radiculopathy — weak triceps, reduced triceps reflex, numb middle finger.

Compression of multiple roots (large central disc herniation, tumor) — a surgical emergency; signs are lower motor neuron.

Hemisphere lesions cause ipsilateral limb ataxia; a stroke here can swell and compress the fourth ventricle and brainstem.

The vermis controls the trunk and gait; alcohol damages the anterior vermis, giving a wide-based gait with fairly normal arm coordination.

The intact side pulls the uvula away from the lesion; recurrent laryngeal involvement causes hoarseness.

Usually injured during a posterior-triangle node biopsy; trapezius weakness causes shoulder droop and lateral scapular winging.

The tongue deviates toward the side of the lesion, with atrophy and fasciculations.

The pupillary fibers run on the surface of the nerve, so compression dilates the pupil early. A painful pupil-involving palsy is an aneurysm until proven otherwise.

Microvascular ischemia affects the core of the nerve and tends to spare the superficial pupillary fibers.

The superior oblique depresses the adducted eye; patients tilt the head away from the weak side. Head trauma is the most common acquired cause.

The lateral rectus fails. Raised intracranial pressure can stretch CN VI — a false localizing sign.

A lower motor neuron lesion weakens the whole half of the face including the forehead; branches to stapedius and the chorda tympani may be involved.

Asymmetric sensorineural hearing loss with tinnitus; larger tumors compress CN V (reduced corneal reflex) and the cerebellum.

Foot drop with weak eversion but normal inversion (tibialis posterior is supplied by the tibial nerve).

Classically vitamin B12 deficiency — sensory ataxia with a positive Romberg plus upper motor neuron signs.

Weak quadriceps with a lost patellar reflex; hip adduction (obturator nerve) is normal, which helps separate it from an L4 radiculopathy.

Mild ptosis with a small pupil. Look for a Pancoast tumor at the lung apex, carotid dissection (with neck pain) or a lateral medullary stroke.

Motor fibers for the whole half-body are packed together, so a small lacune weakens face, arm and leg equally, without cortical signs.

Weak knee extension, reduced patellar reflex and numbness of the medial leg; hip adduction (obturator nerve, L2–L4) may also be weak, unlike a femoral nerve lesion.

Foot drop with weak inversion (tibialis posterior, L5) as well as dorsiflexion distinguishes an L5 root from a common fibular nerve lesion.

Crossed pain/temperature loss (spinal trigeminal tract vs spinothalamic tract), nucleus ambiguus signs and ipsilateral Horner — with no limb weakness.

Serratus anterior palsy after axillary surgery or a chest tube — medial winging when pushing against a wall.

Traction on an abducted arm, a Pancoast tumor or a cervical rib; T1 involvement can add Horner syndrome.

The MCA supplies the face and arm areas of the motor and sensory cortex, the language areas and the optic radiations. Eyes deviate toward the lesion.

Same motor and field deficits as a dominant MCA stroke, but the nondominant parietal lobe gives neglect and anosognosia instead of aphasia.

Ipsilateral CN XII, contralateral hemiparesis (pyramid) and contralateral loss of vibration/proprioception (medial lemniscus).

Numbness of the lateral 3½ digits with the palm spared, nocturnal symptoms and thenar weakness; Tinel and Phalen signs.

The MLF links the abducens nucleus to the opposite medial rectus subnucleus. Bilateral INO in a young adult suggests multiple sclerosis.

The crossing nasal fibers carry both temporal fields; a pituitary adenoma or craniopharyngioma compresses them from below.

A lesion in front of the chiasm affects only one eye and causes a relative afferent pupillary defect; pain on eye movement suggests optic neuritis.

Compression of the dorsal midbrain (often a pineal tumor) impairs vertical gaze and the pupillary light reflex.

Occipital infarction gives a contralateral homonymous hemianopia; the macula is often spared because the occipital pole also receives MCA blood.

Midshaft humerus fracture or "Saturday night palsy" — wrist drop with triceps usually spared.

Weak plantarflexion, reduced ankle reflex, numbness of the lateral foot and sole.

Gluteus medius and minimus can't stabilize the pelvis, so it drops on the unsupported side during stance.

A central cavity interrupts the crossing pain/temperature fibers at the affected levels on both sides; touch and vibration are spared.

VPL (body) and VPM (face) sit side by side, so a small thalamic infarct causes pure sensory loss of the whole opposite side, face included.

Weak plantarflexion and inversion with numbness of the sole; at the tarsal tunnel only the sole is affected.

Clawing of the ring and little fingers, numb medial 1½ digits and a positive Froment sign (weak adductor pollicis).

An expanding supratentorial mass pushes the uncus onto CN III (blown pupil first) and the midbrain — a neurosurgical emergency.

Traction that widens the neck–shoulder angle (shoulder dystocia) — "waiter's tip" posture.

Ipsilateral CN III palsy with contralateral hemiparesis — the CN III fascicles cross the cerebral peduncle.