CNRN common mistakes
CNRN · Understand the traps and how to avoid them.
On posturing vignettes, the candidate labels an image-free description of abnormal flexion (arms flexed, legs extended) as decerebrate and extension posturing as decorticate, or places the lesion on the wrong side of the neuraxis.
The two terms are memorized as a pair rather than being tied to the corticospinal and rubrospinal tracts; under time pressure the labels get swapped.
What to do instead
Anchor each pattern anatomically: decorticate = lesion ABOVE the red nucleus (cortex/hemisphere) with the red nucleus still driving flexor tone -> arms flexed; decerebrate = lesion AT or BELOW the red nucleus (brainstem) -> all four limbs extended, and it carries the worse prognosis. Rehearse by drawing the tract once, then self-test with five fresh descriptions.
Records a Glasgow Coma Scale as a single summed number (e.g., "GCS 7") and scores the verbal component as 1 or 5 in an intubated patient instead of marking it as untestable.
Prehospital and hand-off habits default to a total score, and the intubated verbal subscore has no obvious "correct" number, so the candidate guesses.
What to do instead
Always document the three components (E4 V5 M6). For an intubated patient write V1T or "V = T, untestable" and never let the tube score as orientation or as a 1. Remember a total score cannot be computed when one component is untestable, and that eye and motor scores carry the prognostic weight.
On NIHSS items, awards 0 when a deficit cannot be tested, credits improvement seen on a second attempt, or is surprised that a patient with a large posterior circulation stroke scores near 0.
The scale is treated as a general "stroke severity" score rather than a weighted anterior-circulation scale with specific scoring conventions.
What to do instead
Score what the patient actually did on the FIRST attempt and record untestable items as untestable (UN), never as 0. Learn the structural blind spots: dysarthria/ataxia/visual field items are under-weighted and posterior circulation deficits can produce a low score despite devastating deficits, so a score of 0 does not rule out stroke.
Waits for a blown pupil or a low Glasgow score as the trigger to escalate, and does not connect hypertension with widened pulse pressure plus bradycardia and irregular respirations to rising intracranial pressure.
Pupil change is the sign most often memorized and pictured, so it is treated as the first sign, while Cushing triad is filed away as a "late" curiosity and not recognized when it appears.
What to do instead
Build the timeline explicitly: earliest = declining level of consciousness and new headache/vomiting, then pupil change (ipsilateral blown pupil from CN III compression, with contralateral hemiparesis), and Cushing triad last and most ominous. Treat a new bradycardia-plus-hypertension pattern in a neuro patient as herniation until proven otherwise and escalate immediately rather than rechecking in an hour.
For hyponatremia after subarachnoid hemorrhage, selects fluid restriction as the first intervention without first establishing volume status.
SIADH and cerebral salt wasting are both "hyponatremia after SAH" and the SIADH treatment (restrict fluids) is the one most strongly memorized.
What to do instead
Differentiate by volume: CSW = hypovolemic, high urine output, high urine sodium, needs volume replacement with isotonic saline; SIADH = euvolemic, needs restriction. In the post-SAH patient, restricting fluid in a volume-depleted patient worsens vasospasm risk. The safe rule: assess volume status before choosing any sodium strategy.
In autonomic dysreflexia, reaches for an antihypertensive first and only then looks for the cause, or leaves the patient flat while working up the trigger.
The dramatic blood pressure reading drives action, and the noxious-stimulus sequence is recalled as a list rather than an ordered algorithm.
What to do instead
Order the response: sit the patient upright (at least 90 degrees, legs dependent) and loosen tight clothing FIRST, then search for and remove the noxious stimulus in the standard order - bladder (kinked or blocked catheter, distention, UTI), bowel impaction, skin (pressure injury, ingrown toenail, tight devices) - and use rapid-onset antihypertensives only if BP stays elevated after the stimulus is relieved. Remember the risk applies to injuries at T6 or above.
Treats a neuromuscular patient's oxygen saturation as the respiratory monitor and is reassured by a normal SpO2 while the patient is developing ventilatory failure.
Pulse oximetry is the default monitor on every unit, and neuromuscular respiratory failure is a pump failure that does not desaturate until very late.
What to do instead
Trend bedside pulmonary mechanics instead: vital capacity (falling toward 15-20 mL/kg or a rapid serial decline), negative inspiratory force/maximal inspiratory pressure, and count-then-speak in one breath. Also separate the patterns - myasthenia gravis is fatigable, bulbar-predominant and descending; Guillain-Barre is ascending with areflexia and autonomic instability - because the triggers and monitoring differ.
Reads a burst-suppression pattern on EEG as ongoing seizure activity, or treats triphasic waves as ictal, and considers a single normal 30-minute routine EEG sufficient to exclude seizures.
Any high-amplitude or rhythmic pattern gets filed as "abnormal = seizure," and the time-limited nature of routine EEG is underappreciated.
What to do instead
Attach each pattern to its meaning: burst suppression is suppressed background interrupted by bursts and is a therapeutic target in coma/barbiturate therapy, not a seizure; generalized 3-Hz spike-and-wave is absence; triphasic waves indicate metabolic encephalopathy. Because interictal routine EEG can be normal, nonconvulsive status requires prolonged or continuous video-EEG monitoring.
Answers phenytoin administration questions with a dextrose diluent or a rate faster than 50 mg/min, and forgets the in-line filter; similarly handles mannitol without filtering or monitoring serum osmolality.
These drugs look like routine IV pushes, and the incompatibility and rate ceilings are memorized as trivia rather than as safety rules.
What to do instead
Fix the hard rules: phenytoin goes in normal saline only (it precipitates in dextrose), through a filter, no faster than 50 mg/min in adults with continuous BP and ECG monitoring, and extravasation causes purple glove syndrome; fosphenytoin is dosed in phenytoin equivalents and can run faster. Mannitol requires a filter, must be inspected for crystals (warm if present), and is titrated against serum osmolality/osmol gap with monitoring for rebound and renal effects.
Levels the external ventricular drain transducer at the mid-axillary line, leaves the drain unclamped during repositioning or transport, or lets the patient's head of bed be lowered without adjusting the system.
The mid-axillary reference is the habit carried over from arterial and central venous pressure monitoring, and drain management is treated as a setup task rather than a continuous safety responsibility.
What to do instead
Zero and level the EVD at the tragus (external auditory meatus) with the patient's head in the ordered position, re-level whenever the head of bed changes, clamp the system before repositioning, transport, or any maneuver that changes the column height, and keep drainage at the prescribed pressure/height to avoid over-drainage, pneumocephalus, or herniation. Apply the same clamp-before-move rule to lumbar drains.