The cerebellum is a complex structure located in the posterior cranial fossa. It has connections to the brainstem, basal ganglia, and cerebral cortex and plays a vital role in the coordination of movements. 

Lesions of the cerebellum produce neurological signs which are easy to recognize and have been shown to have excellent inter-rater reliability.[1] These signs evolve secondary to lesions in the vestibulocerebellar, vestibulospinal, and cerebellar ocular motor tracts. However, many signs such as gait ataxia and nystagmus may present in non-cerebellar lesions. A detailed neurological exam, combined with a good history, assists in accurate localization.

Early and accurate identification of these signs helps facilitate appropriate diagnostic testing and management, especially in time-sensitive emergency situations such as an acute cerebellar infarction. Utilizing a combination of signs at the bedside, such as HINTS testing is useful in broadly differentiating neurological signs of peripheral nervous system etiology from central etiologies (such as cerebellar lesions).[2]

Common cerebellar neurological signs are as follows:

Extraocular movements:

• Nystagmus: The pattern of nystagmus is different in etiologies of central origin, such as a cerebellar lesion as compared to etiologies of peripheral origin such as vestibulopathy. In etiologies of peripheral origin, the nystagmus is unidirectional irrespective of the direction of gaze and worsens when the patient directs their gaze towards the healthy ear (Alexander's law).[3][4]

• Impaired smooth pursuits: In cerebellar lesions, patients are unable to track objects with smooth eye movements. Instead, catch-up saccades are the presentation.[5]

• HINTS exam: HINTS exam is a combination of three maneuvers (Head Impulse test, Nystagmus, test of Skew) to help differentiate vertigo of central etiology from vertigo of peripheral etiology. A detailed description of the HINTS exam appears in PubMed in the article published by Kattah JC et al.[6]  
  • Head Impulse test: In vestibular disorders that cause vertigo, the head impulse test is often positive. Performance of this test is as follows:
    • The examiner sits across the patient and holds the patient's head in between both palms. The examiner asks the patient to fix the gaze on the examiner's nose. The head is rotated slightly laterally about 10-20 degrees to one side. Subsequently, the examiner brings the head back to the primary position in a swift motion while continually observing the patient's eyes. An individual with an intact vestibular system (and thus an intact vestibulo-ocular reflex) can maintain his or her gaze on the examiner's nose. In a patient with unilateral vestibular damage, when the head rotates to the primary position from the side with the vestibular lesion, a corrective horizontal saccade is seen. The test is then repeated for the contralateral side. 
    • It is important to rule out lesions of the cervical spine or paraspinal musculature before performing this test. If the examiner is doubtful about the integrity of the neck or the spine, it is best to avoid this test altogether. 
  • Nystagmus: In comparison to nystagmus of peripheral etiology as described above, the nystagmus of central etiology has the following features:
    • Bi-directional (gaze-evoked): The direction of nystagmus changes with the direction of gaze
    • Central nystagmus may also be vertical, which is uncommon in nystagmus of peripheral etiology. 
  • Test of Skew: In lesions involving the brainstem, vertical malalignment of the eyes may present (skew deviation). The alternate cover test can demonstrate this. In this test, the examiner asks the patient to look straight ahead and then alternately covers each eye at a time. If a skew deviation exists, a corrective vertical or oblique saccadic movement is appreciated.

• If any of the three tests point to a central etiology, the likelihood of a central etiology such as a posterior circulation stroke should merit strong consideration.[7][6] It is important to note that every test should be taken in the right clinical context and should not be used as the sole criteria to confirm or refute a diagnosis without considering the history, the rest of the neurological exam, and other investigations. 

Scanning speech: Cerebellar disorders can cause ataxic speech, also known as scanning speech, where the patient usually breaks words into respective syllables.[8]

Dysmetria [9]:

• Finger to nose test: This can be tested in the upper limb by having the patient reach out and touch the examiner's index finger with his or her index finger and then touch his or her nose with the same finger. In a patient with a lesion in the cerebellar hemisphere, the ipsilateral arm will manifest an intention tremor while nearing the target. This tremor occurs as a result of overshooting or undershooting of the patient's index finger due to improper coordination of movements. 

• Heel to shin test: For the lower extremities, the examiner asks the patient to move his or her heel across the shin in a proximal to distal motion. In a hemispheric cerebellar lesion, the patient will not be able to trace the shin in a straight line and will move the heel side to side.

Adiadochokinesia (dysdiadochokinesia) [5]:

• Patients with cerebellar lesions are unable to execute rapid alternating movements properly. The examiner asks the patient to place the palm on the knee and then perform rapid alternate pronation and supination of the forearm. Affected individuals will have difficulty in executing such alternating movements. The movements will appear jerky and irregular. 

Rebound phenomenon [5]:

• With elbows resting on the legs on the table, the examiner asks the patient to flex the elbows against the examiner's resistance. The examiner then abruptly stops providing resistance. Unaffected patients can contract the antagonist muscle(triceps) so that there is none to minimal flexion at the elbow. In individuals with cerebellar lesions, there is exaggerated flexion of the ipsilateral elbow due to the failure of timely contraction of the antagonist muscle. This phenomenon often presents exaggeratedly in spastic limbs. Other upper extremity joints can also undergo testing in this fashion with similar results.

Intention tremor [10]:

• Intention tremor is a kinetic tremor (most prominent when performing a task) the previously mentioned finger to nose test can elicit this sign. The tremor worsens as the patient approaches the examiner's finger. 

Ambulation:

• Stance and posture: In cerebellar lesions, patients tend to have a broad-based stance. The examiner may notice side to side or back and forth swaying of the body while the patient is standing; this is known as titubation.[5]

• Gait: The gait in cerebellar lesions is reminiscent of acute alcohol intoxication.[5] The patient tends to stagger or sway side to side and walks with a broad base; this is known as ataxic gait.[11]

• Tandem walk: Individuals with cerebellar lesions are unable to walk in tandem.[12] The test is performed as follows: The examiner asks the patient to walk in a straight line with the heel of the leading foot touching the toes of the lagging foot as if walking on a tightrope. This sign may also be seen in sensory ataxia or vestibulopathy. Thus, it is essential to check for other signs such as the Romberg's sign to differentiate sensory ataxia or vestibulopathy from cerebellar ataxia. 
• Absence of Romberg's sign: The examiner asks the patient is asked to stand with eyes open, feet close together, and arms by the side. The patient is then asked to close the eyes. Romberg's sign is positive if there is disproportionate swaying or patient falling with eyes closed as compared to eyes open. This sign is present in lesions of the sensory afferent pathway or the vestibular system. Excessive swaying, even with eyes open, can be seen in cerebellar lesions.[13]

Hypotonia:

• Damage to half of the cerebellum can lead to ipsilateral hypotonia

Cerebellar mutism [14]:

• If an injury occurs to the central cerebellum, such as from tumor or surgery, a patient may have mutism for days to indefinitely after the injury.

A pneumonic to remember some of the cerebellar signs is DANISH

• Dysdiadokinesia / dysmetria
• Ataxia
• Nystagmus
• Intention tremor
• Speech - slurred or scanning
• Hypotonia